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Article

Open-Ocean Carbonate System and Air–Sea CO2 Fluxes Across a NE Atlantic Seamount Complex (Madeira–Tore, August 2024)

by
Marta Nogueira
1,* and
Alexandra D. Silva
1,2
1
IPMA, I.P., Instituto Português do Mar e da Atmosfera, I.P., Av. Alfredo Magalhães Ramalho 6, 1495-165 Algés, Portugal
2
Terminal de Cruzeiros do Porto de Leixões, CIIMAR/CIMAR LA, Centro Interdisciplinar de Investigação Marinha e Ambiental, Universidade do Porto, 4450-208 Matosinhos, Portugal
*
Author to whom correspondence should be addressed.
Oceans 2025, 6(3), 46; https://doi.org/10.3390/oceans6030046
Submission received: 16 April 2025 / Revised: 18 June 2025 / Accepted: 24 June 2025 / Published: 17 July 2025
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Abstract

This study focused on the carbonate system dynamics and air–sea CO2 fluxes in the open-ocean waters of the Madeira–Tore Seamount Complex during August 2024. Surface water properties revealed pronounced latitudinal gradients in sea surface temperature (21.9–23.1 °C), salinity (36.2–36.7), and dissolved oxygen (228–251 µmol Kg−1), influenced by mesoscale eddies and topographically driven upwelling. Despite oligotrophic conditions, distinct phytoplankton assemblages were observed, with coccolithophores dominating southern seamounts and open-ocean stations, and green algae and diatoms indicating episodic nutrient input. Surface total alkalinity (TA: 2236–2467 µmol Kg−1), dissolved inorganic carbon (DIC: 2006–2183 µmol Kg−1), and pCO2 (467–515 µatm) showed spatial variability aligned with water mass characteristics and biological activity. All stations exhibited positive air–sea CO2 fluxes (2.8–11.5 mmol m−2 d−1), indicating the region is a CO2 source during summer. Calcite and aragonite saturation states were highest in stratified, warmer waters. Principal Component Analysis highlighted the role of physical mixing, carbonate chemistry, and biological uptake in structuring regional variability. Our findings emphasize and contribute to the complex interplay of physical and biogeochemical drivers in modulating carbon cycling and ecosystem structure across Atlantic seamounts.

1. Introduction

The process known as ocean acidification is occurring more rapidly now than at any other period in Earth’s history [1], as approximately 30% of anthropogenic emissions of carbon dioxide (CO2) to the atmosphere are absorbed by the ocean each year [2,3]. To improve our understanding of the global carbon cycle, accurate assessment of ocean carbonate chemistry variability and sea–air CO2 fluxes is critical. The oceanic carbonate system is undergoing an unprecedented change, with open-ocean pH decreasing on average by approximately 0.0018/year over the past 15–30 years [4]. These changes are not spatially or temporally uniform. Due to regional differences in ocean chemistry, circulation, and biological activity, the patterns of acidification and CO2 flux show significant heterogeneity. Each ocean basin and hemisphere contributes very differently to both the global carbon inventory and its associated uncertainties [3]. Among these, the North Atlantic has been recognized as one of the largest ocean sinks for both natural and anthropogenic CO2, accounting for approximately 25% of the ocean’s total uptake [5]. In subtropical zones (e.g., off the Iberian Peninsula, Azores region), increased TA has been reinforcing ocean acidification from surface ocean uptake of atmospheric CO2 [6]. Increasing attention has been directed to its subregions, such as the North–East (NE) Atlantic, where complex circulation patterns and dynamic physical–biogeochemical interactions further influence CO2 fluxes and acidification trends [7]. The NE–Atlantic region includes several dynamic oceanographic systems, including the Canary Current, the Azores Current, and deep-water masses like the North Atlantic Deep Water (NADW). These systems are essential for transporting heat, nutrients, and gases, such as carbon dioxide (CO2), across ocean basins, thus influencing the global carbon cycle [8,9]. The NE–Atlantic functions as a carbon sink, particularly in the subpolar and temperate zones, absorbing atmospheric CO2 through both physical and biological processes [10], making it a key region for long-term carbon storage and climate regulation.
An important feature of this region is the presence of numerous seamounts—underwater volcanic mountains. Due to their isolation and topographic complexity, these features create unique biogeochemical and ecological hotspots by promoting the vertical transport of nutrients and dissolved gases, which in turn influences local productivity, phytoplankton dynamics, and air–sea gas exchange [11,12,13,14]. Phytoplankton productivity directly affects CO2 dynamics by altering the balance of DIC in the water column through photosynthesis and respiration. Among phytoplankton groups, coccolithophores are known to be highly sensitive to ocean acidification [15]. Their importance lies in their dual contribution to carbon sequestration (via the biological carbon pump) and to surface ocean alkalinity through calcification. Furthermore, their abundance and diversity are strongly influenced by oceanographic conditions, including nutrient availability and vertical mixing [16]. Studies on phytoplankton communities in these areas are essential for understanding the NE Atlantic’s biogeochemical dynamics in the global carbon cycle.
Located between 33° N to 38° N, the Horseshoe Seamount Complex—including Madeira–Tore and adjacent seamounts (e.g., Ampère)—stands out due to its proximity to both oligotrophic gyres and productive upwelling zones. It is a key area for understanding the interactions between geology, oceanography, and biogeochemistry, and may act as a potential sentinel of climate change and acidification in the NE Atlantic. This study aims to 1. assess the spatial variability of surface carbonate system parameters and air–sea CO2 fluxes across the Madeira–Tore seamounts; 2. characterize phytoplankton functional group distributions under different oceanographic conditions and their relationship with carbonate chemistry; and 3. understand the role of mesoscale dynamics in shaping surface water chemistry. Characterizing these regional processes is essential for improving global projections of carbon uptake under climate change.

