*Article* **Physical and Biological Features of the Waters in the Outer Patagonian Shelf and the Malvinas Current**

**Pavel A. Salyuk 1,\* , Sergey A. Mosharov <sup>2</sup> , Dmitry I. Frey 2,3 , Valentina V. Kasyan <sup>4</sup> , Vladimir I. Ponomarev <sup>1</sup> , Olga Yu. Kalinina <sup>2</sup> , Eugene G. Morozov <sup>2</sup> , Alexander A. Latushkin <sup>3</sup> , Philipp V. Sapozhnikov <sup>2</sup> , Sofia A. Ostroumova 2,5, Nadezhda A. Lipinskaya <sup>1</sup> , Maxim V. Budyansky <sup>1</sup> , Pavel V. Chukmasov <sup>6</sup> , Viktor A. Krechik 2,7, Michael Yu. Uleysky <sup>1</sup> , Pavel A. Fayman <sup>1</sup> , Alexander Yu. Mayor <sup>8</sup> , Irina V. Mosharova <sup>2</sup> , Anton D. Chernetsky <sup>2</sup> , Svetlana P. Shkorba <sup>1</sup> and Nikita A. Shved <sup>4</sup>**


**Abstract:** The aim of this study is to trace how the fine-thermohaline and kinematic structure, formed over a section along 45.8◦ S in the interaction zone of the outer Patagonian Shelf (PS) and Malvinas (Falkland) Current (MC) System waters, affect the spatial distribution of bio-optical characteristics, phyto/zooplankton, birds, and marine mammals. For the first time, simultaneous multidisciplinary observations at high spatial resolution (~2.5 km) were performed in this region during the cruise of the R/V "Akademic Mstislav Keldysh" in February 2022. A fine structure of alternating upwelling and downwelling zones over the PS and slope was identified, which resulted from the interaction between the MC inshore branch (MCi), bottom topography, and wind. This interaction significantly affects all the physical, and optical characteristics analyzed in the work, as well as the biota of the region. It was found that the euphotic zone is larger in the downwelling zones than in the upwelling zones, and all spatially local maxima of phytoplankton photosynthetic efficiency are observed in the zones between upwelling and downwelling. Phytoplankton along the section were represented by 43 species. A total of 30 zooplankton species/taxa were identified. Three species of marine mammals and 11 species of birds were recorded in the study site. Most of the phytoplankton species list were formed by dinoflagellates, and picoplankton *Prasinoderma colonial* quantitatively dominated everywhere. Two floristic and three assemblage groups were distinguished among the analyzed phytoplankton communities. High phytoplankton biodiversity was observed above the PS and low above the PS edge and in the MCi core. Copepods mostly dominated in zooplankton. Subantarctic species/taxa of zooplankton concentrated in the nearshore waters of the PS, while Antarctic species/taxa were most abundant in the zone between the MCi and the MC offshore branch (MCo). The relative abundance of birds in the PS was several times higher than in the MCo. The minimum abundance of birds was in the MCi in the zone of the strongest upwelling identified above the PS edge.

**Keywords:** Malvinas Current; submesoscale; water mass; Lagrangian analysis; upper mixed layer; euphotic layer; bio-optical; fluorescence; primary production; SADCP; CTD

**Citation:** Salyuk, P.A.;

Mosharov, S.A.; Frey, D.I.; Kasyan, V.V.; Ponomarev, V.I.; Kalinina, O.Y.; Morozov, E.G.; Latushkin, A.A.; Sapozhnikov, P.V.; Ostroumova, S.A.; et al. Physical and Biological Features of the Waters in the Outer Patagonian Shelf and the Malvinas Current. *Water* **2022**, *14*, 3879. https://doi.org/10.3390/ w14233879

Academic Editor: Guangyi Wang

Received: 9 September 2022 Accepted: 25 November 2022 Published: 28 November 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

#### **1. Introduction**

The Malvinas Current (MC) in the Southwest Atlantic, which transports Subantarctic waters to the north, makes a significant contribution to the formation of zones of high productivity in the waters along the Patagonian Shelf [1]. The cool, nutrient-rich MC branches off from the Antarctic Circumpolar Current [2] and splits into two main currents: inshore and offshore [3–5]. The inshore branch of the current is formed along the edge of the Patagonian Shelf. The offshore branch flows in the deep ocean. The MC exists as two jets, which merge near 44◦ S as the continental slope steepens [6]. South of this location, significant variability of the current was observed based on direct velocity measurements on a mooring array [7].

The waters of the MC strongly interact with the warmer fresh waters of the Patagonian Shelf [8–10]. They play an important role in the spreading and migration of various marine organisms in the Southwest Atlantic [11]. The interaction of different water masses leads to a high species biodiversity in this region [12]. This may include species from mid-latitudes, Subantarctic, and Antarctic regions.

Alternating zones of upwelling and downwelling are formed as a result of the influence of the Patagonian slope on the dynamic characteristics of the baroclinic MC [13,14]. In addition, the effect of wind, which modifies upwelling processes and can enhance them, is an important factor affecting slope upwelling [15]. Upwelling leads to the transport of the waters from the underlying layers, which are rich in nutrients and microelements, to the euphotic layer. This is accompanied by an increase in the abundance of phytoplankton [16], which is the primary link in the food chain. This process is manifested in the satellite sea color data, as the formation of a pronounced band of increased concentration of chlorophyll*a* (chl-*a*) observed approximately from (50◦ S, 62◦ W) to (38◦ S, 56◦ W) [17,18], which corresponds to a spatial scale of about 1300 km. Chl-*a* concentration maps based on satellite data make it possible to study the formation of a zone of high chl-*a* concentration west of the MC inshore branch [16,19].

Zooplankton is the most important intermediate link in the food chain between primary producers and higher trophic levels of the pelagic marine ecosystem [20]. The biomass of zooplankton determines the amount of resources available to fish, birds, and marine mammals [21]. Regular studies of phytoplankton and zooplankton communities are important because most planktonic organisms are characterized by short life cycles and quickly respond to environmental changes [22], including changes in anthropogenic impact [23] and climate [24].

The Patagonian Shelf is a region of intense fishing [25] and many oil and gas fields [26]. This may have an impact on biodiversity and status of all trophic levels in the marine ecosystem. At the same time, decadal variations in the circulation have been observed in the region, associated with a more distant propagation of the subtropical waters of the Brazil Current to the south [27,28]. A long-term increase in the temperature gradient on the slope of the Patagonian Shelf has been recorded, which can affect the environmental conditions of the development of phytoplankton cells [29]. Further climate change is predicted that would lead to reduced fish catches in the Southwest Atlantic [25]. An integrated view of Atlantic coastal Patagonian ecosystems, including the physical environment, biodiversity, and the main ecological processes, together with the ecosystem changes derived by them and anthropogenic impacts are presented in the book [30], where the past, present, and future of ecosystem functions and services in the Patagonian coastal ecosystems are described.

Thus, a significant influence of oceanographic processes on the functioning of marine ecosystems of various trophic levels occurs in the study site. Therefore, the ongoing biological processes must be studied together with the multiscale variability of physical processes in the ocean. Such multidisciplinary investigations were performed in February 2022 during cruise 87 of the R/V "Akademik Mstislav Keldysh", in which a section along 45.8◦ S has been made. The presence of scientists of various specialties allowed us to perform integrated research in the area linking the oceanographic variables of the MC over the Patagonian Shelf and slope with the optical and biological studies.

The novelty of the study is based on the analysis of the first simultaneous multidisciplinary observations of physical oceanography conditions, optical variables, and marine biology aspects in a high-productive region of the Patagonian Shelf. The data were collected at very high spatial resolution (~2.5 km between stations on average), which is much higher than all previously published surveys [3–5,14]. These new data allowed us to determine narrow upwelling and downwelling zones not fully revealed by satellite data and available numerical models.

The study aims to trace how the fine-thermohaline and kinematic structure formed in the interaction zone of the outer Patagonian Shelf and how inshore Malvinas Current branch waters affect the spatial distribution of bio-optical characteristics, phytoplankton, zooplankton, birds, and marine mammals.

#### **2. Materials and Methods**

Oceanographic, optical, and biological data for the analysis were collected on cruise 87 of the R/V "Akademik Mstislav Keldysh" on 21–22 February 2022 (end of summer in the Southern Hemisphere) over the section from the waters of the Patagonian Shelf to the intersection of the inshore and offshore jets of the MC at 45.8◦ S. A total of 16 stations were made from 60.4◦ W to 59.6◦ W and continuous hydrographic and bio-optical records of water properties in the upper layer were made using a flow-through system along the route of the ship from 60.4◦ W to 58.4◦ W. In addition, satellite data of chl-*a* concentration and sea surface temperature were used for the analysis. Summary information on the stations and continuous measurements is presented in Table 1.


**Table 1.** Summary of completed oceanographic and biological studies.

Note(s): \* PS is the Patagonian Shelf, PSe is the PS edge, MC is the Malvinas Current, (MCi) is the MC inshore branch, (MCo) is the MC offshore branch, ZB is the zone between inshore and offshore branches of the MC. \*\* Distance to the previous station. (1) Continuous ADCP and flow-through measurements on the ship's route. (2) In-situ CTD and bio-optical profiling. (3) Phytoplankton activity and primary production analysis. (4) Phytoplankton species analysis. (5) Zooplankton species analysis. (6) Observations of birds and mammals.

#### *2.1. Field Oceanographic and Optical Measurements*

#### 2.1.1. Vertical Profiling at Oceanographic Stations

Vertical profiles of oceanographic variables such as temperature (*T* in ◦C), salinity (*S* in practical salinity units (PSU)), and potential density (*σ* in kg/m<sup>3</sup> ) of seawater, have been measured at stations. Optical parameters were also determined: chl-*a* fluorescence intensity (*Fchl* in relative units), phycoerythrin fluorescence intensity (*Fpe* in relative units), fluorescence intensity of colored dissolved organic matter (CDOM) (*FCDOM* in relative units), seawater turbidity (*Turb* in formazin turbidity units (FTU)), the beam attenuation coefficient at 660 nm (*BAC*<sup>660</sup> in FTU), and the photosynthetically active radiation value (*PAR* in µmol photons/m2/s). The *PAR* measurements were carried out only during daylight hours. The depth of the euphotic zone (*Zeu*) was determined from the vertical profiles of *PAR* as 1% of the *PAR* incident on the sea surface.

The measurements were performed by an instrument set consisting of three probes: AML Oceanographic BaseX CTD probe for measuring oceanographic variables (*T*, *S* and calculated *σ*, and calculated potential temperature *θ* for *θ*-*S* diagrams plotting), measurements were carried out at a frequency of 4 Hz; Turner C6P for measuring *Fchl, FCDOM*, and *Turb* at a frequency of 1 Hz; Kondor instrument to determine the *BAC*<sup>660</sup> at 4 Hz and the *PAR* at 8 Hz. All instruments were calibrated before the expedition. The instrumental complex descended at a speed of 0.5 m/s to a depth of 250 m or to the bottom if the depth was shallower. Only the data measured during downcast of the instruments were used for the analysis.

#### 2.1.2. Measurements of the Current Velocity Vector during the Ship Motion

Direct velocity measurements were carried out by Shipborne Acoustic Doppler Current Profiler (SADCP) system Teledyne RD Instruments Ocean Surveyor (TRDI OS) with a frequency of 76.8 kHz. During the survey the profiler was set in the narrowband mode, which increases the profiling range up to 700 m depth. We set 60 vertical bins 16 m each with an 8 m blank distance immediately below the transducer. The draught of the ship is 6 m, which gives 22 m depth for the center of the first bin (the depth of the uppermost layer of velocity measurements). Time averaging of the raw data was made over 120 s intervals. Since the ship speed varied between 8 and 10 knots, this time average represents an along-track averaging of roughly 500 m. Measurement errors in the amplitude of the horizontal velocities were small, approximately 1–2 cm/s [31]. The TPXO9 model [32] was used to subtract the barotropic tidal velocities at the time of measurements.

#### 2.1.3. Distinguishing Upwelling and Downwelling Zones

The upwelling and downwelling zones were determined based on the analysis of six oceanographic and optical variables over the section: temperature, salinity, density, chl-*a* fluorescence, beam attenuation coefficient at 660 nm, and turbidity. The zones were identified based on the topography of the lower boundary of the upper mixed layer (UML), temperature, salinity, and density contours in the underlying layers. The UML depth (*ZUML*) was estimated by four different methods: (1) is the depth where ∆*T* = 0.5 ◦C relative to the sea surface [33]; (2) is the depth where ∆*T* = 0.8 ◦C relative to 10 m depth [34]; (3) is the depth where potential density gradient is equal to 0.05 kg/m3/m [35]; (4) is the depth where ∆*σ<sup>θ</sup>* is equal to the theoretical difference between *σ<sup>θ</sup>* (*T*10–0.2 ◦C, *S*10) and *σ<sup>θ</sup>* (*T*10, *S*10) for the atmospheric pressure, *T*<sup>10</sup> is the measured temperature at 10 m, S<sup>10</sup> is measured salinity at 10 m [36]. Using four different methods made it possible to increase the accuracy of *ZUML* calculations.

Upwellings correspond to the ascent of the density interface layer and a decrease in the UML thickness, as well as the elevation of the isolines of other measured parameters. In downwelling zones, the opposite behavior of isolines has been observed. Only the most obvious cases of upwelling or downwelling in the UML were selected in the zones of local *ZUML* maxima or minima along the section. Joint analysis of oceanographic and optical variables makes it possible to more accurately identify upwelling and downwelling zones,

and to make the cross-validation of the results. Some zones of vertical motions are better manifested in oceanographic characteristics, while others are more clearly identified from the optical parameters.

