Oceans, Volume 5, Issue 4 (December 2024) – 15 articles

The ocean is increasingly affected by the rise in maritime activities. Increased anthropogenic pressures have led to environmental impacts and also intensified competition for space and resources among various socioeconomic sectors. To mitigate these impacts on marine ecosystems and reduce conflicts, management tools and processes such as marine protected areas (MPAs) and maritime spatial planning (MSP) have become more prevalent. Trade-offs are inherent to these, and necessary to meet specific conservation and socioeconomic goals. In response, understanding and managing these trade-offs has become crucial to achieving ocean sustainability. This study performs a bibliometric review to identify the types of trade-offs discussed in the marine literature and examines their operationalization for the conservation and sustainable use of marine resources. The analysis reveals that trade-offs, particularly those between conservation and development, and the interests of specific stakeholders, are most frequently addressed in the trade-off literature and are typically approached through integrative methods. This comprehensive examination highlights the significance of recognizing and addressing trade-offs to achieve effective marine management and conservation, aligning ecological integrity with socioeconomic interests. Full article

Filter-feeding sponges capture suspended food particles from ambient water, but little is known about the response of sponges to high food concentrations causing superfluous feeding. Here, through several experimental assays, we studied the relationship between algal concentration in the water column, filtration rate, respiration rate, and specific growth rate in single-osculum Halichondria panicea demosponge explants. Laboratory experiments showed that sponge explants filter the ambient seawater at a maximum rate when exposed to naturally (low) algal concentrations, whereas high algal concentrations resulted in superfluous feeding and reduced filtration rates. Explants maintained at algal concentrations above the incipient overloading concentration level showed that growth rates were in fair agreement with the maximum possible weight-specific growth rate of about 4% d⁻¹. Although the filtration rate became reduced due to overloading, the oxygen extraction efficiency increased, and therefore superfluous feeding did not cause reduced growth. This suggests that H. panicea and probably other sponges have adapted to low algal concentrations, displaying continuous maximum filtration rates. Osculum closure reflects a protection mechanism rather than a physiological regulatory response to high algal concentrations. Full article

Gaining robust in situ estimates of the growth rate of marine fish larvae is essential for understanding processes controlling year-class success and developing sustainable management strategies to maintain good environmental status. We measured the growth rate of Atlantic herring (Clupea harengus) larvae in the laboratory and compared it to the activity of aminoacyl-tRNA synthetases (AARS). Larvae were reared under controlled conditions for 20 days at three temperatures (7, 12, and 17 °C) using different prey concentrations (0.1, 0.3, and 2 prey·mL⁻¹) of the copepod Acartia tonsa. The relationship between specific growth rates (SGR) and specific AARS activities was best described by a linear function—SGR = −0.1031 + 0.0017 · spAARS, r² = 0.71, p < 0.05—when only larvae fed ad libitum were considered regardless of the temperature. When larvae fed with low concentrations of food were included in the analysis, the relationship was SGR = −0.0332 + 0.0010 · spAARS, r² = 0.42, p < 0.05. This latter slope was rather low compared to other studies performed in zooplankton. We suggest protein degradation during the early life stages of fish as the cause of this low slope. We also studied SGR under food deprivation and the effect on specific AARS activities. We found rather high specific AARS activities in small individuals of early stages of fish, also suggesting protein degradation. Further research about protein degradation and turnover rates is needed in order to use AARS activity as a proxy for growth rates in field-caught larvae. Full article

Mesozooplankton are highly responsive to environmental changes, making their population dynamics and species composition important indicators of large-scale oceanic conditions. This study investigates the mesozooplankton composition, emphasizing biomass values across all three marine reporting units in the Romanian Black Sea from 2013 to 2020, covering 45 stations that represent transitional, coastal, and marine waters during the warm season. It also examines environmental parameters, such as temperature, salinity, and nutrient levels associated with eutrophication, that impact the mesozooplankton, as well as the environmental status of the pelagic habitat in this region, influenced by various hydrological and anthropogenic factors. Statistical analyses, including multivariate methods, were employed to investigate correlations between mesozooplankton biomass and environmental parameters, facilitating ecological assessments in accordance with the Marine Strategy Framework Directive (MSFD) Descriptors 1 (Biodiversity) and 5 (Eutrophication) using indicators such as Mesozooplankton bBiomass, Copepoda Biomass, and Noctiluca scintillans iomass. The results indicated that Mesozooplankton and Copepoda Biomass did not reach good ecological status (GES). However, the Noctiluca scintillans Biomass indicator demonstrated that all marine reporting units achieved GES during the warm seasons. These findings underscore the dynamic nature of pelagic habitats and highlight the importance of ongoing monitoring to inform policy and conservation efforts. Full article