2. Materials and Methods

2.1. Study Area

This study focuses on the Horseshoe seamount chain, located along the Azores–Gibraltar fracture zone in the NE Atlantic Ocean (Figure 1). This tectonic boundary marks the transition between oceanic and continental crust (e.g., [17]) and lies within the Portuguese Exclusive Economic Zone (EEZ) and the Portuguese Extended Continental Platform. The target areas include the Madeira–Tore complex and the adjacent Ampère Seamount (35°04′ N and 12°57′ W), both recognized for their unique geological formations and their role in supporting ecologically and oceanographically significant habitats [17]. The Madeira–Tore complex is considered one of the most important deep-sea ecosystems in Portuguese waters, covering 197,431 km2, with depths ranging from 25 to over 4500 m. It comprises 17 seamounts of varying geological composition and age [17,18,19,20,21]. These features create diverse seafloor topographies and habitat types, supporting unique biological communities.
The region lies within the North Atlantic subtropical gyre and is influenced by the Canary Current System, one of the world’s four major Eastern Boundary Upwelling Ecosystems [22,23]. The near-surface wind regime across most of the study area (10°–30° N) is dominated by seasonal variations in the intensity of northeast trade winds, which modulate surface productivity, mixed layer dynamics, and northward dispersal processes [19,24]. Within the seamounts of the Horseshoe complex, Gorringe Bank has received particular attention for its unique geological formation and biodiversity. It spans approximately 23,000 km2 and has been designated a Site of Community Importance [25] under the Natura 2000′s network (PTCON0062). Its steep flanks, flat summits, and interaction with regional currents contribute to localized upwelling and enhanced biodiversity [19].
Although TA variations in the Atlantic subtropical gyres (30° S–30° N) are associated with salinity changes [26], the vertical distribution of the carbonate system parameters is primarily influenced by the structure of the water mass. Additionally, biological and biogeochemical processes, such as the production and decomposition of organic matter, the formation and dissolution of calcium carbonate, and the distinct carbonate characteristics imprinted during the formation of each water mass, collectively determine the observed vertical profiles [27]. González-Dávila et al. [28] highlighted that seasonal variations in surface inorganic carbon represent a residual signal resulting from the combination of biological and physical processes, including net community production, air–sea CO2 gas exchange, vertical mixing, and horizontal advection.
For this study, seamounts were selected based on distinct geomorphological features and ecological contexts. The selection considered summit shape (flat vs. pointed), depth range, exposure to dominant water masses, and logistical considerations (e.g., steaming time between sites). The selected seamounts were (i) Gettysburg (Gorringe Bank; height: 4950 m; type: flat); (ii) Josephine (height: 2500 m; type: flat); (iii) Lion (height: 2200 m; type: pointed); and (iv) Ampère (height: 4390 m; type: pointed). These sites represent a gradient of summit morphologies, depths, and relative isolation, which are factors potentially influencing biogeochemical cycles and ecological connectivity [18].

2.2. Surface Physical and Biogeochemical Sampling and Analysis

Sampling was conducted between 2 August and 10 August 2024 onboard RV Mário Ruivo during the CMT-AMP campaign. The survey began at Josephine Seamount (stations 1, 2, and 3), followed by Lion Seamount (stations 4, 5, and 6), a mid-point station between Madeira and Ampère Seamount (station 7), Ampère Seamount (stations 8 and 9), and concluded at Gettysburg peaks of Gorringe Seamount (stations 10 and 11).
Water for biogeochemical analysis was collected with Niskin bottles at the surface. Simultaneously, surface temperature (°C) and salinity (<1 m depth) were recorded at each site using a CTD (SBE-19plus V2 SeaCAT profiler from Sea-Bird Scientific, (Sea-Bird Electronics, Inc. (SBE), Bellevue, USA). Salinity surface bottles were also collected to validate the CTD measurements; these samples were measured in the laboratory with a Guildline Autosal 8400B (Guildline Instruments Limited, Smiths Falls, Ontario, Canada) (calibrated against IAPSO standard seawater).
Dissolved oxygen (DO) was determined by the Winkler titration method [29] using a whole-bottle manual titration. The method’s coefficient of variation ranged from 0.08% to 0.25%, ensuring high precision. Apparent Oxygen Utilization (AOU) was defined as the difference between the saturated oxygen concentration at the temperature and salinity of the water according to [30] and the measured DO concentration in the same sample.
Chlorophyll (Chla) was analyzed in 90% acetone extracts after filtration through Whatman GF/F filters, following the methodology of [31]. Fluorescence was measured using a Hitachi F-7000 fluorometer, calibrated with commercial chlorophyll-a solutions (Sigma Chemical Co., St. Louis, MI, USA).
Samples for phytoplankton were field preserved with neutralized formaldehyde and analyzed in the laboratory using the Utermöhl method [32], following 100 mL settling. Observations were made with an inverted microscope (Leica DMi8, Leica Biosystems, W. Lake Cook Road, Deer Park, United States) equipped with phase contrast and bright-field illumination at magnifications of 100×, 200×, and 400×. Cell abundances were expressed in cells L−1. Whenever possible, species were identified at the lowest taxonomic level. Although Emiliania huxleyi and certain Gephyrocapsa species (both genera belonging to the family Noelaerhabdaceae) were counted separately, they were grouped for statistical analysis to better explore ecological relationships, given the morphological similarities that complicate identification by light microscopy.
Total alkalinity (TA) was determined via automatic titration with HCl to an endpoint of pH 4.5 [33]. Validation was performed using QUASIMEME standards and certified reference materials (CRM) supplied by A.G. Dickson (Scripps Institution of Oceanography, San Diego, MI, USA), ensuring an accuracy of ±2 μmol Kg−1. The Excel program CO2SYS version 3.0 [34] was used for computation of the partial pressure of CO2 (pCO2) and Dissolved Inorganic Carbon (DIC), with the carbonic acid dissociation constants of Lueker et al. [35], the HSO4- dissociation constant of Dickson [36], and the value of [B]T determined by Lee et al. [37]. In situ calcite (ΩCa) and aragonite (ΩAr) saturation states were calculated from TA and DIC using the program CO2SYS.
Total pH (pHT) was collected in triplicate from each Niskin bottle and measured in the laboratory using 10 cm cylindrical optical glass pathlength cells, following the spectrophotometric procedure of Clayton and Byrne [38]. pH values are reported on the total scale. Precision was estimated from the replicates and was ±0.001.
Simple pairwise correlation analyses (Pearson method) were conducted to examine the relationships between all parameters.

2.3. Ancillary Data

2.3.1. Satellite Data

Daily satellite data for sea surface temperature (SST), sea surface salinity (SSS), and current velocities, including Northward sea water velocity, (vo), Eastward sea water velocity (uo), and Upward sea water velocity (wo), were extracted from the IBI-MFC model (Copernicus Product ID: IBI_MULTIYEAR_PHY_005_002), which is based on the NEMO v3.6 general circulation model. The dataset assimilates satellite SST, altimetry, and in situ profiles, providing Level 4 ocean physics fields for the Iberia–Biscay–Ireland region. The data have a horizontal resolution of ~0.083° (1/12°) and daily temporal resolution spanning from 2 to 10 August 2024. All data were processed using ODV (v.5.8.1).
Sea water velocity was determined using northward sea water velocity (vo) and eastward sea water velocity (uo) through linear interpolation with the QGis plugin VectorFieldCalc v. 1.5.4 (Mauro Alberti/VectorFieldCalc GitLab).