#### *2.2. Shipboard Biological Measurements*

#### 2.2.1. Chlorophyll-*a* and Primary Production

Water samples were collected at 11 stations in the study site from a depth of 5 m using the flow-through systems installed on the research vessel (Table 1). The samples were divided into subsamples, which were used for measurement of different productivity parameters (chlorophyll concentrations, experimental carbon fixation estimations (primary production), active fluorescence of chlorophyll).

Primary production (*PP*) was measured onboard using the <sup>14</sup>C uptake method [37]. Water samples were collected in 50-mL flasks and incubated for 3 h in an original laboratory phytoincubator with individually adjustable LED illumination and temperature maintenance using a HAILEA-100 laboratory cooler. After incubation, flasks were filtered onto a 0.45-µm "Vladipore" membrane (Vladimir, Russia). The radioactivity of the samples was determined using a Triathler (Hidex, Turku, Finland) liquid scintillation counter.

Chl-*a* concentration was measured fluorometrically with correction for phaeopigments [38]. Water samples (500 mL) were filtered onto Whatman GF/F (glass-fiber filters) under a low vacuum (0.2 atm). For extraction, Chl-*a* filters were placed in acetone (90%). After 24 h in the cold (4 ◦C) and dark conditions, the extracts were measured with a MEGA-25 fluorometer (Moscow, Russia) before and after acidification with 1 N HCl [39]. The fluorometer has been pre-calibrated using a chl-*a* standard (Burlington, VT, USA, Sigma-Aldrich).

Biomass-specific *PP* (*P B* in [mgC/mg chl-*a*/day]) was calculated by dividing *PP* rates by the chl-*a* concentration in the respective sample [40].

Active chl-*a* fluorescence was measured using an ultrasensitive WATER-PAM fluorometer (Walz, Effeltrich, Germany). The samples were exposed in the fluorometer to light intensity (*E*) as to <sup>14</sup>C uptake and steady fluorescence (*Ft*) and maximum fluorescence (*F<sup>m</sup>* 0 ) measurements. The effective quantum efficiency of photosystem II (PSII) was determined as follows [41]:

$$
\Delta F/Fm' = (F\_m' - F\_t)/F\_m'$$

The relative electron transport rate (*rETR*, relative unit) was calculated from the equation:

$$rETR = \Delta F/F\_m ' \times E \times 0.5 \text{ \AA}$$

where coefficient 0.5 takes into account the distribution of photons between photosystems I and II [42]. *rETR* is a measure of the rate of linear electron transport through PSII, which is correlated with the overall photosynthetic performance of the phytoplankton [43]. *E* is the level of natural illumination of the sample in the chamber of the PAM fluorimeter (µmol photons/m2/s), and coefficient 0.5 takes into account the distribution of photons between photosystems I and II [42]. To determine *rETR*, water samples were exposed at 8 levels of light intensity in the range from 0 to 1500 µmol photons/m<sup>2</sup> per second, including the light intensity corresponding to the natural light at the sampling point, and applied in the incubator for radiocarbon measurements. The *rETR* value reflects the rate of conversion of solar energy into chemically bound energy in cells, which provides the processes of organic matter biosynthesis by phytoplankton.

#### 2.2.2. Phytoplankton Species

Phytoplankton samples were taken at stations using the Apshtein medium plankton net (mesh size 35 µm, the diameter of the upper ring 400 mm) from depths of 0–50 m. This method was used to obtain an integral sample covering a continuous range of depths. In contrast to the method of collection with the help of Niskin bottles, total sampling over a layer of the water column makes it possible to cover the layers where phenomena

that are significant for the structural features of phytoplankton, but poorly detected by hydrophysical instruments, take place. We are talking, for example, about layers with a high concentration of heterotrophic flagellates. In order to avoid clogging the cells of the Capron tree, the plankton net was lifted at a speed of no more than 0.5 m/s. At those stations, network clogging did not occur: none of the microalgae species identified in the samples produced a large amount of exopolymers. The method of sampling the water column layer (individual depths) using a fine-mesh net makes it possible to quantify the structure of the community from the standpoint of the relative abundance of species [44,45]. In this case, this form of a result was relevant for us, since we did not set ourselves the task of identifying the commercial characteristics of communities. This study did not take into account the absolute values of the abundance and biomass of phytoplankton at individual depths.

Two samples were taken from each location. One of them was viewed live, immediately after selection noting the status of the abundance of various species of microphytes (planktonic microalgae). The primary count of microalgae differentially by species and photo-documentation of microalgae was performed based on the material of this sample using a ToupCam 5.1 MP digital microscope camera at magnifications 200×, 400×, and 1000×. The second sample was fixed with an ethanol solution at a final concentration of 70% for transportation to the onshore laboratory of the Shirshov Institute.

Here, 0.25 mL was taken from a concentrated sample with a volume of 25 mL and placed on a glass slide (75 × 25 mm, Citoglas, ApexLab, Moscow, Russia), covered with a large coverslip (24 × 50 mm, Premier Cover Glass, Leica Biosystems, Wetzlar, Germany). Microalgae were counted differentially by species, in strips along the glass slide, based on a total number of 400 specimens [46].

Identification of organisms to a particular species was established using modern atlases and guides, as well as articles on the phytoplankton of the Atlantic and a review of some genera inhabiting oceanic waters and containing species diagnoses [47–58].

#### 2.2.3. Zooplankton Species

Zooplankton samples were obtained at 10 stations by a Multinet [59] opening/closing net system (0.25 m<sup>2</sup> aperture) equipped with five nets of 150 µm mesh-size. Additionally, larger and rarer taxa were collected by a pelagic Double Square net (DSN) (505 µm mesh, 1.0 m<sup>2</sup> inlet area) [60] equipped with a pterygoid deepener weighing 24 kg (Hydrobios, Altenholz, Germany) by performing oblique tows from 200 m to the surface or from near-bottom to the surface at depths <200 m at an average speed of 1.5 knots. The DSN net was equipped with a water flow counter (Hydrobios, Altenholz, Germany). Species were identified to the lowest possible taxonomic level using stereoscopes SZX7 and SZ51 (Olympus, Tokyo, Japan) using special identification keys [61–63]. Copepods, as a dominant group of zooplankton were subjected to a more detailed taxonomical processing. Other zooplanktonic crustaceans were grouped into a class or family. The abundance of zooplankton taxa was determined from each nets sample; it is expressed as individuals per cubic meter (ind./m<sup>3</sup> ) and calculated using the Zooplankton Methodology Collection and Identification [64].

#### 2.2.4. Statistical Analysis of Phyto- and Zooplankton Communities

The algorithms of the Plymouth routines in multivariate ecological research (PRIMER-6 software package [65]) were used for the ecological analysis of the collected data. The Shannon-Weaver index (*H*0 ) was used for calculating the diversity of phytoplankton at stations. The Pielou index (*J* 0 ) and the probability of interspecific encounters (*PIE*) were used to determine the evenness of species structure. The latter index shows the probability that two microalgae randomly found in the same field of view of the microscope at a working magnification of 400× belong to different species. The Sørensen index was used for calculating the similarity of communities at stations, according to the qualitative criteria, while the Bray-Curtis index was used according to the quantitative criteria. The

construction of models with the identification of community groups was carried out using the MDS and clustering methods. The ANOSIM procedures (PRIMER-6) were used to check the reliability of the identification of floristic and assemblage groups. The SIMPER procedure (PRIMER-6) was used for identifying groups of species that characterize one or another group of assemblages.

To compare zooplankton communities, a non-metric cluster and multidimensional scaling (MDS) analyses were performed using the PRIMER-6 computer package. Species abundance data were log10(x + 1) transformed, and a station similarity matrix was generated using the Bray–Curtis metric. Cluster analysis was then applied using group average sorting. To test for significant numerical differences between identified clusters, ANOSIM was conducted on log-transformed abundance data. MDS was performed on the similarity matrix. A SIMPROF test was conducted (α = 0.01) to determine the statistically significant differences between clusters [65]. Diversity was assessed using the Shannon–Weaver index (*H*0 ).

#### 2.2.5. Birds and Mammals

Onboard ship observations of marine mammals and birds were performed at the study site. The observations were carried out continuously during the daylight hours at winds (Beaufort scale < 5) and visibility of more than 50 m by two observers simultaneously from the port and starboard sides of the vessel from the direction-finding deck located at a height of 17 m above the sea level. The total observation time was 10 h 32 min. Relative abundance in units of birds per hour was calculated for the analysis of birds.

#### *2.3. Satellite Sea Surface Temperature and Chlorophyll-a*

We used satellite data on February 18, 2022, for additional analysis of the spatial fields of the chl-*a* concentration (*Chlsat*) and sea surface temperature (*SST*) in the study area: *Chlsat* were calculated from the OLCI radiometer Level-2 data (Sentinel-3B satellite) using the standard OC4ME global empirical algorithm with ~300 m resolution; *SST* values were obtained from the VIIRS radiometer Level-2 data (Suomi-NPP satellite) at 800 m resolution. Chl-*a* data were downloaded through the Ocean Virtual Laboratory web portal (https://ovl.oceandatalab.com/ accessed on 25 November 2022), which uses CREODIAS Finder service (https://finder.creodias.eu/ accessed on 25 November 2022) and *SST* data from Ocean Color Web (https://oceancolor.gsfc.nasa.gov/ accessed on 25 November 2022), where the data are processed by standard algorithms with recommended quality flags set.

It was possible to qualitatively calculate the analyzed parameters in the absence of atmospheric cloudiness. The time difference between satellite and ship measurements was 3 days selecting the closest in time satellite images. This prevents a correct comparison of the measured values. However, it is acceptable for the analysis of the similarity of the observed spatial structures. Additionally, we used native resolution satellite data from a single satellite image without additional merging with other data and interpolation allowing the resolving of fine structure features. Application of satellite data is helpful to avoid misinterpretation of in-situ measurements along the section, as it becomes possible to analyze the instantaneous image of manifestations of oceanographic processes in the upper layer of the sea.

#### *2.4. Simulated Oceanographic Data and Bottom Topography*

Lagrangian analysis has been applied to study the origin of waters in the study site, in which geostrophic velocities calculated from the AVISO satellite altimetry data (0.25◦ × 0.25◦ ) were used to calculate particle trajectories. A large number of synthetic particles (the region is shown in Figure 1a) has been seeded daily with tracers on a grid of 700 × 700 points and their trajectories have been computed back in time for a fixed period of time [66]. The period of integration back-in-time in our study was specified from 21 February 2020, to 21 February 2022 (two-year period). One gets the so-called origin Lagrangian map (O-map) by marking the particles with different colors, which arrived from the northern, western, southern, and eastern boundaries of the study site in the past.

upper layer of the sea.

*2.4. Simulated Oceanographic Data and Bottom Topography*

western, southern, and eastern boundaries of the study site in the past.

with a resolution of 1 min (~1.85 km) were used to plot bottom topography maps.

**Figure 1.** Map of the study site: Lagrangian map of the origin of waters (**a**); a diagram of currents in the southwestern part of the Atlantic on 21 February 2022 (**b**); map of the study site bathymetry and locations of stations (**c**). In Figure (**a**) blue color indicates the flow of water from the south, cyan is related to the flow from the west, red: from the north, yellow: from the east, green: from the Patagonian Shelf, white: from the Antarctic shelf or ice.

was 3 days selecting the closest in time satellite images. This prevents a correct comparison of the measured values. However, it is acceptable for the analysis of the similarity of the observed spatial structures. Additionally, we used native resolution satellite data from a single satellite image without additional merging with other data and interpolation allowing the resolving of fine structure features. Application of satellite data is helpful to avoid misinterpretation of in-situ measurements along the section, as it becomes possible to analyze the instantaneous image of manifestations of oceanographic processes in the

Lagrangian analysis has been applied to study the origin of waters in the study site, in which geostrophic velocities calculated from the AVISO satellite altimetry data (0.25° × 0.25°) were used to calculate particle trajectories. A large number of synthetic particles (the region is shown in Figure 1a) has been seeded daily with tracers on a grid of 700 × 700 points and their trajectories have been computed back in time for a fixed period of time [66]. The period of integration back-in-time in our study was specified from 21 February 2020, to 21 February 2022 (two-year period). One gets the so-called origin Lagrangian map (O-map) by marking the particles with different colors, which arrived from the northern,

The data from the bathymetric database ETOPO1 (Earth topography 1 arc minute)

The data from the bathymetric database ETOPO1 (Earth topography 1 arc minute) with a resolution of 1 min (~1.85 km) were used to plot bottom topography maps.

#### **3. Results**

#### *3.1. Common Oceanographic and Bio-Optical Characteristics of the Study Site*

Results of the Lagrangian analysis of backward particle trajectories from 21 February 2022, with superimposed velocity vectors of the geostrophic currents calculated from the AVISO data, are shown in Figure 1a,b. The study site is marked with a rectangle. Figure 1c shows the bottom topography based on the ETOPO1 digital bathymetry data and the position of the oceanographic section marked with a black dotted line. The stations and track of continuous SADCP measurements are indicated.

The data can be divided into "shelf" (stations 7393–7400) at depths in the range of 130–150 m and "slope" stations (st. 7401–7408) at depths of 280–1000 m. St. 7400 and 7401 can also be classified as "shelf edge". It is seen from Figure 1 that the MC transports Subantarctic waters from the Drake Passage to the study site. South of 43◦ S, the MC splits into two jets: inshore and offshore.