The shipping industry faces the challenge of expanding its capabilities and mitigating its environmental impact. The utilization of adhesive bonding techniques has the potential to facilitate the construction of lighter ships with a reduced carbon footprint. In this work, a complete methodology has been established to study the properties of an adhesive bond between naval steel and the behavior of the bulk adhesive when introduced into seawater. Thermogravimetric analysis (TGA) has been used to determine the thermal properties. Single-lap-joint and tensile tests have been performed showing the negative influence of temperature on the mechanical properties. Seawater absorption has been studied by gravimetric tests. Differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and mechanical bulk tests have been carried out before and after aging. The water absorption is Fickian, and when a constant load is applied, the absorption occurs faster and in greater quantity, decreasing the stiffness and strength. An increase in the glass transition temperature was observed in the absence of a load, with no discernible change upon the application of load condition. Also, aged specimens with constant load exhibited a reduction in water content. These phenomena may be attributed to several diffusion processes. Full article

The decline of natural coral reefs has spurred interest in artificial reef construction and reef rehabilitation efforts. However, many restoration programs are failing because of poor water quality and localized pollution factors. This paper examines one etiology of coral restoration failure, highlighting the need for careful site selection based on hydrodynamic, biological, and pollution criteria. Using Kish Island as a case study, we outline a procedure for determining placement of artificial reefs to minimize external pollution impacts, which could be applied to any reef site worldwide. Our integration of computational modeling and geographic information systems enables the tracking of pollution dispersion and increases the power of restoration efforts. The RQ Plume modeling, used alongside compound pollutant modeling, can identify hotspots with the highest potential to degrade marine environments, particularly reefs. Conservation efforts should prioritize pollution mitigation, as seen in the case of sunscreen pollutants at Kish Island, where regulatory action and public education can contribute to reef protection. This comprehensive approach underscores the critical role of water quality in successful reef restoration and conservation strategies. Full article

There is increasing interest in the role that seagrasses play in storing carbon in the context of climate mitigation, but many knowledge gaps in the factors controlling this storage exist. Here, we provide a small case study that examines the role of infaunal biodiversity in influencing seagrass and the carbon stored in its sediments. A total of 25 species of invertebrate were recorded in an intertidal Zostera marina meadow, where these species were dominated by polychaete worms with no bivalves present. We find organic carbon storage (within the top 20 cm) measured by AFDW to be highly variable within a small area of seagrass meadow ranging from 2961 gC.m⁻² to 11,620 gC.m⁻² with an average (±sd) of 6460² ± 3274 gC.m⁻². Our analysis indicates that infaunal communities are significantly and negatively correlated with this sediment organic carbon. However, this effect is not as influential as hypothesised, and the relatively small sample size of the present study limits its ability to provide strong causality. Other factors, such as algal abundance, curiously had a potentially stronger influence on the carbon in the upper sediments. The increasing richness of infauna is likely reducing the build-up of organic carbon, reducing its ecosystem service role. We believe this to likely be the result of bioturbation by specific species such as Arenicola marina and Ampharete acutifrons. A change in sediment organic carbon suggests that these species could be key drivers of bioturbator-initiated redox-driven organic matter turnovers, influencing the microbial processes and remobilizing sediment compounds. Bioturbators should be considered as a limitation to C_org storage when managing seagrass C_org stocks; however, bioturbation is a natural process that can be moderated when an ecosystem is less influenced by anthropogenic change. The present study only provides small-scale correlative evidence with a range of surprising results; confirming these results within temperate seagrasses requires examining this process at large spatial scales or with targeted experiments. Full article

Perfluorooctanoic acid (PFOA) has been widely studied due to its environmental persistence and bioaccumulation potential, raising concerns about its effects on aquatic life. This research evaluates the impact of PFOA on the antioxidant defenses and stress response systems of the mussel Mytilus galloprovincialis. Mussels were exposed to three concentrations of PFOA (1, 10, and 100 µg·L⁻¹) over 28 days. Several biomarkers, including glutathione S-transferase (GST), superoxide dismutase (SOD), catalase (CAT), lipid peroxidation (LPO), total antioxidant capacity (TAC), vitellogenin (VTG), ubiquitin (UBI), and caspase-3 (CASP) were analyzed. The results suggest stress responses, particularly in animals exposed to higher concentrations, as shown by GST and SOD activities which increased according to PFOA concentrations. Additionally, oxidative stress markers such as MDA and CAT showed variable responses depending on the exposure concentration tested. This study underscores the need for further investigation into the effects of PFOA on mollusks but also the need to unveil gender-specific responses in aquatic organisms exposed to this contaminant. The concentrations of PFOA used in our research are lower than those examined in previous studies, providing crucial insights into the impacts of even minimal exposure levels. It highlights the potential of M. galloprovincialis as a bioindicator in environmental monitoring programs, providing crucial insights for environmental management and policymaking regarding regulating and monitoring PFOA in marine settings. Consequently, in a country where seafood consumption is the second largest in Europe, implementing environmental policies and regulatory measures to manage and monitor PFOA levels in marine environments is crucial. Full article