2.3.2. Atmospheric Data

Atmospheric CO2 data were obtained from the Izana Global GHG Reference Station (Tenerife, Canary Islands, Spain; 28.3090° N–16.499° W), part of the IZO co2–NOAA Global Monitoring Laboratory (weekly dataset) (https://gml.noaa.gov/data/dataset.php?item=izo-co2-flask, accessed 21 March 2025) [39]. Wind speed was measured continuously by the vessel’s meteorological station at 10 m above the ocean surface (resolution ~1 min), and averaged daily mean values were used for CO2 flux calculations.
The North Atlantic Oscillation index (NAO), defined as the normalized sea-level pressure difference between the Azores and Iceland, is a key indicator of atmospheric variability in the Northern Hemisphere. It is linked to the strength of westerly winds over the North Atlantic. In this study, the NAO index was used to explore its potential relationship with pCO2 availability. Monthly NAO values were obtained from the NOAAClimate Prediction Center [40] (https://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/norm.nao.monthly.b5001.current.ascii, accessed 20 March 2025).

2.3.3. CO2 Flux Calculations

Net sea-air CO2 flux (FCO2) was estimated using the following expression:
FCO2 = α k (ΔpCO2) sea-air
where α is the solubility of CO2 in seawater (mol L−1 atm−1) [41], k is the CO2 gas transfer velocity (cm h−1), and ΔpCO2 (μatm) is the sea-air CO2 partial pressure difference.
The k dependence on wind speed has been calculated using the relationship formulated by Wanninkhof et al. [42]. The wind speed daily mean values obtained from the ship meteorological station were used for FCO2 calculations. ΔpCO2 was calculated as the difference between the sea surface pCO2 and the atmospheric pCO2 mean value of 423.0 μatm, obtained from the Izana Station (Canary Islands) for August 2024 [39]. A negative flux indicates the ocean acting as an atmospheric sink, whereas a positive flux denotes it acting as a source.

3. Results

3.1. Physical and Biogeochemical Water Parameters

Sea surface variables such as temperature (SST), salinity (SSS), oxygen (DO), and chlorophyll a (Chla) are presented in Figure 2 and Figure 3 and Table 1. The results showed that SST (Figure 2A) ranged from 21.86 °C to 23.14 °C across the Madeira–Tore Seamount Complex and increased from north to south. The highest values were recorded at Madeira–Ampère open-ocean station (station 7, 23.14 °C) and further east at Lion Seamount (station 5_mid slope W, 23 °C). Generally, lower SST values were observed at higher latitude seamounts (~36° N, between −14° W and −11° W), specifically at Josephine (~22 °C, mid-slope East station 1 and summit station 2) and at Gettysburg Peak (~21.9 °C, station 11). This pattern aligns with SST satellite data (Figure 3A), which showed a clear temperature gradient during the sampling period, with cooler temperatures in the north (towards 37° N) and warmer temperatures in the south (towards 33° N).
Salinity also showed a north-to-south increasing gradient (Figure 2B and Figure 3B), with values ranging from 36.21 to 36.74. The highest value was observed at Ampère Seamount (station 8, mid slope West, 36.74), while lower values were observed at Josephine Seamount (station 2, summit: 36.21; station 3: 36.22) further north (Figure 2). The spatial variability in SSS likely reflects the influence of mesoscale features similar to those affecting SST, in addition to precipitation and evaporation [26,43].
Surface waters in the Madeira–Tore complex were oversaturated with oxygen (AOU < 0), with DO concentrations ranging from 228 to 251 µmol Kg−1 (Figure 2C). Higher values were observed at eastern and southeastern seamounts, such as station 4 (Lion summit 1: 251 µmol Kg−1) and station 2 (Josephine summit: 247 µmol Kg−1). Lower values were found at lower latitudes, particularly at the Madeira–Ampère open-ocean station (station 7: 228 µmol Kg−1) and Ampère Seamount (stations 8 and 9: 228 µmol Kg−1).
Chlorophyll-a concentrations were very low, ranging from 0.01 to 0.04 mg.m−3 (Figure 2D). The highest values were determined at the inner stations of the study area: Lion Seamount summits (station 4 and station 6), Josephine mid-slope west (station 1), and Madeira–Ampère open-ocean station (station 7). The lowest values were observed at the slope station of the Gettysburg Peak summit (station 10) and at Josephine summit (station 2), located in the outer northern stations of the study area.
Figure 4A illustrates the spatial distribution of sea surface water velocity field (m s−1) and flow direction in the study area. A complex dynamic is observed with multiple mesoscale eddies and jet-like features, enhancing horizontal mixing. The strongest currents (~5 m s−1) appeared in narrow bands around 34° N–35° N, suggesting the presence of jets moving from southwest to northeast over the Madeira–Ampère open-ocean station and Ampère Seamount. There are regions with high variability in current direction and velocity. The velocity vectors do not show a uniform northward or southward dominance. In the western and central areas (14° W–16° W), flow tends to be eastward to north-eastward, with localized regions of north-westward and south-westward deflection around eddies. To the east (~10° W–12° W), a large cyclonic feature dominates (e.g., mesoscale vortices), inducing northward flow on its western edge and southward flow on its eastern edge.
Figure 4B illustrates the corresponding upward water movement (wo). Red to green areas indicate upwelling, bringing deeper, cooler, nutrient-rich water to the surface, whereas blue zones indicate downwelling. Most of the region displayed patchy upwelling activity, especially between 33° N and 36° N, over or downstream of Gettysburg Peak, Ampère Seamount, and the Madeira–Ampère open-ocean station. These signatures are indicative of topographically driven upwelling or internal waves, also supporting nutrient uplift.
Figure 4C shows a dominant wind direction from NNE (around 39°), with most wind speeds in the 5–10 m s−1 range during the sampling period. These prevailing winds are likely one of the primary drivers of the upwelling signatures observed in Figure 4A,B.