Figure 2 shows the measured SADCP northern current velocity component (*V* in cm/s, Figure 2a) and eastern current velocity component (*U* in cm/s, Figure 2b) over the section along 45.8◦ S with marked positions of vertical profiles at stations. One can see the northward propagation of two jets of the MC (Figure 2a) and anticyclonic circulation between the jets with intensification on the western side. In this work, the jet boundaries are chosen along the 40 cm/s isoline. *Water* **2022**, *14*, x FOR PEER REVIEW 10 of 36

**Figure 2.** Meridional (**a**) and zonal (**b**) components of SADCP velocity measurements along the section of the study site. **Figure 2.** Meridional (**a**) and zonal (**b**) components of SADCP velocity measurements along the section of the study site.

The inshore branch is located close to the shelf and deepens to about 450 m. The western boundary of this branch in the surface layer is located near station 7397, and the eastern boundary is near station 7406. The width of the branch at the sea surface is about 40 km. The core of the current was recorded at stations 7400–7404, where the maximum velocities exceed 60 cm/s. The offshore branch of the current extends deeper than 750 m

The satellite data on the concentration of chl*-a* (*Chlsat*) and *SST* are presented in Figure 3, for an additional description of the study site. The section shows alternating high and low values of chl*-a* concentrations and *SST* values, which are associated with upwelling

The inshore branch is located close to the shelf and deepens to about 450 m. The western boundary of this branch in the surface layer is located near station 7397, and the eastern boundary is near station 7406. The width of the branch at the sea surface is about 40 km. The core of the current was recorded at stations 7400–7404, where the maximum velocities exceed 60 cm/s. The offshore branch of the current extends deeper than 750 m below the zone of correct SADCP measurements. The width of the offshore branch here at the sea surface is about 40 km similar to the inshore branch. *Water* **2022**, *14*, x FOR PEER REVIEW 11 of 36

> The satellite data on the concentration of chl-*a* (*Chlsat*) and *SST* are presented in Figure 3, for an additional description of the study site. The section shows alternating high and low values of chl-*a* concentrations and *SST* values, which are associated with upwelling and downwelling manifestations in the surface layer of the sea. In general, both parameters decrease with distance from the shelf due to the influence of both jets of the MC. High *Chlsat* values are observed not only over the shelf, but also approximately over the entire inshore jet of the MC, decreasing towards its eastern part. and downwelling manifestations in the surface layer of the sea. In general, both parameters decrease with distance from the shelf due to the influence of both jets of the MC. High *Сhlsat* values are observed not only over the shelf, but also approximately over the entire inshore jet of the MC, decreasing towards its eastern part.

**Figure 3.** Satellite maps of chlorophyll*-a* concentration (*Chlsat*) obtained by OLCI/Sentinel-3B (**a**) and sea surface temperature (*SST*) obtained by VIIRS/SNPP (**b**) on 18 February 2022. **Figure 3.** Satellite maps of chlorophyll-*a* concentration (*Chlsat*) obtained by OLCI/Sentinel-3B (**a**) and sea surface temperature (*SST*) obtained by VIIRS/SNPP (**b**) on 18 February 2022.

Figures 4a–c show the vertical distributions of temperature, salinity, and density, respectively, and Figure 4d shows spatial variations of calculated UML thickness (*ZUML*).

13 °C. The thermocline coincides with the position of the pycnocline, and their depth changes in accordance with the variations between the upwelling and downwelling

*3.2. Oceanographic Variables*

zones.

#### *3.2. Oceanographic Variables*

Figure 4a–c show the vertical distributions of temperature, salinity, and density, respectively, and Figure 4d shows spatial variations of calculated UML thickness (*ZUML*). The thickness of the UML varies within 25–55 m; seawater temperature in the UML is 11–13 ◦C. The thermocline coincides with the position of the pycnocline, and their depth changes in accordance with the variations between the upwelling and downwelling zones.

At stations 7400–7401, upwelling manifested itself most clearly in the UML as the ascent of the lower boundary of the mixed layer. Above the shelf break at a depth of about 80 m, this process is reflected by the elevation of the 6 ◦C isotherm at station 7400. An increase in the 6.5 ◦C isotherm at depths of 70–80 m at station 7397 is also clearly seen. Two additional upwellings at stations 7403–7405 and 7407–7408 can be distinguished from the upward deflection of the isopycnals. Downwelling was revealed at stations 7393–7396, and also at stations 7402 and 7406, due to the lowering of the thermocline and pycnocline and the corresponding increase in the thickness of the upper mixed layer.

In general, the differences in the UML thickness between upwelling and downwelling zones are significant. In the first case, they are in the range of ~25–35 m, and in the second case, in the range of 40–55 m. It is noteworthy that upwelling and downwelling zones change at the boundaries of the inshore jet of the MC. At the western boundary, upwelling in the current changes to downwelling next to the current. On the contrary, on the eastern boundary, downwelling in the current changes to upwelling. Another feature is downwelling at station 7402, located in the middle of the inshore jet core zone.

#### *3.3. Biooptical Characteristics*

Figure 5 shows sections of bio-optical characteristics, in which zones of upwelling and downwelling of UML waters were distinguished. Additionally, two upwelling zones in the middle layer (50–120 m) have been revealed: above the Patagonian Shelf and above the Patagonian continental slope. These zones correspond to the peculiarities revealed in the analysis of the oceanographic variables shown in Figure 4. The *Fpc* distribution is not shown here because it is very similar to the *Fchl* distribution, and the differences can only be seen in the scatterplots that will be shown in the following sections.

Increased values of *Fchl* and *BAC*<sup>660</sup> are observed in the UML at stations 7393–7396 on the Patagonian Shelf and at stations 7400–7401 above the shelf edge. The local maximum of these values above the shelf edge is due to a decrease in the UML thickness during upwelling and a corresponding increase in the concentration of suspended phytoplankton and non-algal particles. Lower turbidity values are mostly observed in the upwelling zones in the UML rather than in adjacent downwelling zones. As the distance from the shelf increases in the eastern direction, the *FCDOM* values decrease. A distinctive feature of the vertical distribution of CDOM from the other considered bio-optical characteristics is the absence of high *FCDOM* values in the UML. In general, this vertical distribution is more uniform. At the same time, higher CDOM concentrations are observed under the pycnocline in the core of the inshore jet of the MC at stations 7400–7404, which may be associated with the intrusion of shelf waters into this layer.

**Figure 4.** Zonal sections of: temperature, °C (**a**); salinity, PSU (**b**); density, kg/m3 (**c**); and spatial variations of upper mixed layer depth (UML), estimated by method 1 [33], 2 [34], 3 [35] or 4 [36] (**d**). **Figure 4.** Zonal sections of: temperature, ◦C (**a**); salinity, PSU (**b**); density, kg/m3 (**c**); and spatial variations of upper mixed layer depth (UML), estimated by method 1 [33], 2 [34], 3 [35] or 4 [36] (**d**).

At stations 7400–7401, upwelling manifested itself most clearly in the UML as the ascent of the lower boundary of the mixed layer. Above the shelf break at a depth of about 80 m, this process is reflected by the elevation of the 6 °C isotherm at station 7400. An

**Figure 5.** Sections of optical characteristics: intensity of chl*-a* fluorescence, *Fchl* (**a**); beam attenuation coefficient at a wavelength of 660 nm, *BAC*<sup>660</sup> (**b**); turbidity, *Turb* (**c**); intensity of fluorescence of colored dissolved organic matter, *FCDOM* (**d**). Black dashed line indicates the euphotic depth *Zeu*. **Figure 5.** Sections of optical characteristics: intensity of chl-*a* fluorescence, *Fchl* (**a**); beam attenuation coefficient at a wavelength of 660 nm, *BAC*<sup>660</sup> (**b**); turbidity, *Turb* (**c**); intensity of fluorescence of colored dissolved organic matter, *FCDOM* (**d**). Black dashed line indicates the euphotic depth *Zeu*.

#### *3.4. Water Mass Classification* The waters over the Patagonian Shelf (yellow, orange and red color in Figure 6) are

summary table for the identified types of water.

*3.4. Water Mass Classification*

The joint analysis of oceanographic and bio-optical characteristics allows a more detailed classification of water masses. Figure 6 shows scatter diagrams for individual quantities and the corresponding distribution of the selected zones over the section. Figure 6a is a standard θ-S diagram, Figure 6b is a scatter plot of chlorophyll-a and phycoerythrin fluorescence intensities, the changes in which may indicate a different ratio of pigments in phytoplankton communities, Figure 6c shows CDOM fluorescence intensity versus salinity, which is also used to separate waters, especially in coastal areas [67,68], Figure 6d is a schematic representation of the section with highlighted water masses. Table 2 is a summary table for the identified types of water. generally fresher and less dense than those over the shelf slope part (cyan and blue color in Figure 6) of the section (Figures 4b,c and 6a). In general, the waters over the PS slope can be related to the MC system. The boundary between the PS and MC system waters is observed from the bottom (stations 7396–7397) to the surface (stations 7400–7401). This boundary passes through value S = 33.78 PSU, which allowed us to separate clusters in Figure 6a,c. Such separation is possible due to the ascent of bottom waters near stations 7396–7397 towards the shelf edge. In turn, there is the upwelling elevation of denser saline waters on the continental slope. These processes lead to the displacement of less dense waters in the upper mixed layer to the east.

The joint analysis of oceanographic and bio-optical characteristics allows a more detailed classification of water masses. Figure 6 shows scatter diagrams for individual quantities and the corresponding distribution of the selected zones over the section. Figure 6a is a standard θ-S diagram, Figure 6b is a scatter plot of chlorophyll-a and phycoerythrin fluorescence intensities, the changes in which may indicate a different ratio of pigments in phytoplankton communities, Figure 6c shows CDOM fluorescence intensity versus salinity, which is also used to separate waters, especially in coastal areas [67,68], Figure 6d is a schematic representation of the section with highlighted water masses. Table 2 is a

*Water* **2022**, *14*, x FOR PEER REVIEW 15 of 36

**Figure 6.** Scatter plots of "potential temperature—salinity" (**a**); "phycoerythrin—chl-*a* fluorescence intensity" (**b**); "CDOM fluorescence intensity—salinity" (**c**); and a scheme of the distribution of identified water masses along the section and vertical directions in the UML (**d**). Colors are described in Table 2. **Figure 6.** Scatter plots of "potential temperature—salinity" (**a**); "phycoerythrin—chl-*a* fluorescence intensity" (**b**); "CDOM fluorescence intensity—salinity" (**c**); and a scheme of the distribution of identified water masses along the section and vertical directions in the UML (**d**). Colors are described in Table 2.

The waters over the Patagonian Shelf (yellow, orange and red color in Figure 6) are generally fresher and less dense than those over the shelf slope part (cyan and blue color in Figure 6) of the section (Figures 4b,c and 6a). In general, the waters over the PS slope can be related to the MC system. The boundary between the PS and MC system waters is observed from the bottom (stations 7396–7397) to the surface (stations 7400–7401). This boundary passes through value S = 33.78 PSU, which allowed us to separate clusters in Figure 6a,c. Such separation is possible due to the ascent of bottom waters near stations 7396–7397 towards the shelf edge. In turn, there is the upwelling elevation of denser saline waters on the continental slope. These processes lead to the displacement of less dense waters in the upper mixed layer to the east.


**Table 2.** Identified water masses along the Section.

The waters of the Patagonian shelf can be divided into three subtypes, according to Figure 6b,c. Waters highlighted by yellow color correspond to a decrease in salinity below 33.675 PSU, which is observed at stations 7393–7396 in the depth range of 50–100 m associated with the water intrusion from the distant part of the shelf. Waters marked in red are deeper than 50–100 m at stations 7393–7396, where suspended and dissolved organic matter ascends from the bottom. In the waters marked in orange, phytoplankton communities function, for which the ratio between the fluorescence intensities of phycoerythrin and chlorophyll-a changes and leads to the identification of cluster in Figure 6b, and in general, the highest *Fchl* values are observed in these waters.

Seawaters belonging to the Malvinas Current system can be divided into waters located above the boundary of the Patagonian Shelf, mainly in the zone of action of the coastal jet (cyan color), and into waters located above the deep part of the section between the coastal and offshore branches of the Malvinas Current (blue color). The separation boundary between them in salinity is 33.95 PSU, which made it possible to separate clusters in the "*FCDOM*-*S*" scatterplot (Figure 6c).

#### *3.5. Variations in the Euphotic Depth and the Upper Mixel Layer Depth*

Vertical profiles of various variables at all stations are shown in Figure 7a. In addition, the purple horizontal line indicates the depth where 1% of the *PAR* incident on the surface is observed (euphotic depth). Figure 7b shows the distribution of *ZUML* and the depth of the euphotic zone (*Zeu*) along the section.

One can see in Figure 7 that at most stations where downwelling was identified, the euphotic depth is nearly in the middle of the phytoplankton layer (where increased *Fchl* values are observed). At all stations where upwelling was recorded, the euphotic depth was located below the phytoplankton layer. This feature is apparently related to the vertical displacement of the layer with phytoplankton and the change in the UML thickness.

In upwelling zones, the euphotic zone is generally deeper than in downwelling zones, despite the fact that in upwellings, a local increase in *Fchl* and *BAC*<sup>660</sup> values can be observed in the surface layer (Figure 7a,b). As an example, this can be observed at nearby stations 7400, 7401, and 7402. The highest chl-*a* values at stations 7400 and 7401 are higher than at 7402. However, on the contrary, the integral concentration of chl-*a* in the layer 0–100 m, is higher at station 7402 than at 7400 and 7401. Therefore, the depth of the euphotic zone at station 7402 is lower than at 7400 and 7401.

zone at station 7402 is lower than at 7400 and 7401.

