Bivalve aquaculture-environment interactions in the context of climate change

Impacts of climate change on water quality, benthic mussels, and suspended mussel culture in a shallow, eutrophic estuary

Climate change is a global problem that causes severe local changes to marine biota, ecosystem functioning, and ecosystem services. The Limfjorden is a shallow, eutrophic estuary influenced by episodic summer hypoxia with an important mussel fishery and suspended mussel culture industry. Three future climate change scenarios ranging from low greenhouse gas emissions (SSP1-2.6), to intermediate (SSP2-4.5) and very high emissions (SSP5-8.5) were combined with nutrient load reductions according to the National Water Plans to investigate potential impacts on natural benthic mussel populations and suspended mussel culture for the two periods 2051-2060 and 2090-2099, relative to a reference period from 2009 to 2018. The FlexSem model combined 3D hydrodynamics with a pelagic biogeochemical model, a sediment-benthos model, and a dynamic energy budget - farm scale model for mussel culture. Model results showed that the Limfjorden was sensitive to climate change impacts with the strongest responses of physics and water quality in the worst case SSP5-8.5 scenario with no nutrient reductions. In the two low emissions scenarios, expected improvements of bottom oxygen and Chlorophyll a concentrations due to reduced nutrient loads were counteracted by climate change impacts on water physics (warming, freshening, stronger stratification). Hence, higher nutrient reductions in the Water Plans would be needed to reach a good ecological status under the influence of climate change. Suspended mussel culture was intensified in all scenarios showing a high potential harvest, whereas the benthic mussels suffered from reduced food supply and hypoxia. Provided the environmental changes and trends in social demands, in the future, it is likely that suspended mussel cultivation will become the primary source of mussels for the industry. Model scenarios can be used to inform managers, mussel farmers, fishermen, and the local population on potential future changes in bivalve harvesting and ecosystem health, and to find solutions to mitigate climate change impacts.

Histopathological changes in the greenshell mussel, Perna canaliculus, in response to chronic thermal stress

Climate change associated temperature challenges pose a serious threat to the marine environment. Elevations in average sea surface temperatures are occurring and increasing frequency of marine heatwaves resulting in mortalities of organisms are being reported. In recent years, marine farmers have reported summer mass mortality events of the New Zealand Greenshell mussel, Perna canaliculus, during the summer months; however, the etiological agents have yet to be determined. To elucidate the role of thermal stress, adult P. canaliculus were exposed to three chronic temperature treatments: a benign control of 17 °C and stressful elevations of 21 °C and 24 °C. Eight mussels per treatment were collected each month throughout a 14-month challenge period to identify and investigate histopathological differences among P. canaliculus populations exposed to the three temperatures. Histopathology revealed several significant deleterious alterations to tissues associated with temperature and exposure time. Increasing temperature and progression of time resulted in 1) an increase in the number of focal lipofuscin-ceroid aggregations, 2) an increase in focal hemocytosis, 3) an increase in the thickness of the sub-epithelial layer of the intestinal tract and 4) a decreased energy reserve cell (glycogen) coverage in the mantle. Prolonged exposure, irrespective of temperature, impacted gametogenesis, which was effectively arrested. Furthermore, increased levels of the heat shock protein 70 kDa (HSP 70) were seen in gill and gonad from thermally challenged mussels. The occurrence of the parasite Perkinsus olseni at month 5 in the 24 °C treatment, and month 7 at 21 °C was unexpected and may have exacerbated the fore-mentioned tissue conditions. Prolonged exposure to stable thermal conditions therefore appears to impact P. canaliculus, tissues with implications for broodstock captivity. Mussels experiencing elevated, temperatures of 21 and 24 °C demonstrated more rapid pathological signs. This research provides further insight into the complex host-pathogen-environment interactions for P. canaliculus in response to prolonged elevated temperature.

Assessing shellfish farming-mediated benthic impacts based on organic carbon flux simulation and composition of macrofaunal community

Marine benthic environments serve as the ultimate sink for sediment organic matter (SOM), but shellfish farming can potentially disturb the natural sink of seston, altering ecosystem functioning. Understanding the potential disturbance of a shellfish farm and its ecological effects is therefore important for a responsible management of shellfish-mediated marine ecosystem. In this study, the variations in benthic organic carbon flux of a bottom-based Manila clam (Ruditapes philippinarum) farm in Laizhou Bay, China were estimated by using a carbon flux model coupled with hydrodynamic and individual growth models. SOM and macrofaunal community were monitored for 3 years to investigate their changes to the carbon fluxes. Model simulations illustrated that the carbon flux in an area of 247 km² was altered due to seston depletion and biodeposition, which caused decrease and increase in SOM in different areas, respectively. Cluster analysis based on taxonomic composition of macrofaunal community divided the sites into four groups, which corresponded with predicted changes of carbon flux. Increased carbon flux caused higher disturbance level (indicated by AMBI) to the macrofaunal community but increased species richness, abundance, and Shannon-Wiener index, suggesting the community was both disturbed and benefited from clam farming. This study confirmed that the benthic organic carbon flux is a key factor causing differences in SOM and macrofaunal community outside the farm, and thus can be used as an efficient method for estimating the benthic impacts of shellfish farming both in and outside the farming area.