3.2. Phytoplankton Community

Surface phytoplankton communities showed variability in both composition and abundance across the studied seamounts, slightly increasing from north to south (Table 1 and Table 2). Lion and Ampère had the highest abundances and diversity. Total phytoplankton cell counts ranged from 920 cells L−1 (station 4) to 47,387 cells L−1 (station 6), both recorded at Lion summits. The Madeira–Ampère open-ocean station (#7) recorded the second highest cell concentration, at 22,293 cells L−1.
Coccolithophores and dinoflagellates were the most abundant groups, with coccolithophores reaching 32,907 cells L−1 at station 6 (Lion summit 1) and dinoflagellates peaking at 6080 cells L−1 at station 9 (Ampère summit). Coccolithophores were the only group present at all stations, whereas dinoflagellates were mostly concentrated at Ampère Seamount and absent from Josephine. Emiliania huxleyi, a key coccolithophore, was present at all stations along with several species of Gephyrocapsa, mainly G. muelerae and G.ericsonii. The highest concentrations of E. huxleyi (6400 cells L−1) and Gephyrocapsa spp. (2133 cells L−1) were recorded at station 6 (Lion summit 1). Coronosphaera mediterranea was only observed at the surface in this station, reaching 7520 cells L−1. Discosphaera tubifer was most abundant at the Madeira-Ampère open-ocean station (station 7, which recorded the highest SST at 23.1 °C), followed by mid-slope stations (stations 1, 3, 8, 10) and Gettysburg summit (station 11).
The dinoflagellate community was characterized by bloom-forming autotrophic and mixotrophic species, some capable of forming cysts. Gymnodinium, Scripsiella, Ceratium, and Heterocapsa, were the most abundant genera.
The third most abundant group was green algae (Chlorophyceae), which reached a maximum of 4240 cells L−1 at station 6 (Lion summit 1). They were present across all seamounts except at the western slopes of Josephine and Lion Seamounts (stations 3 and 5, respectively), and on the summits of Ampère (station 9) and Gettysburg (station 11). Green algae (Chlorophyceae) included Pyramimonas spp.
Other phytoplankton groups, in decreasing order of abundance, included diatoms, ciliates, and cryptophytes. At station 6 (Lion summit 1), diatoms and ciliates reached their highest concentrations, with 3040 cells L−1 and 2880 cells L−1, respectively. Diatoms were also recorded at station 7 (Madeira–Ampère open-ocean). Species were classified into two orders: Pennales, which included medium to large chain-forming Pseudo-nitzschia species from the seriata group (station 6, Lion summit 1), and Centrales, which included larger chain-forming species such as Proboscia alata and Hemiaulus spp. Cryptophytes were only found at the southern stations, in surface samples at the Madeira–Ampère open-ocean station (# 7) and at Ampère (mid-slope W, station 8).
The taxonomic composition of samples revealed a diverse community, with a total of 31 distinct genera and 37 species or species groups identified across six major categories. Coccolithophores and Dinoflagellates exhibited the highest species richness, each represented by 11 and 10 genera, respectively, and 13 species or species groups. Diatoms contributed seven genera and eight species or species groups, including the broad categories of Pennales and Centrales, which encompass various species. It is important to note that the use of “spp.” denotes multiple species within a genus. The same with green algae, cryptophyceae, and ciliates, each of which represents more than one species.

3.3. Inorganic Carbon System and Atmospheric Fluxes

The inorganic carbon system parameters determined and derived are presented in Figure 5 and Table 1.
Surface total alkalinity (TA) ranged from 2236 to 2467 µmol Kg−1, showing moderate north–south variability (Figure 5A). The lowest values were observed at higher latitudes (37° N), at Josephine Seamount slope stations (stations 1 and 2), and the highest further south, at Lion summit 1 (station 6). Results also showed that Josephine Seamount had ~160 µmol Kg−1 lower TA compared to the other seamounts.
Surface water pHT ranged from 7.951 to 8.001 (Figure 5B). Highest pH values were observed at Lion summit 1 (station 6: 8.001), followed by Gettysburg Peak mid slope west (station 10: 7.999), while the lowest was observed at Josephine mid slope west (station 1).
Dissolved inorganic carbon (DIC) varied from 2006 to 2183 µmol Kg−1, with the lowest at Josephine Seamount slope west (station 1) and increasing in other seamounts, with the highest values observed at Lion summit 1 (station 6) and Ampère mid slope west (station 9) (Figure 5C).
Surface pCO2 values ranged from 467.1 µatm to 514.8 µatm (Figure 5D). The lowest value was observed in one of the northern seamounts, at Josephine summit (station 2), while the highest was at Lion mid-slope west (station 5), further south. Average values of pCO2 for each seamount revealed that Gettysburg and Josephine presented the lowest values, 478.8 µatm and 481.5 µatm, respectively, while pCO2 was relatively constant and higher—an average value of 493.7 ± 0.6 μatm—along the other seamounts and the Madeira–Ampère open-ocean station. Consequently, the ocean–atmosphere pCO2 gradient (ΔpCO2) was positive at all seamounts, reaching an average of 63.0 ± 14.2 µatm.
Air–sea CO2 flux calculations used daily mean values of wind speed, which were variable (ranging between 5.78 and 8.60 m s−1) and resulted in positive flux values varying from 2.76 to 11.47 mmol m−2 day −1 (Figure 5E). CO2 emissions were highest (9.82 mmol m−2 day−1) at Ampère Seamount (33° N), while Josephine Seamount (37° N) presented the lowest emissions (3.66 mmol m−2 day −1).
Surface saturation of calcite (ΩCa) and aragonite (ΩAr) values ranged from 3.86 to 4.84 and from 2.53 to 3.18, respectively, and increased from north to south (Figure 5F,G). The lowest ΩCa and ΩAr values were observed at Josephine Seamount in the mid-slope west (station 1: 3.86 and 2.53), while the highest values were observed at Lion summit 1 (station 6: 4.84 and 3.18). ΩCa and ΩAr values were relatively constant for the other seamounts and at the Madeira–Ampère open-ocean station, at 4.63 ± 0.02 and 3.04 ± 0.02, respectively.

3.4. Principal Components Analysis

Principal component analysis (PCA) (Past 5.1 software [44]) summarized the complex relationships between physical–biogeochemical variables and total phytoplankton and explained 87% of the total variability. PC1 accounted for 54%, PC2 for 21%, and PC3 for 13% (Figure 6).
PC1 explained the majority of the variance and reflects biological productivity and physical forcing, separating productive southern stations (Lion, Ampère, and open-ocean) from less productive northern stations (Josephine). PC2 and PC3 further emphasized the role of carbonate chemistry and salinity in shaping the observed patterns, revealing distinct groups of samples based on these parameters. PC2 highlighted carbonate chemistry variability, primarily driven by total alkalinity (TA) and DIC, and linked to water mass origin or nutrient influence. PC3 separated samples based on differences in salinity (S) and ΔpCO2, suggesting a conservative behavior between these two variables.
The PCA confirmed the distinct environmental profiles of northern vs. southern seamounts and the combined influence of conservative mixing, stratification, and biological uptake.