**Figure 7.** Vertical profiles of chl*-a* fluorescence (green), euphotic depth (purple), beam attenuation coefficient at 660 nm (black), temperature (red), and salinity (blue) at stations taken during daylight hours days (**a**). Variations in the euphotic depth (*Zeu*) and UML depth (*ZUML*) (**b**). Examples of *Zeu* and *Z*UML determination are presented for stations 7400 and 7402, respectively. **Figure 7.** Vertical profiles of chl-*a* fluorescence (green), euphotic depth (purple), beam attenuation coefficient at 660 nm (black), temperature (red), and salinity (blue) at stations taken during daylight hours days (**a**). Variations in the euphotic depth (*Zeu*) and UML depth (*ZUML*) (**b**). Examples of *Zeu* and *Z*UML determination are presented for stations 7400 and 7402, respectively.

100 m, is higher at station 7402 than at 7400 and 7401. Therefore, the depth of the euphotic

#### *3.6. Phytoplankton Biological Activity*

Primary production (*PP*) in the surface layer varied from 10.5 to 35.8 mgC/m<sup>2</sup> per day (on average 23.4 <sup>±</sup> 8.6 mgC/m<sup>2</sup> per day) (Figure 8). The maximum values were revealed at the shelf station 7396 (35.8 mgC/m<sup>2</sup> per day) and in the inshore branch of the MC at stations 7400–7405 (23.5–27.9 mgC/m<sup>2</sup> per day). A notable decrease in *PP* (10.5–15.3 mgC/m<sup>2</sup> per day) was observed at the shelf edge (station 7499) and in the zone between the inshore and offshore branches (stations 7407–7408). The distribution of chl-*a* in the surface layer over the section was similar: an obvious trend towards a decrease from the shelf to the extreme oceanic station (from 3.57 to 1.06 µg/L). At the same time, unlike the pattern of distribution of *PP*, the concentration of chl-*a* in the inshore branch of the MC beyond the shelf edge remained at the same level (2.26–2.47 µg/L). shelf edge remained at the same level (2.26–2.47 µg/L). The ratio of *rETR* and *P<sup>B</sup>* values measured at the same light intensity for each subsample characterizes the photosynthetic efficiency of phytoplankton at different stations of the section, i.e., the degree of consuming the light energy for photosynthesis of organic matter. The photosynthetic efficiency (*PB*/*rETR*) ranged from 0.022 to 0.051 (mean 0.032 ± 0.009) (Figure 8).

Primary production (*PP*) in the surface layer varied from 10.5 to 35.8 mgC/m<sup>2</sup> per day (on average 23.4 ± 8.6 mgC/m<sup>2</sup> per day) (Figure 8). The maximum values were revealed at the shelf station 7396 (35.8 mgC/m<sup>2</sup> per day) and in the inshore branch of the MC at stations 7400–7405 (23.5–27.9 mgC/m<sup>2</sup> per day). A notable decrease in *PP* (10.5–15.3 mgC/m<sup>2</sup> per day) was observed at the shelf edge (station 7499) and in the zone between the inshore and offshore branches (stations 7407–7408). The distribution of chl*-a* in the surface layer over the section was similar: an obvious trend towards a decrease from the shelf to the extreme oceanic station (from 3.57 to 1.06 µg/L). At the same time, unlike the pattern of distribution of *PP*, the concentration of chl*-a* in the inshore branch of the MC beyond the

*Water* **2022**, *14*, x FOR PEER REVIEW 18 of 36

*3.6. Phytoplankton Biological Activity*

**Figure 8.** Phytoplankton photosynthetic efficiency (*P<sup>B</sup>*/*rETR*) and primary production (*PP*) in the upper 5-m water layer. **Figure 8.** Phytoplankton photosynthetic efficiency (*P <sup>B</sup>*/*rETR*) and primary production (*PP*) in the upper 5-m water layer.

*3.7. Phytoplankton Communities* A total of 43 species and subspecies of microalgae were noted at eight stations where phytoplankton has been studied (Table 1). Dinoflagellates (Myzozoa: Dinophyceae) predominated qualitatively among them (in qualitative terms according to the number of species): 27 species and subspecies (62.8% of the total list of phytoplankton). This was The ratio of *rETR* and *P <sup>B</sup>* values measured at the same light intensity for each subsample characterizes the photosynthetic efficiency of phytoplankton at different stations of the section, i.e., the degree of consuming the light energy for photosynthesis of organic matter. The photosynthetic efficiency (*P <sup>B</sup>*/*rETR*) ranged from 0.022 to 0.051 (mean 0.032 <sup>±</sup> 0.009) (Figure 8).

#### followed by diatoms (Ochrophyta: Bacillariophyceae) 12 (27.9%), pyramimonads (Chlo-*3.7. Phytoplankton Communities*

rophyta: Pyramimonadophyceae) 2 (4.7%), and prasinophytes (Chlorophyta: Prasinophyceae), represented by the species *Pterosperma* cf. *polygonum*, and silicoflagellates (Ochrophyta: Dictyochophyceae) *Dictyocha fibula*. Heterotrophs significantly prevailed in the list of dinoflagellates: they accounted for 19 species or 41.9% of the entire noted flora. In turn, at the level of genera, *Protoperidinium* was the most widely represented with 12 species and subspecies (27.9%, which is over a quarter of the entire flora). Thus, the phytoplankton species spectrum was represented mainly by dinoflagellates, and most of them had no adjustment to photosynthesis. A total of 43 species and subspecies of microalgae were noted at eight stations where phytoplankton has been studied (Table 1). Dinoflagellates (Myzozoa: Dinophyceae) predominated qualitatively among them (in qualitative terms according to the number of species): 27 species and subspecies (62.8% of the total list of phytoplankton). This was followed by diatoms (Ochrophyta: Bacillariophyceae) 12 (27.9%), pyramimonads (Chlorophyta: Pyramimonadophyceae) 2 (4.7%), and prasinophytes (Chlorophyta: Prasinophyceae), represented by the species *Pterosperma* cf. *polygonum*, and silicoflagellates (Ochrophyta: Dictyochophyceae) *Dictyocha fibula*.

Only one species, the small-celled coccoid pyramimonad *Prasinoderma coloniale*, was found at all eight stations of the section. The other seven species are *Dinophysis rotundata*, *Tripos lineatus*, *T. fusus*, *Protoperidinium curvipes*, *P. pyriforme*, *P.* cf. *subsphaericum and*  Heterotrophs significantly prevailed in the list of dinoflagellates: they accounted for 19 species or 41.9% of the entire noted flora. In turn, at the level of genera, *Protoperidinium* was the most widely represented with 12 species and subspecies (27.9%, which is over a quarter of the entire flora). Thus, the phytoplankton species spectrum was represented mainly by dinoflagellates, and most of them had no adjustment to photosynthesis.

Only one species, the small-celled coccoid pyramimonad *Prasinoderma coloniale*, was found at all eight stations of the section. The other seven species are *Dinophysis rotundata*, *Tripos lineatus*, *T. fusus*, *Protoperidinium curvipes*, *P. pyriforme*, *P.* cf. *subsphaericum and Rhizosolenia hebetate*. These species had the status of widespread since they were noted at four of seven stations, but not everywhere.

The number of species at stations varied from four (in the core of the inshore branch) to 24 (at station 7395 in the downwelling zone above the shelf). On average, there were 12.5 species and subspecies per location. Structural diversity detected using the ShannonWeaver index (*H*0 ) ranged from 0.725 (in the core of the inshore branch) to 2.038 (at the eastern periphery of the jet), with an average value of 1.467. The evenness of the species abundance, according to the Pielou index (*J* 0 ), varied from 0.386 (at the westernmost point above the shelf in the downwelling zone) to 0.830 (at the eastern boundary of the jet), averaging 0.617. The probability of detecting two different species in one field of view ranged from 0.344 (at station 7400 in the upwelling zone above the shelf break) to 0.785 (in the downwelling zone at the eastern boundary of the jet), on average 0.582. In general, all the described indicators (species richness of communities, their diversity, and evenness of the species richness) were at relatively low levels.

Phytoplankton in the study site was not very rich in species, and in some places composition was poor (above the shelf edge and slope and especially in the core of the inshore jet), not particularly diverse, while at many locations the structure of communities was strongly influenced by the first most abundant species. At all stations, the pyramimonad *Prasinoderma coloniale* dominated in number. It was represented both among free-floating phytoplankton and as part of microaggregates. Small (up to 3–5 µm in diameter) spherical cells of this species were often found in living form and in the composition of spreading pellets of planktonic crustaceans. The cells proliferated again as part of aggregates built on the basis of such pellets. The contribution of this species to the total number of cenoses at the stations was 37.5–80.5%, 60.7% on average. The quantitative structure of communities at the stations is given in Table 3.

**Table 3.** Structure of phytoplankton communities at stations. The contributions (in % of the total number) formed by representatives of various large taxonomic groups are given. Specific species are indicated for the groups represented by one species.


The total contribution of *Protoperidinium* species, widely represented at some stations, varied from 4.2% to 18.2% of the total abundance (9.3% on average). They played a particularly significant role in the total abundance at some locations over the shelf (in the downwelling zone and in the transition zone to upwelling), and in the core of the inshore branch. The number of species of this genus was especially high in the downwelling zone over the shelf (8–10), as was the total number of heterotrophic dinoflagellates (12–14). The total contribution of heterotrophic forms to the abundance of communities was especially high at station 7395 (downwelling over the shelf, 25.18%), as well as on the eastern periphery of the jet (22.64%) and, on its border (25%). On average over all samples, heterotrophs accounted for 17.72% of the total number of microalgae.

The average similarity of communities at the stations in terms of the composition of the microalgae flora was 26.07%. Assemblages of different locations, in general, differed significantly in composition. Structurally (based on the relative abundance of species), their average similarity was even lower, only 20.24%.

Two floristic groups (Figure 9a) can be distinguished based on the similarity of composition (total correlation *R* = 0.85, *p* = 0.5%). Among them, the Af group was represented over the shelf and at the eastern boundary of the inshore branch; it was characterized by the presence of *Paralia* sp., *Dinophysis rotundata*, *Tripos lineatus*, *Rhizosolenia hebetata*, *Protoperidinium curvipes*, *P. pyriforme*, *P. cassum*, *P.* cf. *subsphaericum,* and *P. oviforme*. Group Bf was found above the shelf break and slope; it was represented by *Tripos lineatus, T. fusus*, and *Stephanopyxis nipponica*.

**Figure 9.** Dendrograms showing the grouping of phytoplankton communities according to qualitative features (floristic groups (**a**)) and quantitative structure (groups of assemblages (**b**)). Corresponding *ϴ*-*S* diagram (**c**) and "phycoerythrin—chl-*a* fluorescence intensity" scatter plot (**d**) for different phytoplankton assemblages where colors are associated with phytoplankton assemblage groups. **Figure 9.** Dendrograms showing the grouping of phytoplankton communities according to qualitative features (floristic groups (**a**)) and quantitative structure (groups of assemblages (**b**)). Corresponding *θ*-*S* diagram (**c**) and "phycoerythrin—chl-*a* fluorescence intensity" scatter plot (**d**) for different phytoplankton assemblages where colors are associated with phytoplankton assemblage groups.

Within the section, it was possible to distinguish three groups of assemblages based on species ratios by relative abundance (total *R* = 0.93, *p* = 0.1%) (Figure 9b). In particular, the Am group, which united the assemblages above the shelf (in the downwelling zone and in the transition zone to upwelling), was presented (in descending order of significance) by *Paralia* sp., *Tripos lineatus*, *Dinophysis rotundata*, *Protoperidinium curvipes*, *P.* cf. *subsphaericum*, *Rhizosolenia hebetata*, *P. pyriforme*, *Dinophysis acuminata*, *Protoperidinium pellucidum*, *P. solidicorne,* and *Oblea rotunda*. This group can be considered as characteristic of the Patagonian Shelf. Groups Am and Bm were distinguished in the zone of local maxima of *Fchl* and *BAC*660.

In addition, Figure 9c shows scatterplots of oceanographic and bio-optical characteristics according to clustering by groups of assemblages where a significant separation is also seen, both in oceanographic conditions (*T*, *S*) and in the ratio of fluorescence intensity of various phytoplankton pigments. At the same time, such a clear separation did not happen for floristic groups, the same points belonged to different clusters, or different points were present in the same cluster.

Assemblages above the shelf break and slope were united in the Bm group characterized as equally significant by *Tripos lineatus, T. fusus*, and *Stephanopyxis nipponica*. Communities at the periphery of the inshore branch and in the region of its eastern boundary (in the deep-water region) belong to the Cm group of assemblages; this was presented in descending order of importance, by *Dinophysis rotundata, Neoceratium pentagonum, Protoperidinium curvipes*, and *Thalassiosira eccentrica*. Assemblage in the core of the inshore branch was not included in any of the groups due to extremely low number of species. *Water* **2022**, *14*, x FOR PEER REVIEW 22 of 36

#### *3.8. Zooplankton Species 3.8. Zooplankton Species*

A total of 30 zooplankton species/taxa were identified. The composition of zooplankton at stations varied from 9 (st. 7393) to 22 species/taxa (st. 7406 and 7408). The species diversity calculated using the Shannon-Weaver index (*H*0 ) ranged from 0.984 (st. 7402) to 2.751 (st. 7406). A total of 30 zooplankton species/taxa were identified. The composition of zooplankton at stations varied from 9 (st. 7393) to 22 species/taxa (st. 7406 and 7408). The species diversity calculated using the Shannon-Weaver index (*H'*) ranged from 0.984 (st. 7402) to 2.751 (st. 7406).