The current study looks at how the characteristics of Arabian Sea tropical cyclones (TCs) change over time. The results show that in the pre-monsoon (April–June) and the post-monsoon (October–December), the activity of TCs > 34 knots, including cyclonic storm (CS), severe cyclonic storm (SCS), very severe cyclonic storm (VSCS), extreme severe cyclonic storm (ESCS), and super cyclonic storm (Sup. CS), has significantly increased, while the tendency of TCs < 34 knots, depressions and deep depressions (Ds) over the Arabian Sea has only slightly increased. Most of the TC activity in the first two decades (1982–2001) over the Arabian Sea activated on the eastern side, while in the last two decades (2002–2021), there was an expansion toward the southwest region of the Arabian Sea, especially in the post-monsoon season. The composite analysis of environmental parameters over the Arabian Sea reveals that the negative anomalies of outgoing longwave radiation (OLR) and the positive anomalies of relative humidity at 500 hPa (RH–500 hPa) in the first decade (1982–1991) and the second decade (1992–2001) are more concentrated on the eastern side of the Arabian Sea, leading to increased activity for TCs. Decades three (2002–2011) and four (2012–2021) demonstrated a wide distribution of weak vertical wind shear (VWS) and strong convection (OLR and RH–500 hPa) over the Arabian Sea basin. This led to TCs occurring more frequently and stronger, especially in the post-monsoon season. SST over the Arabian Sea was sufficient for tropical storm activity (≥26.5 °C) for both typical seasons. Full article

Invasive marine invertebrates are increasingly recognized as a potential disturbance to coastal ecosystems. We sought to better document the taxonomic composition of subtidal communities around Long Island to obtain a baseline that can be used to monitor current and future invasions of non-indigenous species. We placed settlement blocks at 18 sites along the coast of Long Island, New York, for three months. After recovering blocks at 12 sites, we analyzed the taxonomic composition of fouling communities on the blocks. We observed 64 invertebrate and 3 algal taxa, with large variation in taxon richness among sites. Multivariate analyses revealed that although taxon composition was significantly dissimilar between north and south shores, variation in dissimilarity did not differ significantly between shores. The high variability in taxon composition observed among sites indicates that additional research is needed to expand our knowledge of invertebrate diversity in the waters surrounding Long Island. Adding more sites and replicate blocks within sites could improve future sampling designs. This research will benefit continuing efforts to monitor, manage, and prevent the establishment of marine invasive species. Full article

Understanding the ecological dynamics of zooplankton communities is crucial to precisely assessing the health of marine ecosystems and their management. Metabarcoding has contributed to a better understanding of biodiversity in marine environments. However, this methodology still requires protocol optimisation. Here, we used a complementary approach combining molecular and morphological identification methods to identify the zooplankton community inhabiting the Berlengas Archipelago, Portugal. The presence of non-indigenous species was also assessed. The results showed that the metabarcoding approach outperformed the classical morphological identification method, detecting more species with higher resolution. Nevertheless, the classical method was able to identify species that were not detected by the molecular approaches, probably due to a lack of reference data in the databases. The comparison between different molecular approaches showed that COI and bulk DNA gave better results than 18S rRNA and eDNA by detecting higher species diversity. However, complementarities were observed between them. Molecular tools also proved effective in identifying several potential non-indigenous species, identifying, for the first time, several potentially unreported NIS inhabiting the Portuguese marine ecosystems. Overall, our results confirmed the importance of combining both classical and molecular methods to obtain a more refined assessment of the zooplanktonic communities in marine environments. Full article