4. Discussion

The Madeira–Tore complex revealed a dynamic interplay between physical and biogeochemical processes, with clear latitudinal and longitudinal gradients shaping environmental conditions and influencing phytoplankton community structure. Spatial variability in SST and SSS (Figure 2A,B and Figure 3A,B) reflected distinct water masses characteristics, modulated by regional circulation patterns, variable upwelling intensity, and the influence of seamount topography on mesoscale dynamics. Cooler and less saline waters were observed over northern seamounts (Josephine, Gettysburg), while southern (<35° N) sites (Lion, Ampère) exhibited warmer and more saline conditions. These gradients are consistent with previous reports of weaker summer upwelling between 33° N and 37° N, leading to greater SST seasonal amplitudes in central latitudes [24]. The southern stations, being closer to the influence of the Canary Current, Azores Current, and subtropical gyre waters, reflect these warm, saline intrusions [18].
The sea surface velocity field (Figure 4A) revealed considerable horizontal variability in both flow intensity and direction, suggesting intensified mesoscale activity and mixing throughout the region. Peak velocities reached 0.5 m s−1, which, given regional references for eddies in the Canary and Azores currents [45,46], can be classified as moderate to strong. These velocities typically correspond to the presence of mesoscale eddies, jets, or meandering currents, features known to enhance horizontal mixing. Moderate velocity cells in proximity to seamounts suggest seamount–eddy interaction, and the patchy upward motion signatures (positive wo values, Figure 4B) are indicative of topographically driven upwelling or internal waves, supporting nutrient uplift [47]. The prevailing NNE wind direction (Figure 4C) further supports wind-driven upwelling as a key driver in the region.
For instance, over Josephine Seamount, upward velocities coincided with lower SST, lower SSS, and elevated dissolved oxygen (DO), a combination suggesting active vertical exchange and surface water renewal. The lowest TA, DIC, and phytoplankton abundances are potentially explained by a weaker bottom-up gradient associated with a thinner mixed-layer (averaged 8 m depth) [48]. The biogeochemical signal at Josephine, coupled with high DO, suggests that physical forcing rather than biological uptake largely controls surface carbonate chemistry in the region. All stations exhibited oxygen oversaturation, although DO was slightly lower at the easternmost stations, possibly due to, e.g., biological respiration.
In contrast, southern seamounts like Lion and Ampère, as well as the intervening open-ocean station, featured higher SST and SSS, deeper stratification (mixer layer averaging 20–25 m depth), and stronger horizontal flows. These conditions were supported by oversaturation with respect to calcite and aragonite (Ω > 1) and elevated carbonate saturation states (ΩCa and ΩAr). These settings favored coccolithophore dominance, particularly Emiliania huxleyi, which is consistent with their enhanced calcification under elevated CO2 and temperature [49,50]. The strong correlations between salinity and TA (r2 = 0.895, p < 0.01, n = 11), ΩCa (r2 = 0.814, p < 0.01, n = 11), and ΩAr (r2 = 0.810, p < 0.01, n = 11, p < 0.01, n = 11) reinforce the dominance of conservative mixing in shaping carbonate chemistry in stratified waters across the Atlantic subtropical gyres (30° S–30° N) [43]. Coccolithophores have a competitive advantage, as they rely on low-efficiency carbon concentrating mechanisms (CCMs), which allow them to thrive under elevated pCO2 [51]. Species composition (Table 2) provided additional insights into physical and biogeochemical conditions, with taxa such as Discosphaera tubifera, a k-strategist coccolithophore, abundant at station 7 (an open-ocean station and the warmest), suggesting stable, low-nutrient conditions, potentially influenced by the Azores Current [52]. In contrast, Coronosphaera mediterranea was only observed at station 6 (Lion), likely indicating episodic nutrient enrichment [53]. Dinoflagellates, intermediate in CCM efficiencies, were absent at Josephine but present at southern stations, pointing to the inferred increasing stratification. Bloom-forming taxa, associated with these conditions, include autotrophic and mixotrophic species from e.g., Ceratium, Scripsiella, Heterocapsa, and Gymnodinium genera. Their photosynthetic performance may be susceptible to carbon limitation, making them beneficiaries of rising atmospheric CO2 [51].
Despite overall oligotrophic conditions (as inferred from low chlorophyll-a and cell counts), localized blooms were detected at Lion and the open-ocean station, probably due to episodic nutrient enrichment. Diatoms, characterized by high-efficiency CCMs, were restricted to stations 6 and 7. The species found are linked to upwelling (e.g., Pseudonitzchia spp.), and genera (e.g., Hemiaulus and Proboscia) are often found during upwelling relaxation phases, which further suggests pulse-like nutrient delivery, potentially during upwelling–relaxation cycles [54]. Green algae, widely distributed except at some summit and west-slope stations, may reflect local variations in water velocity. Their presence, along with that of diatoms, signals episodic upwelling or nutrient enrichment. These maxima illustrate how physical forcings, such as current intensification, stratification, and vertical displacement, drive shifts in community structure, favoring different phytoplankton functional groups. Enhanced wind forcing from the NNE (Figure 4C) likely contributed to episodic nutrient pulses that modulate phytoplankton succession.
The carbonate system spatial gradients aligned with the physical patterns (Figure 5, Table 1). Despite low biological activity, surface pCO2 exceeded atmospheric levels (423 µatm at Izana station), indicating net oceanic CO2 export during the August 2024 survey. A north–south contrast in surface pCO2 was evident, with higher variability in the northern stations regulated by physical forcing (whereas biological influence intensifies toward the south). This variability likely reflects the complex interaction of physical (e.g., solar heating, mixed-layer dynamics, air–sea gas exchange) and biological (photosynthesis, respiration, calcification) processes, along with vertical inputs of CO2-rich subsurface waters [5,55].
Although pHT exhibited minor spatial variation (mean = 7.987 ± 0.015), it was slightly lower than summer values for nearby regions—such as the Strait of Gibraltar and the Canary Islands [56]—possibly reflecting increasing atmospheric CO2 (from 411 µatm in 2020 to 420 µatm in 2024, based on Izana station data, [39]). Strong positive correlations of pHT with carbonate saturation states, ΩCa (r2 = 0.829, p < 0.01, n = 11) and ΩAr (r2 = 0.821, p < 0.01, n = 11), support the role of carbonate saturation in modulating pHT variability [57]. The weak correlation with AOU suggests physical over biological control of surface pHT variability during the study period. TA values were comparable to historical baselines [6,26], and the strong correlation with salinity (r2 = 0.895, p < 0.01, n = 11) highlights the dominant influence of conservative mixing across subtropical gyres [43], with minimal contribution from biological activity [58]. DIC values were consistent with regional baselines [56]. Strong correlations with TA (r2 = 0.997, p < 0.01, n = 11), salinity (r2 = 0.906, p < 0.01, n = 11), ΩCa (r2 = 0.941, p < 0.01, n = 11), and ΩAr (r2 = 0.942, p < 0.01, n = 11), while showing no significant relationship with AOU, further confirmed that conservative mixing, not remineralization, dominates the control of surface DIC across the region.
The calculated CO2 fluxes (FCO2) averaged 9.38 ± 8.03 µmol m−2 day−1, exceeding historical regional estimates (Table 3). These elevated fluxes, positively correlated with salinity (r2 = 0.769, p < 0.01, n = 11), which may reflect a combination of reduced buffering capacity and enhanced stratification-induced warming due to a smaller mixed layer depth, typical of summer periods [48,59]. This underscored the role of conservative mixing in modulating fluxes in stratified regions with limited upwelling. However, in areas subject to localized wind-driven vertical transport (e.g., under persistent NNE winds, Figure 4C), CO2 from below may also contribute to increasing sea-air fluxes.
Overall, the variability among seamounts reflected a complex combination of physical forcing (e.g., eddy activity, upwelling, mixing) and hydrodynamics (e.g., Canary current). A positive NAO index was observed in July and August 2024 (value: 1.459 and 0.6325, respectively) [40], and as expected, positive patterns of ΔpCO2 were observed, promoting CO2 release [59].