Zooplankton abundance ranged from 543 to 3067 ind/m<sup>3</sup> (Figure 10) with an average value of 1804 <sup>±</sup> 951 ind/m<sup>3</sup> . The maximum abundance values were located in the inshore branch of the MC in the area of the upwelling and downwelling zones, with high *Fchl* values (st. 7400–7402), while the minimum abundance values were observed in the center of the inshore branch of the MC (st. 7404). Copepods and euphausiids larvae were the dominant zooplankton components at all stations. Euphausiids larvae were found in the nearshore waters of the Patagonian Shelf. Other non-copepod groups were not as abundant (Figure 10). Zooplankton abundance ranged from 543 to 3067 ind/m<sup>3</sup> (Figure 10) with an average value of 1804 ± 951 ind/m<sup>3</sup> . The maximum abundance values were located in the inshore branch of the MC in the area of the upwelling and downwelling zones, with high *Fchl* values (st. 7400–7402), while the minimum abundance values were observed in the center of the inshore branch of the MC (st. 7404). Copepods and euphausiids larvae were the dominant zooplankton components at all stations. Euphausiids larvae were found in the nearshore waters of the Patagonian Shelf. Other non-copepod groups were not as abundant (Figure 10).

**Figure 10.** Contribution (%) of the major taxonomic groups to the total zooplankton abundance (ind/m<sup>3</sup> ). **Figure 10.** Contribution (%) of the major taxonomic groups to the total zooplankton abundance (ind/m<sup>3</sup> ).

Three (A-C) significant groups were identified (total *R* = 0.68, *p* = 0.05%) in the zooplankton community structure (Figure 11), which also clearly separated as seen in *ϴ-S*  diagram (Figure 11c). Group A was detected in the nearshore waters of the Patagonian Shelf (st. 7393, 7395, 7397 and 7398) and mainly composed by copepod *Calanoides acutus*. Larvae of euphausiids, amphipods, and ostracods were present in this community; abundance was higher in the waters of the Patagonian Shelf. Group B was restricted in the inshore branch of the MC in the upwelling and down-Three (A–C) significant groups were identified (total *R* = 0.68, *p* = 0.05%) in the zooplankton community structure (Figure 11), which also clearly separated as seen in *θ-S* diagram (Figure 11c). Group A was detected in the nearshore waters of the Patagonian Shelf (st. 7393, 7395, 7397 and 7398) and mainly composed by copepod *Calanoides acutus*. Larvae of euphausiids, amphipods, and ostracods were present in this community; abundance was higher in the waters of the Patagonian Shelf.

welling zones (st. 7400–7402). It was presented by Subantarctic species of the copepod genera *Calanus*, followed by euphausiids, salps, chaetognaths, appendicularians, amphi-

Group C was identified in the zone between the inshore and offshore branches of the MC (st. 7406 and 7408) and presented by copepod *Calanoides acutus* and Antarctic oceanic species/taxa [69,70] copepod genera *Candacia*, *Euchirella*, *Haloptilus*, *Paraeuchaeta*, *Rhincalanus*, and euphausiids *Thysanoessa macrura*, Pteropod shells, siphonophores. One station (st. 7404) located in the center of the inshore branch of the MC was not included in any of the clusters and was characterized by mixed Subantarctic and Subtropical spe-

cies/taxa with lower abundance and species richness (Figure 10).

were abundant in the surface layer of 100-0 m.

1

**Figure 11.** Cluster (**a**) and MDS (**b**) analyses of zooplankton collections (by species abundance), and *θ*-*S* scatter plot, where the colors of points correspond to the groups in (**a**–**c**).

Group B was restricted in the inshore branch of the MC in the upwelling and downwelling zones (st. 7400–7402). It was presented by Subantarctic species of the copepod genera *Calanus*, followed by euphausiids, salps, chaetognaths, appendicularians, amphipods, and polychaets. The deep-sea copepods of the genera *Paraeuchaeta* and *Rhincalanus* were abundant in the surface layer of 100-0 m.

Group C was identified in the zone between the inshore and offshore branches of the MC (st. 7406 and 7408) and presented by copepod *Calanoides acutus* and Antarctic oceanic species/taxa [69,70] copepod genera *Candacia*, *Euchirella*, *Haloptilus*, *Paraeuchaeta*, *Rhincalanus*, and euphausiids *Thysanoessa macrura*, Pteropod shells, siphonophores. One station (st. 7404) located in the center of the inshore branch of the MC was not included in any of the clusters and was characterized by mixed Subantarctic and Subtropical species/taxa with lower abundance and species richness (Figure 10).

#### *3.9. Observations of Marine Mammals and Sea Birds*

Summary results of observations of marine mammals and birds are shown in Figure 12. In the study site, we observed marine mammals belonging to the baleen whales Mysticeti and the toothed whales Odontoceti. Baleen whales were represented by one species: Antarctic minke whale *Balaenoptera bonaerensis* belonging to the family Balaenopteridae. Two species of recorded toothed whales belonged to two different families: the Peale's

*Water* **2022**, *14*, x FOR PEER REVIEW 24 of 36

dolphin *Lagenorhynchus australis* in the dolphin family Delphinidae and the southern bottlenose whale *Hyperoodon planifrons* belonging to the family Ziphiidae. rences/one individual and one occurrence/one individual, respectively. In the upwelling zone, one individual of the petrel family was noted.

offshore branches of the MC, respectively. On the contrary, some species of this family appeared more often over one branch of the MC with respect to another; this is typical for the shelf zone and the offshore branch. An increase in the number of occurrences in these regions was noted for the soft-plumaged petrel *Pterodroma mollis* and the great shearwater *Ardenna gravis* from five to 19 individuals and from two to seven individuals, respectively. The Atlantic petrel *Pterodroma incerta* and the great-winged petrel *Pterodroma macroptera* were noted only by single occurrences over the offshore branch, namely two occur-

**Figure 12.** The ratio of the bird species at each site and sightings of marine mammals. **Figure 12.** The ratio of the bird species at each site and sightings of marine mammals.

The albatross family, second in terms of the number of individuals encountered, was represented by two species. A change in the occurrence of the black-browed albatross *Thalassarche melanophris* has been established in the different sites of the MC, as discussed above for some species of the petrel family. This species is characterized by a decrease in Two species, the Antarctic minke whale and the Peale's dolphin, were found in the shelf zone of the MC, one occurrence/one individual and two occurrences/five individuals, respectively. The southern bottlenose whale was noted in the offshore branch of the MC (one occurrence/one individual).

The birds encountered in the MC region were represented by two orders and four families. The order Procellariiformes included the following families: albatrosses Diomedeidae, petrels Procellariidae, and southern storm petrels Oceanitidae. The order Charadriiformes was represented by the skua family Stercorariidae. A total of 10 species of birds were recorded, with a total number of 422 individuals.

It was found that the petrel family was the most numerous and diverse in terms of the species composition. A total of 256 individuals belonging to five species of this family were

recorded. However, there was a change in the occurrence of some species in different parts of the section across the MC. For example, the number of white-chinned petrel *Procellaria aequinoctialis*, the most abundant bird species, decreased by more than 40 times, from 202 individuals in the shelf zone to four and five individuals in the inshore and offshore branches of the MC, respectively. On the contrary, some species of this family appeared more often over one branch of the MC with respect to another; this is typical for the shelf zone and the offshore branch. An increase in the number of occurrences in these regions was noted for the soft-plumaged petrel *Pterodroma mollis* and the great shearwater *Ardenna gravis* from five to 19 individuals and from two to seven individuals, respectively. The Atlantic petrel *Pterodroma incerta* and the great-winged petrel *Pterodroma macroptera* were noted only by single occurrences over the offshore branch, namely two occurrences/one individual and one occurrence/one individual, respectively. In the upwelling zone, one individual of the petrel family was noted.

The albatross family, second in terms of the number of individuals encountered, was represented by two species. A change in the occurrence of the black-browed albatross *Thalassarche melanophris* has been established in the different sites of the MC, as discussed above for some species of the petrel family. This species is characterized by a decrease in the number of individuals in the transition from the shelf zone to the inshore branch of the MC from 153 to three individuals, and to one individual during the transition to the offshore branch of the current, respectively. Solitary specimens of the sooty albatross *Phoebetria fusca* have been recorded over the inshore and offshore branches. Over the inshore branch, two juveniles belonging to the so-called giant albatrosses were noted, but it was not possible to establish their species identity. The family Oceanitidae was not numerous. Among them, two specimens of Wilson's storm-petrel *Oceanites oceanicus* were noted in the offshore branch and one specimen of the black-bellied storm-petrel *Fregetta tropica* in the inshore branch. The only member of the Stercorariidae family, the Antarctic Skua *Stercorarius antarcticus*, was recorded in the shelf zone of the MC.

The average number of birds encountered per hour was calculated for each site. The largest number of birds were found in the shelf zone: 363 individuals (72.6 birds per hour); in the coastal branch, 20 individuals (10.3 birds per hour); and in the offshore branch, 38 individuals (19 birds per hour). In the shelf zone, over a distance of several kilometers, fishing boats were seen, around which a large number of birds were circling. The ratio of the bird species at each site and sightings of marine mammals are presented in Figure 12.

#### **4. Discussion**

#### *4.1. Types of Water Masses and Hydrodynamic Manifestations of Oceanographic Characteristics*

The combined use of hydrophysical and bio-optical data, as well as model data from the Lagrangian analysis, allows better separation of water masses. The division of the waters over the section according to temperature, salinity, and optically active substances into a shelf and open-sea waters is consistent with the results reported in [71,72], where the boundary between the shelf waters and the waters of the Malvinas Current system is salinity isoline S ≈ 33.8 PSU (in this work we assume 33.78 PSU). The ranges of temperaturesalinity values for the Malvinas Current obtained in this work are consistent with the data in [6,71]. In addition, a similar spatial separation was also obtained in our work based on the Lagrangian analysis data (Figure 1). In this case, intrusions of water masses in both directions can be observed, both of the shelf waters into the system of the Malvinas Current, and of waters of the Malvinas Current into the shelf region. Both processes are seen on the Lagrangian maps in Figure 1. Intrusion of shelf waters into the open sea is presented in the results of our in situ observations (Figures 4–6). The strong intrusion of MC waters into the shelf is accompanied by the upward displacement of high saline, low temperature, and high nutrient waters reported in [73] indicating the upwelling of deep nutrients on a similar section near 47◦ S, where the slope of the continental margin is less abrupt.

The coastal branch of the Malvinas Current is well described in [4,5]. However, in this study, we found the unusual phenomenon that velocities in the coastal branch were higher than in the offshore branch of the Malvinas Current, and in addition, the coastal branch was located very close to the shelf edge (Figure 2). This can potentially lead to an increase in the vertical motion of water masses. In general, SADCP measurements in the Malvinas Current are rarely performed. Some results are reported in [4,5,74], but even there they were not accompanied by detailed CTD measurements, which can provide complete information about the vertical structure of waters in the zone of alternating upwellings/downwellings.

Satellite data of medium spatial resolution make it possible to see not just one band of elevated concentrations of chlorophyll-a, which belong to the well-known upwelling along the Patagonian Shelf break [16–18]. A complex pattern of several bands of varying chlorophyll-a concentrations and surface layer temperature is also seen, which indicates the action of complex dynamic processes in the study site leading to a series of successive upwellings and downwellings. In general, this suggests that the spatial resolution of the survey should be even better than 2–5 km (0.5–1 km), which can be achieved using submersible autonomous vehicles.

At the same time, satellite data still do not give a complete pattern of the ongoing processes, since they do not allow us to analyze the vertical variability of oceanographic and bio-optical characteristics. They also do not allow a correct assessment of the euphotic depth in this area, which will be discussed in more detail in the next section. Nevertheless, satellite data are very helpful as an additional source of data for analysis to highlight hydrophysical structures and corresponding fronts on the map, and to analyze the horizontal motion of water masses. Modern implementations of global reanalyses are also insufficient for a comprehensive study of thin vertical and horizontal oceanographic structures and processes in the study area since they lack spatial resolution and accurate bathymetry.

The detailed section made it possible to distinguish the submesoscale structure of alternating upwelling and downwelling zones, which was not the case in previous works, where the analysis focused on larger scales. Similar alternations are presented in measured in situ data [14,75], in model data [76], and similar manifestations in the satellite images are visible in [3,77]. They can be caused by the alternation of zones of divergence and convergence of the current velocity vector to the left and to the right of the MC inshore jet velocity maximum. The data presented here do not reveal erosion of the pycnocline caused by upwelling unlike the results presented in [14] at 43◦ S and in [76] at 51◦ S, which were performed on the Patagonian Shelf. It is important to note that in the cited works, erosion of the pycnocline occurred during upwelling over the shelf, and not over the outer boundary of the shelf.

Upwelling and downwelling zones in some cases are better manifested in the oceanographic variables (temperature, salinity, density), and in other cases in the optical characteristics presented here. For example, the *BAC*<sup>660</sup> and turbidity sections show the ascent of turbid bottom waters from the Patagonian Shelf and their displacement towards its edge (from the bottom parts of stations 7396–7397 to the surface parts of stations 7400–7401). Downwelling between stations 7402 and 7403 in the surface layer of 50 m, which is displaced in the underlying layers at station 7401, is better pronounced in the *BAC*<sup>660</sup> data. Downwelling at station 7406 is seen from all presented optical characteristics except *FCDOM*. Therefore, in such works, it is important to have a large number of automatically measured vertical profiles of marine environment parameters in order to increase the reliability of the data.