In Cartesian coordinates $\left(x,y,z\right),$ the gradient Richardson number $Ri$ is the ratio between the square of the buoyancy frequency N and the square of the vertical shear S, $Ri=N^2/S^2$, where $N^2=-\left(g/\rho \right)\partial \rho /\partial z$ and $S^2={\left(\partial u/\partial z\right)}^2+{\left(\partial v/\partial z\right)}^2$, with ρ potential density, $\left(u,v\right)$ the horizontal velocity components and g gravity acceleration. In isopycnic coordinates $\left(x,y,\rho \right)$, $Ri$ is expressed as the ratio between ${M^2\equiv N}^{-2}$ and the squared diapycnal shear ${S_{\rho }}^2={\left(\rho /g\right)}^2\left[{\left(\partial u/\partial \rho \right)}^2+{\left(\partial v/\partial \rho \right)}^2\right]$, $Ri=M^2/{S_{\rho }}^2$. This could suggest that a decrease (increase) in stratification brings a decrease (increase) in dynamic stability in Cartesian coordinates, but a stability increase (decrease) in isopycnic coordinates. The apparently different role of stratification arises because S and $S_{\rho }$ are related through the stratification itself, $S_{\rho }=S/N^2$. In terms of characteristic times, this is equivalent to $\tau \equiv S_{\rho }={t_o}^2/t_d$, which is interpreted as a critical dynamic time $\tau$ that equals the buoyancy period $t_o\equiv N^{-1}$ normalized by the ratio $t_d/t_o$, where $t_d=S^{-1}$ is the deformation time. Here we follow simple arguments and use field data from three different regions (island shelf break, Gulf Stream and Mediterranean outflow) to endorse the usefulness of the isopycnal approach. In particular, we define the reduced squared diapycnal shear ${{\sigma }_{\rho }}^2={S_{\rho }}^2-M^2$ and compare it with the reduced squared vertical ${\sigma }^2=S^2-N^2$, both being positive (negative) for unstable (stable) conditions. While both Ri and ${\sigma }^2$ remain highly variable for all stratification conditions, the mean ${{\sigma }_{\rho }}^2$ values approach ${S_{\rho }}^2$ with increasing stratification. Further, the field data follow the relation ${{\sigma }_{\rho }}^2=\left(1-Ri\right)/\left(N^{2}Ri\right)$, with a subcritical $Ri=0.22$ for both the island shelf break and the Mediterranean outflow. We propose ${{\sigma }_{\rho }}^2$ and ${S_{\rho }}^2$ to be good indexes for the occurrence of effective mixing under highly stratified conditions. Full article

Recent years have seen a notable rise in dolphin-watching boat activities along the Algarve coast in Portugal, potentially affecting the common dolphin (Delphinus delphis) and bottlenose dolphin (Tursiops truncatus) local populations. This study examines the impact of increasing underwater noise levels from these boats on dolphin vocalizations. Field recordings were conducted from June to September 2022, analyzing dolphin whistles in various boat presence scenarios. The results indicate significant changes in whistle-frequency characteristics with boat presence, including increased start, low, and high frequencies, alongside a decrease in the number of inflection points in modulated whistles. The changes might negatively impact dolphin populations viability, underscoring the need for further research. Additionally, improved mitigation strategies may be necessary to reduce the potential negative effects of dolphin watching on cetacean communication and behavior in the Algarve region. Full article

Global climate change is profoundly impacting coral ecosystems. Rising sea surface temperatures, in particular, disrupt coral reproductive synchrony, cause bleaching, and mortality. Oculina patagonica, a temperate scleractinian coral abundant across the Mediterranean Sea, can grow at a temperature range of 10–31 °C. Studies conducted three decades ago documented this species bleaching during the summer months, the same time as its gonads mature. However, the Eastern Mediterranean Sea is experiencing some of the fastest-warming sea surface temperatures worldwide. This study repeated the year-round in situ assessment of the reproductive cycle and gonad development and correlation to summer bleaching. In addition, thermal performance of the holobiont was assessed in an ex situ thermal stress experiment. In situ monitoring revealed no temporal changes in gonad development compared to previous studies, despite sea surface warming and concurrent bleaching. Experimental thermal performance curves indicated that photosynthetic rate peaked at 23 °C, bleached coral area was significant at 29 °C, and peaked at 34 °C. With local sea surface temperature reaching 31 °C, O. patagonica is exposed beyond its bleaching threshold during the summer months in situ. Despite this, O. patagonica maintains gonad development and physiologically recovers at the end of summer demonstrating resilience to current warming trends. Full article

Wind-induced waves can lead to the partial or complete wash-over of beaches, causing erosion that impacts both the landscape and tourist infrastructure. In some regions of the world, e.g., Croatia, this process, which usually occurs during a harsh winter, has a major impact on the environment and the economy, and preventing or reducing this process is highly desirable. One of the simplest methods to reduce or prevent beach erosion is the use of innovative underwater structures designed to decrease wave energy by reducing wave height. In this study, submerged breakwaters are numerically investigated using various topologies, positions, and angles relative to the free surface. Not only is the optimal topology determined, but the most efficient arrangement of multiple breakwaters is also determined. The advantage of newly developed submerged breakwaters over traditional ones (rock-fixed piers) is that they do not require complex construction, massive foundations, or high investment costs. Instead, they comprise simple floating bodies connected to the seabed by mooring lines. This design makes them not only cheap, adaptable, and easy to install but also environmentally friendly, as they have little impact on the seabed and the environment. To evaluate wave damping effectiveness, the incompressible computational fluid dynamics (ICFD) method is used, which enables the use of a turbulence model and the possibility of accurate wave modelling. Full article