5. Conclusions

The Madeira–Tore complex exhibited a clear latitudinal gradient in sea surface temperature, salinity, and carbonate system properties, which modulates surface biogeochemistry and phytoplankton structure. Northern seamounts such as Josephine and Gettysburg were marked by limited stratification, reduced buffering capacity, and low surface biological activity, likely driven by physical processes such as mixing. In contrast, southern seamounts—Lion and Ampère—and adjacent open-ocean areas emerged as biogeochemical hotspots, with higher carbonate saturation states (ΩAr > 3.1), elevated CO2 fluxes, and more diverse and abundant phytoplankton communities, particularly coccolithophores. Despite oligotrophic conditions, episodic enrichment events, suggested by the presence of diatoms and green algae, point to localized upwelling, eddies, or internal wave activity linked to seamount topography. These processes likely enhance vertical nutrient supply and support functional phytoplankton diversity. The consistently FCO2 positive values across all stations indicate that the Madeira–Tore region acted as a net CO2 source to the atmosphere during August 2024, with average fluxes (6.4 mmol m−2 d−1) surpassing historical estimates for the North Atlantic subtropical gyre. This highlights seasonal and latitudinal variability in air–sea carbon exchange in transitional oceanic systems. The Madeira–Tore Seamount Complex is a topographically influenced system that lies between the Canary current upwelling system and the North Atlantic subtropical gyre, with additional influences from the Azores current and mesoscale eddies.
The principal component analysis confirmed that surface variability is structured by both conservative mixing (e.g., SSS and TA) and biological uptake, with stratified southern waters favoring elevated saturation states and coccolithophore dominance. This dataset, detailing spatial variability in carbonate concentrations, community structure, and associated oceanographic conditions, provides a valuable baseline for monitoring regional acidification patterns and assessing ecosystem vulnerability under climate change scenarios. It also offers actionable knowledge to support the designation and adaptive management of Marine Protected Areas (MPAs) and aligns with conservation targets under the EU Marine Strategy Framework Directive (MSFD). Future studies should extend this approach temporally and vertically to capture seasonal dynamics and subsurface processes that may further influence carbon cycling and biological connectivity in seamount ecosystems.