#### *4.2. Relation between the Depths of the Upper Mixed and Euphotic Zones*

It is not obvious at first glance result (Figure 7) that the euphotic zone increases in upwelling zones, and decreases in downwelling zones, despite the fact that the maximum concentrations of chl-*a* are higher in upwelling zones; they appear on satellite data as bands of increased concentrations of chl-*a*.

The effect of the increased maximum of chl-*a* at stations with upwelling can be explained by a decrease in the thickness of the UML and the associated concentration of substances. The effect of shallower euphotic zone at the stations with downwelling may be

associated with the spread of sufficiently high chl-*a* values to a greater depth within the UML; hence, the depth-integrated attenuation of light is more effective than at upwelling stations. During upwelling, the most transparent water of the intermediate layer of the sea, together with the seasonal pycnocline, ascends closer to the surface.

This leads to the paradoxical result that, on the one hand, during upwelling, the maximum chl-*a* values are higher, but the euphotic zone is deeper, due to the fact that the depth-integrated chl-*a* values are lower, since they are observed only within a thinner UML. This is also seen in Figure 7b, where local decreases in the thickness of the UML correspond to increases in the thickness of the euphotic zone at the stations with upwelling. Local increases in the UML thickness correspond to decreases of euphotic zone thickness at the stations with downwelling. This is not so obvious at stations 7403 and 7404, perhaps due to the fact that the upwelling and downwelling processes are not as strong here. In addition, at the first two stations 7393 and 7394 there are also all indications that downwelling may take place there based on the form of profiles in Figure 7a, and relationship between *ZUML* and *Zeu*.

It is important to understand that such a large deepening of the phytoplankton layer at stations 7393, 7394, 7395, 7396, 7397, and 7402 below the *Zeu* is not normal from the point of view of the functioning of phytoplankton cells [78–81] and may be associated with dynamic effects, such as downwelling, which drive substances into deeper layers. A similar profile shape is presented in Figure 3 in [82] in the Drake Passage, where the ACC jet streams exist, the position of which changes in time [5], which can lead to the formation of powerful zones of divergence and convergence, and the corresponding ascends and descends of water masses. A similar analysis of *Zeu* and *ZUML* distribution was carried out in [73], however, with a lower spatial resolution, which did not allow us to connect the identified features in the spatial distribution of *Zeu* and *ZUML* with the fine structure of upwellings and downwellings.

In addition, the revealed phenomenon influences the satellite estimates of the euphotic layer depth in the study area, which is knowingly incorrect in the upwelling and downwelling zones. All modern algorithms for estimating *Zeu* [83,84] use the feedback from satellite estimates of the chl-*a* concentration in the water layer seen from the space; the latter are higher in the case of upwelling zones and lower in downwelling zones. This leads to underestimation of *Zeu* in the upwelling zone and to overestimation of *Zeu* in the downwelling zone. It must be kept in mind that this conclusion is valid for the presented section and is not necessarily true for a different hydrophysical and bio-optical situation.

#### *4.3. Analysis of Photosynthetic Activity of Phytoplankton*

The results presented in 3.6 were approximately in the same range as those found for estuarine phytoplankton and microphytobenthos [85], where values of the *rETR* efficiency for C fixation (*EE*) varied between 0.04 and 0.16. These authors recalculated published data for cultures and presented *EE* values varying between 0.007 and 0.020 for different marine phytoplankton species.

The maximum values of the *P <sup>B</sup>*/*rETR* parameter were found in the outer part of the inshore branch of the MC (stations 7406, 0.051) and in the zone between the inshore and offshore branches (stations 7407–7408). When analyzing the spatial distribution of photosynthetic efficiency values, shown in Figure 8, it should be noted that stations with high *P <sup>B</sup>*/*rETR* were located at the boundaries of upwelling and downwelling zones throughout the entire section from the shelf to the oceanic part. Specific hydrodynamic conditions at the boundaries of zones likely contribute to the increase in the photosynthetic activity of phytoplankton in these regions.

Earlier, one of the authors of this paper reported the results in the Kara Sea, where an increase in *P <sup>B</sup>*/*rETR* in the local areas with strong gradients of oceanographic conditions was caused by various factors: the area of the outer shelf in the zone of mixing of river and sea waters in the estuary of the Yenisei River, and a strong frontal zone at the edge of the shelf that appeared as a result of the interaction of two currents in the upwelling zone above the continental slope [39]. Despite the geographical remoteness of these two regions and the different nature of the gradients of oceanographic variables, an increase in photosynthetic efficiency precisely in the zones of gradients of variables is manifested in both cases. This allows us to assume the general regularity of this phenomenon.

#### *4.4. Phytoplankton Communities*

The work by Antacli et al. [86] reporting detailed studies of phyto- and protozooplankton of the southern part of the Patagonian Shelf in the summer of 2004, provides very detailed data on the species composition of these communities (319 species), their size structure (90% of the total number accounted for picoplankton 2–5 µm in size), as well as the significant spatial heterogeneity of the communities in this vast region. These surveys were performed on four transects across the shelf located between 47◦ and 55◦ S. In turn, our observations covered the marginal and close-to-marginal regions of the Patagonian Shelf, its slope part, as well as the coastal jet of the Malvinas Current up to its eastern periphery. This expands the geographic scope of the results.

Our studies were carried out 18 years later, which gives us a certain opportunity to assess changes in communities over time. It should be noted that almost 20 years later, in the more eastern region, which covers a significantly greater diversity of water masses, the smallest size fraction of phytoplankton also dominated, "headed by a coccal ultraphytoeukaryotic cell (3 µm) (probably chlorophyte/prasinophyte), which was the most important morpho-species in the region (frequency of occurrence FO = 35%, relative abundance RA = 45%)". Based on our data and descriptions of colleagues, we can assume that in the summer of 2004, *Prasinoderma coloniale* developed just as abundantly in the water area studied in [86]. Thus, this species still plays the most significant role in the abundance of phytoplankton in the entire region under consideration.

Against the background of the widespread significant abundance of this species, the phytoplankton size fraction >10 µm made a relatively small total contribution to the community structure, both in 2004 and in 2022 (except for the eastern boundary of the MC). However, these relatively larger forms accounted for the largest part of the species richness of communities in both seasons and both areas. In general, the total number of species noted by us (43) seems significantly less than the sum of dinoflagellates (148) + diatoms (73) encountered by our colleagues in 2004.

It should be taken into account that our observations show an increase in the species richness of communities during the transition to the shelf zone, while in the summer of 2004, four vast areas were studied just above the shelf. This could already be the reason for the higher floristic diversity. In terms of the significant heterogeneity of the community structure in the section studied by us and in the sections made in [86], there is also a certain correlation because assemblages both above the shelf and in the zone of influence of the coastal jet of the Malvinas Current were and remain significantly heterogeneous in space.

The work [87] is also devoted to the study of phytoplankton including the summer one in the southern part of the Patagonian Shelf. However, according to the observations made by the authors in the summer of 2008, in a vast area northwest of the Falkland Islands, the main components of phytoplankton were the coccolithophores Emiliania huxleyi and the primesiophytes Phaeocystis antarctica. The authors attribute their flowering to a notable decrease in the level of nutrients from spring to summer due to the abundant development of diatoms in spring. At the same time, their observations were made in the middle of summer (4–7 January 2008), while our studies on the section from the shelf and through the Malvinas Current were made at the end of summer (21–22 February 2022). The results in [86] were also obtained at the end of summer (18 March–4 April 2004). The observed differences in mass species may well be explained, among other issues, by different stages of seasonal succession of phytoplankton.

#### *4.5. Zooplankton Communities*

The majority of identified zooplankton species (80%) were resident species, which are shared with the Subantarctic zone, and other taxa can be considered advected species common to the Antarctic zone [88]. Previous observations [76] showed "zooplankton hot spot—high biomass and abundance" associated with frontal upwelling. Additionally, all identified groups of zooplankton are well separated in accordance with oceanographic characteristics (Figure 11). All identified groups of zooplankton are well separated in accordance with oceanographic characteristics (Figure 11).

Copepods of the genera *Calanoides* from group A observed in the Patagonian Shelf, and from group C in the zone between inshore and offshore branches of the MC, were the most abundant members of zooplankton communities in the nearshore waters throughout the Antarctic zone and in the south of the Subantarctic zone as described in [88,89], and were also one of the main dominant taxa by biomass in the Drake Passage [90]. They were found at depths of 0–200 m in austral summer and 500–800 m in winter [91].

Copepods of the genera *Calanus* related to Group B in the inshore branch of the MC in the zone of most significant upwelling and downwelling, were found previously in the deep-sea layers (300–500 m) in austral summer [92]. Possibly, deep-sea copepods in the surface waters (0–100 m) can be associated with the local zones of upwelling. The path of possible vertical motions of this group of zooplankton is shown in all sections of oceanographic (Figure 4) and bio-optical (Figure 5) characteristics.

In the study region, the Antarctic oceanic species/taxa were found in the communities of group C, while previously (January 2022) in the same expedition, similar species composition was detected in the water northeast of the South Orkney Islands. This area was influenced by the wide Antarctic Circumpolar Current (ACC) system where water flows are generally directed to the east [93]. Possibly, the eastward currents could transport these species both to the northeast of the South Orkney Islands and to the Patagonian Shelf. Generally, a correlation exists between the faunal and floristic relations in the Southeast Pacific and Southwest Atlantic within the biogeographic Magellanic Province [94]. This is provided by the eastward water mass transport through the Magellan Strait and the Drake Passage [11,95]. The zone of sterile migration of Antarctic species is usually insignificant, but in some cases, it reaches a large extent [88]. The application of mathematical modeling is required for the confirmation of these ideas.

#### *4.6. Observations of Marine Mammals and Sea Birds*

The study site is a very important food base for animals of higher trophic levels. Unfortunately, due to the different efforts of observations at each site and limited data, it was not possible to identify a statistically significant dependence in the distribution of marine mammals and birds on oceanographic and biological parameters. However, we can make a few qualitative conclusions.

All of the found mammal species belong to different families and have different biological traits, including different trophic ecology. Antarctic minke whales feed mainly on zooplankton, while the Peale's dolphins feed on demersal fish, some species of cephalopods and shrimp. The southern bottlenose whale is known for feeding at great depths, deeper than 1000 m, mainly on squid, but also fish, such as the Patagonian toothfish *Dissostichus eleginoides*; therefore, it is not surprising that this species was observed in the offshore branch of the Malvinas Current, where depths reach 3000 m. The distribution of observed species also differs, although in some areas their ranges overlap [96].

Antarctic minke whale is widely distributed in the Southern Hemisphere. The Peale's dolphin mainly lives near the coast and on the shelf of South America and the Malvinas (Falkland) Islands. The southern bottlenose whale has a circumpolar distribution in the Southern Hemisphere in the deep regions [96]. No marine mammals were encountered in the shallow upwelling zone and the inshore branch of the MC.

All species of birds we met are typical for this region of the South Atlantic Ocean [97]. The highest density of concentration of birds encountered was recorded in the shelf part

of the route; a large amount of bioproduction was observed in the same area. At the same time, it is interesting that the minimum density of birds was recorded in the zone of maximum upwelling, where there were also high concentrations of chlorophyll-a and zooplankton. This is probably due to the limited observation time in this relatively small but very dynamic area with high horizontal and vertical velocities of water masses. It is likely that birds prefer less dynamic, but no less productive zones for feeding farther from the shelf slope. In addition, the change in the species diversity of the food supply, characteristic of this area, can also affect the distribution of birds. These hypotheses require further testing and longer observations in this and other areas with similar conditions. Similar results with large variability in the species and quantitative composition of marine mammals and birds within a few tens of kilometers in a highly dynamic area of the Southwest Atlantic across an eddy dipole in the interaction zone between Subtropical and Subantarctic waters are presented in [98].

#### *4.7. Summary Information about Hydro-Physical and Biological Charactristics*

Table 4 presents summary information about identified water masses, seawater vertical velocity directions, groups of assemblages of phytoplankton communities and groups of zooplankton communities, and also the density of observed birds. One can see that the division into different biological groups occurred similarly in accordance with the zones of the Patagonian Shelf, the edge of the shelf, and the zone between the inshore and offshore branches of the MC. In addition, the separation of biological communities is influenced by different water masses, as well as upwelling and downwelling zones.

**Table 4.** Summary information about hydro-physical and biological characteristics of investigated waters. The background colors correspond to the colors that mark the planktonic groups in Figures 9b–d and 11, and ship tracks in Figure 12.


Note(s): \* Upwelling (U) or downwelling (D) in the upper mixed layer. \*\* Groups of assemblages of phytoplankton.

Alternating zones of upwelling and downwelling are formed over the slope of the Patagonian Shelf under the influence of the MC and wind. This is especially pronounced in the region of the inshore jet of the MC. The detected zones significantly influenced all the characteristics analyzed in the work. It is shown that at the boundaries of the inshore jet, upwelling and downwelling alternate in space. At the western boundary of the jet, upwelling in the current changes to downwelling beyond the current, and on the contrary, at the eastern boundary of the jet, downwelling within the current changes to upwelling at the open sea side.