Author Contributions

Conceptualization, M.N. and A.D.S.; methodology, M.N. and A.D.S.; formal analysis, M.N. and A.D.S.; investigation, M.N. and A.D.S.; writing—original draft preparation, M.N. and A.D.S.; writing—review and editing, M.N. and A.D.S.; project administration, M.N.; funding acquisition, M.N. and A.D.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research is a part of the project “Management and monitoring plans for Oceanic Marine Protected Areas: Ecological characterization of seamounts in the Madeira–Tore Geological Complex and adjacent areas (Oceanic MPAs)” and was funded by Fundo Azul, Aviso Convite n. 1/2024.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to thank all the colleagues involved in the CMT-AMP cruise for their support in sample collection, and Ângelo Monteiro for laboratory work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Oceans 06 00046 g001
Figure 1. Map of the study area showing the location of the seamounts surveyed. The inset map (top left) displays the location of the study area in the North Atlantic Ocean, highlighted by the red rectangle. The main map illustrates the bathymetry of the region, with depth indicated by the color gradient (see vertical color bar, right). The seamounts surveyed are labelled with red circles and numbered as follows: 1–3: Josephine Seamount; 4–6: Lion Seamount; 7: Madeira–Ampère open-ocean station; 8–9: Ampère Seamount; 10–11: Gettysburg Peak (Gorringe Seamount). The map also shows the proximity of the seamounts to Portugal and Madeira. Bathymetric data was sourced from GEBCO_2023 Grid in Ocean Data View.
Figure 1. Map of the study area showing the location of the seamounts surveyed. The inset map (top left) displays the location of the study area in the North Atlantic Ocean, highlighted by the red rectangle. The main map illustrates the bathymetry of the region, with depth indicated by the color gradient (see vertical color bar, right). The seamounts surveyed are labelled with red circles and numbered as follows: 1–3: Josephine Seamount; 4–6: Lion Seamount; 7: Madeira–Ampère open-ocean station; 8–9: Ampère Seamount; 10–11: Gettysburg Peak (Gorringe Seamount). The map also shows the proximity of the seamounts to Portugal and Madeira. Bathymetric data was sourced from GEBCO_2023 Grid in Ocean Data View.
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Figure 2. Environmental parameters in the study area. The figure illustrates the spatial distribution of key environmental parameters measured during the survey. (A) Sea Surface Temperature (SST) in degrees Celsius (°C). (B) Sea Surface Salinity (SSS), (C) Dissolved Oxygen (DO) in µmol Kg−1. (D) Chlorophyll a concentration (Chla) in milligrams per cubic meter (mg.m−3). The color scales indicate the respective values for each parameter. Data were processed and visualized using Ocean Data View. A minimal weighted average gridding (quality limit: 0.8) was applied to balance the preservation of the original data structure with improved visual clarity.
Figure 2. Environmental parameters in the study area. The figure illustrates the spatial distribution of key environmental parameters measured during the survey. (A) Sea Surface Temperature (SST) in degrees Celsius (°C). (B) Sea Surface Salinity (SSS), (C) Dissolved Oxygen (DO) in µmol Kg−1. (D) Chlorophyll a concentration (Chla) in milligrams per cubic meter (mg.m−3). The color scales indicate the respective values for each parameter. Data were processed and visualized using Ocean Data View. A minimal weighted average gridding (quality limit: 0.8) was applied to balance the preservation of the original data structure with improved visual clarity.
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Figure 3. Satellite imagery of Sea Surface Temperature (SST) and Sea Surface Salinity (SSS) in the study area. (A) Sea Surface Temperature (SST) in degrees Celsius (°C). (B) Sea Surface Salinity (SSS) from IBI-MFC (average for 2–10 August 2024). Insets show localized comparisons between satellite and in situ observations to validate spatial gradients. Color scales indicating the respective values. The white dots indicate the locations of the seamounts surveyed during the expedition. Data were processed and visualized using Ocean Data View.
Figure 3. Satellite imagery of Sea Surface Temperature (SST) and Sea Surface Salinity (SSS) in the study area. (A) Sea Surface Temperature (SST) in degrees Celsius (°C). (B) Sea Surface Salinity (SSS) from IBI-MFC (average for 2–10 August 2024). Insets show localized comparisons between satellite and in situ observations to validate spatial gradients. Color scales indicating the respective values. The white dots indicate the locations of the seamounts surveyed during the expedition. Data were processed and visualized using Ocean Data View.
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Figure 4. Horizontal water velocity (5 August 2024), vertical water velocity, and wind vectors (average 2–10 August 2024) in the study area. (A) Sea surface water velocity field (m s−1); velocity intensity is indicated using a color gradient ranging from blue (0 m s−1) to red (0.5 m s−1); direction, intensity, and flow patterns are represented by black velocity vectors. (B) Vertical water velocity (wo—upward sea water velocity) (m s−1); positive values indicate upward water movement, and negative values indicate downward movement. The red dots in both panels indicate the locations of the surveyed seamounts. Data were processed and visualized using QGis v3.42 and Ocean Data View. (C) Wind rose diagram showing the frequency distribution of wind directions and wind speed ranges.
Figure 4. Horizontal water velocity (5 August 2024), vertical water velocity, and wind vectors (average 2–10 August 2024) in the study area. (A) Sea surface water velocity field (m s−1); velocity intensity is indicated using a color gradient ranging from blue (0 m s−1) to red (0.5 m s−1); direction, intensity, and flow patterns are represented by black velocity vectors. (B) Vertical water velocity (wo—upward sea water velocity) (m s−1); positive values indicate upward water movement, and negative values indicate downward movement. The red dots in both panels indicate the locations of the surveyed seamounts. Data were processed and visualized using QGis v3.42 and Ocean Data View. (C) Wind rose diagram showing the frequency distribution of wind directions and wind speed ranges.
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Figure 5. Carbonate system parameters in the study area. The figure presents the spatial distribution of key carbonate system parameters measured during the survey. (A) Total alkalinity (TA) in µmol Kg−1. (B) pHT. (C) Dissolved inorganic carbon (DIC) in µmol Kg−1. (D) Partial pressure of CO2 (pCO2) in µatm. (E) CO2 fluxes (FCO2). (F) Saturation state of calcite (ΩCa). (G) Saturation state of aragonite (ΩAr). The color scales indicate the respective values for each parameter. Data were processed and visualized using Ocean Data View. A minimal weighted average gridding (quality limit: 0.8) was applied to balance the preservation of the original data structure with improved visual clarity.
Figure 5. Carbonate system parameters in the study area. The figure presents the spatial distribution of key carbonate system parameters measured during the survey. (A) Total alkalinity (TA) in µmol Kg−1. (B) pHT. (C) Dissolved inorganic carbon (DIC) in µmol Kg−1. (D) Partial pressure of CO2 (pCO2) in µatm. (E) CO2 fluxes (FCO2). (F) Saturation state of calcite (ΩCa). (G) Saturation state of aragonite (ΩAr). The color scales indicate the respective values for each parameter. Data were processed and visualized using Ocean Data View. A minimal weighted average gridding (quality limit: 0.8) was applied to balance the preservation of the original data structure with improved visual clarity.
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Figure 6. Principal component analysis (PCA) of environmental and carbonate system parameters at seamount locations. Left panel—biplot showing the first two principal components (PC1 vs. PC2 [explained variability 77%]) and right panel—biplot showing the first and third principal components (PC1 vs. PC3 [explained variability 67%]). In both biplots, vectors represent the environmental and carbonate system parameters, including Sea Surface Temperature (T), Sea Surface Salinity (S), Dissolved Oxygen (DO), Chlorophyll a (Chla), Total phytoplankton (T Phyto), Total Alkalinity (TA), Dissolved Inorganic Carbon (DIC), partial pressure of CO2 (pCO2), pHT, and saturation states of calcite (ΩCa) and aragonite (ΩAr). PC1 (54%) reflects biological productivity and physical forcing, separating productive southern stations (Lion, Ampère, and open-ocean) from less productive northern stations (Josephine). PC2 (21%) highlights carbonate chemistry variability, primarily driven by alkalinity and DIC. PC3 (13%) captures differences in salinity and related water mass mixing. The length and direction of the vectors indicate the strength and direction of the correlation between each parameter and the principal components. Points represent the seamount locations, labeled as follows: J1–J3: Josephine Seamount; L4–L6: Lion Seamount; A8–A9: Ampère Seamount; G10–G11: Gettysburg Peak (Gorringe Seamount); MA: Madeira–Ampère open-ocean station. The analysis was performed using Plot5.1.
Figure 6. Principal component analysis (PCA) of environmental and carbonate system parameters at seamount locations. Left panel—biplot showing the first two principal components (PC1 vs. PC2 [explained variability 77%]) and right panel—biplot showing the first and third principal components (PC1 vs. PC3 [explained variability 67%]). In both biplots, vectors represent the environmental and carbonate system parameters, including Sea Surface Temperature (T), Sea Surface Salinity (S), Dissolved Oxygen (DO), Chlorophyll a (Chla), Total phytoplankton (T Phyto), Total Alkalinity (TA), Dissolved Inorganic Carbon (DIC), partial pressure of CO2 (pCO2), pHT, and saturation states of calcite (ΩCa) and aragonite (ΩAr). PC1 (54%) reflects biological productivity and physical forcing, separating productive southern stations (Lion, Ampère, and open-ocean) from less productive northern stations (Josephine). PC2 (21%) highlights carbonate chemistry variability, primarily driven by alkalinity and DIC. PC3 (13%) captures differences in salinity and related water mass mixing. The length and direction of the vectors indicate the strength and direction of the correlation between each parameter and the principal components. Points represent the seamount locations, labeled as follows: J1–J3: Josephine Seamount; L4–L6: Lion Seamount; A8–A9: Ampère Seamount; G10–G11: Gettysburg Peak (Gorringe Seamount); MA: Madeira–Ampère open-ocean station. The analysis was performed using Plot5.1.
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Table 1. Environmental and carbonate system parameters at sampling stations. The table presents surface values, and their respective units, for temperature (T), salinity (S), dissolved oxygen (DO), Apparent Oxygen Utilization (AOU), chlorophyll a (Chla), total phytoplankton and per main functional groups (cells L−1), total alkalinity (TA), pHT, dissolved inorganic carbon (DIC), partial pressure of CO2 (pCO2), saturation state of calcite (ΩCa), saturation state of aragonite (ΩAr), and sea-air CO2 flux (CO2 flux) at the sampling stations.
Table 1. Environmental and carbonate system parameters at sampling stations. The table presents surface values, and their respective units, for temperature (T), salinity (S), dissolved oxygen (DO), Apparent Oxygen Utilization (AOU), chlorophyll a (Chla), total phytoplankton and per main functional groups (cells L−1), total alkalinity (TA), pHT, dissolved inorganic carbon (DIC), partial pressure of CO2 (pCO2), saturation state of calcite (ΩCa), saturation state of aragonite (ΩAr), and sea-air CO2 flux (CO2 flux) at the sampling stations.
Stations 1234567891011
SeamountJosephineJosephineJosephine LionLioLionMadeira–AmpèreAmpèreAmpèreGettysburg PeakGettysburg Peak
LocationMid-slope WSummitMid-slope ESummit 2 Mid-slope W Summit 1OpenOceanMid-slope WSummitMid-slope WSummit
Latitude36.60136.63036.65935.2735.3135.2234.41735.04035.05436.56036.528
Longitude−14.28−14.25−14.15−15.65−15.69−15.49−14.10−13.04−12.87−11.66−11.58
Bottom depth [m]1410216140469914577264180149666138664
T [°C]22.0422.1422.4722.6323.0022.9423.1422.6822.7421.8621.98
S36.2836.2136.2236.5436.5636.5736.6736.7436.6936.5736.61
DO [µmol Kg−1] 238247242251237239228228228233231
AOU [µmol Kg−1]−22.69−31.54−27.92−38.03−25.42−27.02−16.61−15.26−15.51−16.58−15.51
Chla [mg m−3]0.0380.0080.0250.0410.0280.0360.0350.0220.0250.0080.012
Total phytoplankton (cells L−1)864018131067920357347,38722,2937467858732534800
Coccolithophores73601493747600277332,90715,0932667202729333520
Dinoflagellates800032008004320272038406080320800
Diatoms0000030409600000
Chlorophytes48032003200424080004800480
Criptophytes0000001440480000
Ciliates0000028801280480000
TA [µmol Kg−1]22362310226824072439246724312452242624372435
pHT 7.9517.9877.9707.9887.9718.0017.9847.9797.9937.9997.995
DIC [µmol Kg−1]20062055202221382174218321582183215121652164
pCO2 [µatm]498467479485515479494506482477481
ΩCa3.864.284.134.564.544.844.664.594.664.614.60
ΩAr2.532.802.702.992.983.183.063.013.063.023.01
CO2 flux [mmol m−2 d−1]4.702.763.516.7510.014.966.2011.478.165.806.23
Table 2. Taxonomic list of genera and species of various phytoplankton and ciliate groups, including coccolithophores (Prymnesiophyceae), dinoflagellates (Dinophyceae), diatoms (Bacillariophyceae), green algae (Chlorophyceae), Cryptophyceae, and Ciliates, at the surface, August 2024.
Table 2. Taxonomic list of genera and species of various phytoplankton and ciliate groups, including coccolithophores (Prymnesiophyceae), dinoflagellates (Dinophyceae), diatoms (Bacillariophyceae), green algae (Chlorophyceae), Cryptophyceae, and Ciliates, at the surface, August 2024.
Coccolithophores Dinoflagellates Diatoms Green Algae
(Prymnesiophyceae)(Dinophyceae)(Bacillariophyceae)(Chlorophyceae)
Calyptrosphaera spp.Azadinium spp.CentralesPyramimonas spp.
Coronosphaera mediterraneaCeratium furcaCocconeis spp.
Discosphaera tubiferCeratium fususHemiaulus spp.Cryptophyceae
Emiliania huxleyiFragilidium spp.Odontella spp.
Gephyrocapsa ericsoniiGymnodinium spp.Proboscia alataCiliates
Gephyrocapsa muelleraeGyrodinium spp.Pennales
Gephyrocapsa oceanicaHeterocapsa spp.Pseudonitzschia spp. seriata group
HolocococcolithsKarlodinium spp.
Ophyaster spp.Oxytoxum spp.
Rhabdosphaera clavigeraScripsiella spp.
Syracosphaera pulchra
Syracosphaera spp.
Table 3. Comparison of pCO2, sea surface temperature (SST), and CO2 fluxes (FCO2) between this work and other studies located in the North Atlantic subtropical gyre.
Table 3. Comparison of pCO2, sea surface temperature (SST), and CO2 fluxes (FCO2) between this work and other studies located in the North Atlantic subtropical gyre.
PeriodLocationpCO2 (μatm) SST (°C)FCO2
(mmol m−2 d−1) 		Reference
2002–2005Atl 20–50° N 370 ± 10 -−1.2 ± 1.5 Schuster & Watson (2007) [60]
Spring 1970–2006 Atl 14–50° N 310 ± 30 -−2.7 ± 2.7 Takahashi et al. (2009) [61]
Autumn 2000–2008 Atl 27–39° N -18.6 ± 0.9 0.2 ± 0.4 Padín et al. (2010) [62]
Spring 2011 SGTW
(19.3–39.1° N, 66.9–30.7° W) 		343 ± 8 19.8 ± 4.1 −5.5 ± 2.2 Burgos et al. (2015) [63]
Summer 2019–2020 28° N–36° N -22.6 ± 1.1 2.15 ± 0.08 Curbelo-Hernández et al. (2021) [64]
Summer 2024 33.5° N–37.5°N 487 ± 14 22.6 ± 0.4 6.4 ± 2.6 This study
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Nogueira, M.; Silva, A.D. Open-Ocean Carbonate System and Air–Sea CO2 Fluxes Across a NE Atlantic Seamount Complex (Madeira–Tore, August 2024). Oceans 2025, 6, 46. https://doi.org/10.3390/oceans6030046

AMA Style

Nogueira M, Silva AD. Open-Ocean Carbonate System and Air–Sea CO2 Fluxes Across a NE Atlantic Seamount Complex (Madeira–Tore, August 2024). Oceans. 2025; 6(3):46. https://doi.org/10.3390/oceans6030046

Chicago/Turabian Style

Nogueira, Marta, and Alexandra D. Silva. 2025. "Open-Ocean Carbonate System and Air–Sea CO2 Fluxes Across a NE Atlantic Seamount Complex (Madeira–Tore, August 2024)" Oceans 6, no. 3: 46. https://doi.org/10.3390/oceans6030046

APA Style

Nogueira, M., & Silva, A. D. (2025). Open-Ocean Carbonate System and Air–Sea CO2 Fluxes Across a NE Atlantic Seamount Complex (Madeira–Tore, August 2024). Oceans, 6(3), 46. https://doi.org/10.3390/oceans6030046

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