It was found that the euphotic zone is larger in the upwelling zones and smaller in the downwelling zones, which is associated with variations in mixed layer depth and depthintegrated abundance of phytoplankton cells and other optically active components of seawater within the UML, which are higher in downwelling zones and lower in upwelling zones. At the same time, on the contrary, the local maxima of the chl-*a* content and the light attenuation coefficient can be higher in the upwelling region than in downwelling due to the concentration of phytoplankton cells and non-living particles in the reduced upper mixed layer. It was found from the collected dataset that all spatially local maxima of phytoplankton photosynthetic efficiency are observed in the highly dynamic zones where upwelling changes to downwelling. A study of photosynthetic efficiency based on parallel measurements of primary production and electron transport rate in photosystem II is new for the Malvinas upwelling area.

Generally, all considered biological communities of different trophic levels were similarly distributed in space in accordance with the zones of the Patagonian Shelf and branches of the MC, as well as in accordance with the corresponding upwelling and downwelling fine structure.

Phytoplankton, studied for the first time in summer both in the marginal regions of the shelf and in the inshore branch of the MC, included more than 40 species. Qualitatively, dinoflagellates predominated, and heterotrophic forms were among them. Overall, adiversity, structural diversity, and heterogeneity scores were low especially in the shelf edge and in the core of the MC inshore branch. Above the Patagonian Shelf, the richness of the flora was high. The picoplankton photosynthetic pyramidomonads *Prasinoderma coloniale* quantitatively dominated everywhere. Two floristic groups have been identified: Af, on the Patagonian Shelf, and Bf, above the shelf slope. Among the communities, against the background of a common dominant, three groups of complexes were distinguished: Am—above the shelf; Bm—above the slope of the shelf; Cm—on the eastern margin of the MC coastal jet.

Zooplankton was mostly dominated by copepods. The average abundance and species diversity of zooplankton increased from the inshore branch towards the offshore branch of the MC. Three major zooplankton communities were identified, which are associated with the physical properties in the inshore and offshore branches of the Malvinas Current. Copepods and euphausiids larvae are concentrated in the water of the Patagonian Shelf. The upwelling and downwelling zones were characterized by the presence of species of Subantarctic communities. In the zone between the inshore and offshore branches of the Malvinas Current zooplankton was mainly composed of the species belonging to the Antarctic communities. Deep-sea species that were found in the surface water of the MC can be associated with the local zones of upwelling.

Marine mammals and birds were more common on the Patagonian Shelf. At the same time, the greatest species diversity of birds was noted in the region of the offshore branch of the MC. The smallest relative abundance of birds was recorded in the inshore branch despite the fact that the strongest upwelling and highly bio-productive waters were observed in this region.

#### **5. Conclusions**

The main findings of the work are the following:


**Author Contributions:** Conceptualization, P.A.S. and D.I.F.; methodology, S.A.M., V.V.K., P.V.S., A.Y.M., P.A.S. and V.I.P.; software, M.Y.U.; validation, V.A.K. and N.A.S.; formal analysis, P.A.S., D.I.F., V.I.P., P.V.S., S.A.O., N.A.L., M.V.B., P.V.C. and P.A.F.; investigation, P.A.S., S.A.M., O.Y.K., P.V.S., A.A.L., I.V.M. and A.D.C.; resources, A.Y.M. and N.A.S.; data curation, P.A.S.; writing—original draft, P.A.S., S.A.M., V.V.K., P.V.S. and P.V.C.; writing—review and editing, P.A.S., V.I.P., E.G.M., S.P.S. and V.V.K.; visualization, P.A.S., V.V.K., A.A.L., S.A.O., N.A.L. and M.V.B.; supervision, P.A.S.; project administration, E.G.M.; funding acquisition, E.G.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the Russian State Tasks FWMM-2022-0033, 0211-2021-008 and 0211-2021-0007 (POI FEB RAS), FMWE-2022-0001 (IO RAS), 122072000067-9 (NSCMB FEB RAS), FNNN-2022-0001, FNNN-2021-0003 (MHI RAS), FFER-2019-0021 (IEE RAS), 0202-2021-0001 (IACP FEB RAS) and grant of the Russian Science Foundation 21-77-20004 (ship operations). Resources of the computing cluster of the POI FEB RAS were also involved.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** No studies involving humans were performed. Birds and mammals were observed using binoculars and photo cameras. No experiments with animals have been carried out.

**Data Availability Statement:** Data are available upon request.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Eugene G. Morozov 1,\* , Dmitry I. Frey <sup>1</sup> , Oleg A. Zuev <sup>1</sup> , Manuel G. Velarde 2,3 , Viktor A. Krechik <sup>1</sup> and Rinat Z. Mukhametianov 1,4**

	- <sup>4</sup> Moscow Institute of Physics and Technology, Institutsky per. 9, Dolgoprudny, 141700 Moscow, Russia

**Abstract:** Supercritical hydraulically controlled overflow of Antarctic Bottom Water from the Weddell Sea has been observed in the Orkney Passage during field measurements in February 2022. The Orkney Passage is the main pathway for the densest layer of Antarctic Bottom Water flow from the Weddell Sea to the Scotia Sea. The bottom current overflows the sill across the passage and flows down from the crest of the sill at 3600 m deeper than 4000 m. The descending flow accelerates because of the difference in the height of the sill and its foot. An estimate of the Froude number of this flow was greater than unity. Near the foot of the slope the kinetic energy of the flow becomes insufficient to continue moving in this regime. The flow slows down, and strong mixing and warming of the bottom water occurs due to the exchange with the surrounding waters. This hydrodynamic phenomenon is called supercritical hydraulically controlled flow. However, the flow of bottom water continues further and eventually fills the abyssal depths of the Atlantic.

**Keywords:** Orkney Passage; hydraulic control; subcritical; supercritical flows

#### **1. Introduction**

The study site is located in the Weddell Gyre, which is a region in the Weddell Sea with specific properties that controls the deep and bottom ocean circulation and affects the global climate with the capability of influencing the global climate on time scales of hundreds to thousands of years [1]. The waters of the Weddell Sea deeper than 3000 m are connected with the Scotia Sea by the passages in the South Scotia Ridge.

Antarctic Bottom Water (AABW) that occupies the bottom layer of the Atlantic Ocean is generally formed in the Weddell Sea. Sinking of this dense water plays a key role in driving the lower limb of the global overturning circulation [2–4]. A major part of the AABW formation occurs in the Weddell Sea [5]. Formation of AABW generally occurs in the southern and western parts of the Weddell Sea over the ice shelves. Low temperatures and saline shelf waters are formed as a result of surface water cooling during ice freezing and brine rejection in coastal polynyas. Formation of highly dense water is maintained by cyclonic circulation in the Weddell Sea, which is driven by western winds in the north and eastern winds in the south of the Weddell Sea. The flow over the South Scotia Ridge represents a significant component of the AABW export from the Weddell Sea [6].

Weddell Sea Deep Water (WSDW) is the coldest part of AABW. Weddell Sea Deep Water is defined as water with potential temperatures between ~0◦C and −0.7 ◦C. This water is formed due to the air–sea–ice interaction at the periphery of the Weddell Sea combined with the upwelling of Weddell Sea Bottom Water [7,8]. The main pathway for the export of WSDW from the Weddell Sea is the Orkney Passage [9]. Part of the bottom water is exported from the Weddell Sea through the South Sandwich Trench and other fractures of the South Scotia Ridge [2].

**Citation:** Morozov, E.G.; Frey, D.I.; Zuev, O.A.; Velarde, M.G.; Krechik, V.A.; Mukhametianov, R.Z. Hydraulically Controlled Bottom Flow in the Orkney Passage. *Water* **2022**, *14*, 3088. https://doi.org/ 10.3390/w14193088

Academic Editor: Fangxin Fang

Received: 22 August 2022 Accepted: 27 September 2022 Published: 1 October 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

#### **2. Hydraulic Control** Fluids can transmit a signal by advection or wave propagation. We consider a water flow

**2. Hydraulic Control** 

of the South Scotia Ridge [2].

We shall explain the physical phenomenon of hydraulic control following [10,11]. Fluids can transmit a signal by advection or wave propagation. We consider a water flow over an underwater ridge or dam. The flow from a deep reservoir overflows the crest of a dam or underwater ridge and spills down the spillway. A hydraulic jump is formed at the base of the spillway, which is an abrupt increase in the fluid depth (or depth of isopycnals) accompanied by turbulence, wave generation and mixing. over an underwater ridge or dam. The flow from a deep reservoir overflows the crest of a dam or underwater ridge and spills down the spillway. A hydraulic jump is formed at the base of the spillway, which is an abrupt increase in the fluid depth (or depth of isopycnals) accompanied by turbulence, wave generation and mixing. If perturbations of the flow appear upstream of the ridge (dam) crest, the waves (surface or internal) are generated, which can propagate in either direction. The flow down-

We shall explain the physical phenomenon of hydraulic control following [10,11].

exported from the Weddell Sea through the South Sandwich Trench and other fractures

If perturbations of the flow appear upstream of the ridge (dam) crest, the waves (surface or internal) are generated, which can propagate in either direction. The flow downstream from the crest is so rapid that waves cannot propagate upstream. In other words, the velocity of the waves is smaller than the velocity of the descending flow. This type of flow is called supercritical. As the flow reaches the hydraulic jump, its velocity decreases and it returns to a subcritical state. In addition, the regime of the flow at the crest is critical, i.e., changing from subcritical to supercritical. Thus, the outflow from the reservoir is said to be choked or hydraulically controlled by the ridge (dam). The size of the dam, water depth and stratification are parameters that control the flow. A scheme of the phenomenon is shown in Figure 1a. stream from the crest is so rapid that waves cannot propagate upstream. In other words, the velocity of the waves is smaller than the velocity of the descending flow. This type of flow is called supercritical. As the flow reaches the hydraulic jump, its velocity decreases and it returns to a subcritical state. In addition, the regime of the flow at the crest is critical, i.e., changing from subcritical to supercritical. Thus, the outflow from the reservoir is said to be choked or hydraulically controlled by the ridge (dam). The size of the dam, water depth and stratification are parameters that control the flow. A scheme of the phenomenon is shown in Figure 1a. If the dam is low and the flow is slow the current just overflows the dam (Figure 1b).

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(**b**)

**Figure 1.** (**a**). Scheme of hydraulic control. Modified from [11]. (**b**). Scheme of laminar current overflowing an obstacle. **Figure 1.** (**a**). Scheme of hydraulic control. Modified from [11]. (**b**). Scheme of laminar current overflowing an obstacle.

If the dam is low and the flow is slow the current just overflows the dam (Figure 1b). Whitehead et al. [12] and Pratt and Whitehead [11] gave examples of hydraulic control in the passage between the Virgin Islands and in the Strait of Denmark; other examples were found in the Strait of Gibraltar [13,14].

This phenomenon has also been studied in laboratory experiments [15]. Even if the flow is subcritical, the flow entrains surrounding waters depending not only on the Froude number but also on the Reynolds number, as shown in [16]. The authors demonstrated this on the example of laboratory experiments and overflow in the Denmark Strait. Field

measurements of a bolus of cold water overflowing the sill in the Denmark Strait and properties of the flow with variable velocity of the overflowing water were reported in [17]. Overflow in the Faroe Bank Channel was studied in [18]. Field measurements of a bolus of cold water overflowing the sill in the Denmark Strait and properties of the flow with variable velocity of the overflowing water were reported in [17]. Overflow in the Faroe Bank Channel was studied in [18]. in [17]. Overflow in the Faroe Bank Channel was studied in [18]. We will show some more illustrative examples. First is the well-known Niagara Falls, which is a surface flow (Figure 2).

Whitehead et al. [12] and Pratt and Whitehead [11] gave examples of hydraulic control in the passage between the Virgin Islands and in the Strait of Denmark; other exam-

This phenomenon has also been studied in laboratory experiments [15]. Even if the flow is subcritical, the flow entrains surrounding waters depending not only on the Froude number but also on the Reynolds number, as shown in [16]. The authors demonstrated this on the example of laboratory experiments and overflow in the Denmark Strait. Field measurements of a bolus of cold water overflowing the sill in the Denmark Strait and properties of the flow with variable velocity of the overflowing water were reported

Whitehead et al. [12] and Pratt and Whitehead [11] gave examples of hydraulic control in the passage between the Virgin Islands and in the Strait of Denmark; other exam-

This phenomenon has also been studied in laboratory experiments [15]. Even if the flow is subcritical, the flow entrains surrounding waters depending not only on the Froude number but also on the Reynolds number, as shown in [16]. The authors demonstrated this on the example of laboratory experiments and overflow in the Denmark Strait.

We will show some more illustrative examples. First is the well-known Niagara Falls, which is a surface flow (Figure 2). We will show some more illustrative examples. First is the well-known Niagara Falls, which is a surface flow (Figure 2).

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ples were found in the Strait of Gibraltar [13,14].

ples were found in the Strait of Gibraltar [13,14].

**Figure 2.** Subcritical and supercritical regimes in the Niagara Falls. **Figure 2.** Subcritical and supercritical regimes in the Niagara Falls. Another example is a flow over an underwater ridge in a less known region in the

Another example is a flow over an underwater ridge in a less known region in the Kara Gates Strait between the Barents and Kara seas [19]. The phenomenon of the flow is very similar to that in the Strait of Gibraltar. A surface flow from the Barents Sea to the Kara Sea forms a hydraulic jump, whereas strong tidal currents over a sill generate largeamplitude internal tides [19–21] (Figure 3). Another example is a flow over an underwater ridge in a less known region in the Kara Gates Strait between the Barents and Kara seas [19]. The phenomenon of the flow is very similar to that in the Strait of Gibraltar. A surface flow from the Barents Sea to the Kara Sea forms a hydraulic jump, whereas strong tidal currents over a sill generate large-amplitude internal tides [19–21] (Figure 3). Kara Gates Strait between the Barents and Kara seas [19]. The phenomenon of the flow is very similar to that in the Strait of Gibraltar. A surface flow from the Barents Sea to the Kara Sea forms a hydraulic jump, whereas strong tidal currents over a sill generate largeamplitude internal tides [19–21] (Figure 3).

**Figure 3.** Field of isopycnals 23.5, 24.5, 25.5, and 26.5 perturbed by the flow in the Kara Gates Strait and internal waves based on numerical calculations using the model in [21,22]. The black color shows the bottom topography as specified in the model (**a**). Distribution of temperature over the towed section (**b**). Isotherms are shown with an interval of 1 ◦C. Thicker lines show isotherms of 1 and 0 ◦C to emphasize the observed effect. The black color shows the bottom profile. Location of the region on a larger scale is shown in the inset.

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region on a larger scale is shown in the inset.

The flow of AABW through the Orkney Passage has been previously studied based on CTD/LADCP casts. A map of stations in different years is shown in Figure 4. The flow of AABW through the Orkney Passage has been previously studied based on CTD/LADCP casts. A map of stations in different years is shown in Figure 4.

**Figure 3.** Field of isopycnals 23.5, 24.5, 25.5, and 26.5 perturbed by the flow in the Kara Gates Strait and internal waves based on numerical calculations using the model in [21,22]. The black color shows the bottom topography as specified in the model (**a**). Distribution of temperature over the towed section (**b**). Isotherms are shown with an interval of 1 °C. Thicker lines show isotherms of 1 and 0 °C to emphasize the observed effect. The black color shows the bottom profile. Location of the

#### **3. Data and Methods 3. Data and Methods**

Seven CTD casts with velocity profiling (LADCP) were made in the cruise in 2022 in the Orkney Passage region. Five stations were made across the passage and two stations down into the Laurie Depression from the central station. The depth of the ridge across the passage is about 3600 m. The sounding stations were displaced to the north relative to the repeated section in order to exclude possible interference with moorings that could be placed in this area. Seven CTD casts with velocity profiling (LADCP) were made in the cruise in 2022 in the Orkney Passage region. Five stations were made across the passage and two stations down into the Laurie Depression from the central station. The depth of the ridge across the passage is about 3600 m. The sounding stations were displaced to the north relative to the repeated section in order to exclude possible interference with moorings that could be placed in this area.

The stations were performed using a lowered acoustic doppler current profiler (LADCP) and conductivity, temperature, and depth (CTD) profilers mounted on a General Oceanics GO1018 rosette water sampler. The water sampler was equipped with a Valeport VA500 altimeter allowing measurements close to the ocean bottom (3–7 m above the seafloor). An Idronaut Ocean Seven 320plus CTD probe was used for the measurements together with an MKplus Deck Unit. The CTD data were collected using the standard package REDAS5. The declared accuracy of CTD measurements is 0.001 °C for temperature and 0.001 mS/cm for conductivity sensors. The LADCP data measured by the TRDI WorkHorse Monitor 300 kHz profiler were processed using the programming package LDEO Software version IX.10 [23]. The accuracy of velocity measurements estimated The stations were performed using a lowered acoustic doppler current profiler (LADCP) and conductivity, temperature, and depth (CTD) profilers mounted on a General Oceanics GO1018 rosette water sampler. The water sampler was equipped with a Valeport VA500 altimeter allowing measurements close to the ocean bottom (3–7 m above the seafloor). An Idronaut Ocean Seven 320plus CTD probe was used for the measurements together with an MKplus Deck Unit. The CTD data were collected using the standard package REDAS5. The declared accuracy of CTD measurements is 0.001 ◦C for temperature and 0.001 mS/cm for conductivity sensors. The LADCP data measured by the TRDI WorkHorse Monitor 300 kHz profiler were processed using the programming package LDEO Software version IX.10 [23]. The accuracy of velocity measurements estimated by the processing program is usually 3–4 cm/s. In the bottom layers due to the bottom track signals, the errors decrease to 1–2 cm/s.

by the processing program is usually 3–4 cm/s. In the bottom layers due to the bottom The temperature section is shown in Figure 5.

track signals, the errors decrease to 1–2 cm/s. The temperature section is shown in Figure 5. One can see in Figures 5 and 6 two characteristic features that are common to all flows in underwater channels. The cold core is displaced to the right side of the channel along the flow. This occurs due to the Ekman circulation, which arises from the bottom friction when the current flows in a narrow channel. A second core appears in the western (left) part of the passage above the bottom, which is displaced to the left of the flow and is located over the western wall of the channel [24]. The minimum potential temperature is −0.54 ◦C, which is close to the previous estimates [6].

**Figure 5.** Section of potential temperature across the Orkney Passage. **Figure 5.** Section of potential temperature across the Orkney Passage.

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**Figure 6.** Longitudinal velocity component (normal to the section) across the Orkney Passage. **Figure 6.** Longitudinal velocity component (normal to the section) across the Orkney Passage.

The velocities of the flow in the passage correspond to similar flows in the southern hemisphere and in particular the flow in the Vema Channel. The core of the maximum The velocities of the flow in the passage correspond to similar flows in the southern hemisphere and in particular the flow in the Vema Channel. The core of the maximum velocity is displaced to the left side relative to the flow.

**Figure 6.** Longitudinal velocity component (normal to the section) across the Orkney Passage. The velocities of the flow in the passage correspond to similar flows in the southern hemisphere and in particular the flow in the Vema Channel. The core of the maximum velocity is displaced to the left side relative to the flow. Let us now consider the downflow of bottom water after overflowing the ridge. Figure 7 shows the flow velocities down the slope. The flow accelerates due to the transition of potential energy on the crest of the ridge into kinetic energy. Due to the expansion of the passage, the flow becomes wider and after flowing down by more than 200 m at a Let us now consider the downflow of bottom water after overflowing the ridge. Figure 7 shows the flow velocities down the slope. The flow accelerates due to the transition of potential energy on the crest of the ridge into kinetic energy. Due to the expansion of the passage, the flow becomes wider and after flowing down by more than 200 m at a distance of about 3500 m, the flow slows down, descending into the deep layers of the ocean and not having enough energy to continue the fast flow. The maximum flow velocities are close to 50 cm/s. The potential temperature behaves similarly, forming a tongue of cold water flowing down the slope (Figure 8). In the tongue of cold water, which rolls down the slope, the temperature gradually increases from −0.54 to −0.45 ◦C. Salinity distribution downslope in the direction of the descending flow is shown in Figure 9. Contour lines of salinity that show the further propagation of the flow downslope are more illustrative than the isotherms because bottom water is warming due to the contact with the overlying waters. The salinity increase is due only to mixing.

Let us now consider the downflow of bottom water after overflowing the ridge. Figure 7 shows the flow velocities down the slope. The flow accelerates due to the transition of potential energy on the crest of the ridge into kinetic energy. Due to the expansion of the passage, the flow becomes wider and after flowing down by more than 200 m at a

velocity is displaced to the left side relative to the flow.

overlying waters. The salinity increase is due only to mixing.

overlying waters. The salinity increase is due only to mixing.

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**Figure 7.** Longitudinal velocity (through flow) section downslope the Orkney Passage. Contour line of 0.356 m/s is additionally shown. **Figure 7.** Longitudinal velocity (through flow) section downslope the Orkney Passage. Contour line of 0.356 m/s is additionally shown. of 0.356 m/s is additionally shown.

distance of about 3500 m, the flow slows down, descending into the deep layers of the ocean and not having enough energy to continue the fast flow. The maximum flow velocities are close to 50 cm/s. The potential temperature behaves similarly, forming a tongue of cold water flowing down the slope (Figure 8). In the tongue of cold water, which rolls down the slope, the temperature gradually increases from −0.54 to −0.45 °С. Salinity distribution downslope in the direction of the descending flow is shown in Figure 9. Contour lines of salinity that show the further propagation of the flow downslope are more illustrative than the isotherms because bottom water is warming due to the contact with the

distance of about 3500 m, the flow slows down, descending into the deep layers of the ocean and not having enough energy to continue the fast flow. The maximum flow velocities are close to 50 cm/s. The potential temperature behaves similarly, forming a tongue of cold water flowing down the slope (Figure 8). In the tongue of cold water, which rolls down the slope, the temperature gradually increases from −0.54 to −0.45 °С. Salinity distribution downslope in the direction of the descending flow is shown in Figure 9. Contour lines of salinity that show the further propagation of the flow downslope are more illustrative than the isotherms because bottom water is warming due to the contact with the

**Figure 8.** Potential temperature distribution over the downslope section of the Orkney Passage. **Figure 8.** Potential temperature distribution over the downslope section of the Orkney Passage. **Figure 8.** Potential temperature distribution over the downslope section of the Orkney Passage.

**Figure 9.** Salinity distribution over the downslope section of the Orkney Passage. **Figure 9.** Salinity distribution over the downslope section of the Orkney Passage.

It was reported in [25] that hydraulically controlled flow over the sills is responsible for bottom mixing above the slope. A local overturning circulation cell is formed over the

= ⁄ඥ′

<sup>ට</sup>వ.ఴ ×భబబ ×బ.బబబభయ భ

We studied the flow of Antarctic Bottom Water from the Weddell Sea to the Scotia Sea through the Orkney Passage. This passage is considered the main pathway for the outflow of Antarctic Bottom Water from the Weddell Sea; hence, it is the main source of Antarctic Bottom Water in the Atlantic. The transport of bottom water through this passage is 5 Sv based on reports in many publications. We made five CTD/LADCP casts to the bottom over the sill of the passage and three casts along the slope downstream. During the overflow of Antarctic Bottom Water from the Weddell Sea over the ridge in the Orkney Passage, a supercritical regime with a Froude number greater than unity was noted. The flow accelerates when descending the slope. Near the foot of the slope there is not enough kinetic energy of the flow to continue moving in this regime. The flow slows down; strong mixing and warming of the bottom water occur due to the exchange with the surrounding

**Author Contributions:** E.G.M., D.I.F., MGV: Conceptualization, original draft preparation, writing; V.A.K., O.A.Z.: original draft preparation; RZM: data procerssing. All authors have read and agreed

≈ 1.0

changes as the velocity increases and the Froude number exceeds unity. The critical regime appears when the Froude number is *Fr* > 1. When the current overflows the sill of

where *v* is the meridional component of velocity, *g*'*H* is the velocity of gravity waves in the shallow flow, *H =* 100 m is the thickness of the flow, *g*'= *g* Δρ ρ is reduced acceleration due to gravity, ρ = 1 g/cm3 is the in situ density, *g =* 9.8 m s−1, and Δρ = 0.00013 g/cm3 in the overflowing current. The critical regime of overflowing appears when the

= .ଷହ

the Orkney Passage the Froude number is calculated as:

waters. This mode is called hydraulically controlled flow.

to the published version of the manuscript.

bottom current exceeds *v =* 0.356 m s−1:

**4. Conclusions** 

It was reported in [25] that hydraulically controlled flow over the sills is responsible for bottom mixing above the slope. A local overturning circulation cell is formed over the slope with increasing vertical velocity that mixes the bottom flow [25,26]. The flow regime changes as the velocity increases and the Froude number exceeds unity. The critical regime appears when the Froude number is *Fr* > 1. When the current overflows the sill of the Orkney Passage the Froude number is calculated as:

$$Fr = v/\sqrt{\mathcal{g'}H}$$

where *v* is the meridional component of velocity, p *g* 0*H* is the velocity of gravity waves in the shallow flow, *H =* 100 m is the thickness of the flow, *g* 0 = *g*∆ρ/ρ is reduced acceleration due to gravity, ρ = 1 g/cm<sup>3</sup> is the in situ density, *g =* 9.8 m s−<sup>1</sup> , and ∆ρ = 0.00013 g/cm<sup>3</sup> in the overflowing current. The critical regime of overflowing appears when the bottom current exceeds *v =* 0.356 m s−<sup>1</sup> :

$$Fr = \frac{0.356}{\sqrt{\frac{9.8 \times 100 \times 0.00013}{1}}} \approx 1.0$$

#### **4. Conclusions**

We studied the flow of Antarctic Bottom Water from the Weddell Sea to the Scotia Sea through the Orkney Passage. This passage is considered the main pathway for the outflow of Antarctic Bottom Water from the Weddell Sea; hence, it is the main source of Antarctic Bottom Water in the Atlantic. The transport of bottom water through this passage is 5 Sv based on reports in many publications. We made five CTD/LADCP casts to the bottom over the sill of the passage and three casts along the slope downstream. During the overflow of Antarctic Bottom Water from the Weddell Sea over the ridge in the Orkney Passage, a supercritical regime with a Froude number greater than unity was noted. The flow accelerates when descending the slope. Near the foot of the slope there is not enough kinetic energy of the flow to continue moving in this regime. The flow slows down; strong mixing and warming of the bottom water occur due to the exchange with the surrounding waters. This mode is called hydraulically controlled flow.

**Author Contributions:** E.G.M., D.I.F., M.G.V.: Conceptualization, original draft preparation, writing; V.A.K., O.A.Z.: original draft preparation; R.Z.M.: data procerssing. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work was supported by the Russian Science Foundation grant 21-77-20004 (ship measurements). Field data processing was supported by the Russian Foundation for Basic Research grant no 20-08-00246.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** No studies involving humans were performed. Birds and mammals were observed using binoculars and photo cameras. No experiments with animals have been carried out.

**Data Availability Statement:** Data are available upon request.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Abbreviations**



### **References**