Influence of biodeposition by suspended cultured oyster on the distributions of trace elements in multiple media in a semi-enclosed bay of China

It remains unclear how the suspended non-fed bivalve mariculture will alter the coastal transfer and cleaning process of trace elements, the non-degradable contaminants, which have been reported to accumulate in sediment from bivalve mariculture areas. Herein, we set up a field in situ comparative test in the suspended oyster (Crassostrea plicatula) farming area (OF) and reference area (RA) of Xiangshan Bay to verify our hypothesis that the biodepositon of suspended oysters would consolidate trace elements from the water column and transport them to the sediment. Distribution of trace elements in multiple media of biodeposits (BDs), settling particles (SPs), sediments (SEs), and seawater demonstrate that the accelerated deposition of BDs which enriched trace elements from the water column by oysters filtering suspended particles led to trace elements accumulation in SEs from OF. Additionally, As, Cd, Co, Cr, Cu, Ni, V, and Zn were strongly regulated by this process with significant (p < 0.05) higher concentrations in SEs from OF (10.96, 0.20, 13.98, 82.40, 38.47, 38.22, 108.57, and 111.20 μg/g, repectively) than those from RA (9.43, 0.13, 11.76, 63.30, 30.34, 29.55, 86.59, and 100.24 μg/g, repectively), but the extent was different for Mn, Mo, Pb, and W with concentrations in SEs from OF (737.37, 0.81, 30.98, and 3.96 μg/g, repectively) and RA (765.25, 0.69, 31.27, and 3.34 μg/g, repectively), especially for Rb and Sr with concentrations in SEs from OF (131.13 and 96.24 μg/g, repectively) and RA (142.21 and 161.10 μg/g, repectively), due to their geochemical and geophysical properties. Moreover, the harvest of hyper-accumulated oysters as a sink for removing trace elements from water column cannot hide the impact of this process.

Increased oyster aquaculture in the Sado Estuary (Portugal): How to ensure ecosystem sustainability?

Aquaculture is one of the fastest growing sectors in the world. However, this may come with a cost, as increasing aquatic production is likely to impose changes in the environment. To ensure ecosystem sustainability, it is essential to think on this larger scale. This study aims to use the Delft3D model suite to evaluate the ecological carrying capacity for bivalve production in the Sado Estuary (Portugal), under present and future conditions (2050). Scenarios for increased oyster production resulted in reductions of chlorophyll a associated with increased nutrient concentrations. In the most extreme production scenario, which considered an increase of 100 ha in production area, a predicted decrease of 90 % in phytoplankton biomass was observed. Climate change (CC) was incorporated as an increase in sea level and water temperature, as well as a reduction in river flow. Under present oyster production conditions, CC revealed contrasting patterns, i.e. an increase in chlorophyll a concentrations and a reduction in nutrients. These results suggest that CC has a positive effect in counteracting the impacts of increased oyster production, however further research is necessary. All scenarios point to reduced dissolved oxygen concentrations, highlighting the need to monitor this parameter. Given the difficulty in defining what are unacceptable impacts to the ecosystem it would be prudent to include a socio-ecological framework in the future, in order to integrate ecosystem services and the perception of local stakeholders.

Epigenetic-associated phenotypic plasticity of the ocean acidification-acclimated edible oyster in the mariculture environment

For marine invertebrates with a pelagic-benthic life cycle, larval exposure to ocean acidification (OA) can affect adult performance in response to another environmental stressor. This carry-over effect has the potential to alter phenotypic traits. However, the molecular mechanisms that mediate "OA"-triggered carry-over effects have not been explored despite such information being key to improving species fitness and management strategies for aquafarming. This study integrated the genome-wide DNA methylome and transcriptome to examine epigenetic modification-mediated carry-over OA impacts on phenotypic traits of the ecologically and commercially important oyster species Crassostrea hongkongensis under field conditions. Larvae of C. hongkongensis were exposed to control pH 8.0 and low pH 7.4 conditions, mimicking near future OA scenario in their habitat, before being outplanted as post-metamorphic juveniles at two mariculture field sites with contrasting environmental stressors for 9 months. The larval carry-over OA effect was found to have persistent impacts on the growth and survival trade-off traits on the outplanted juveniles, although the beneficial or adverse effect depended on the environmental conditions at the outplanted sites. Site-specific plasticity was demonstrated with a diverse DNA methylation-associated gene expression profile, with signal transduction and the endocrine system being the most common and highly enriched functions. Highly methylated exons prevailed in the key genes related to general metabolic and endocytic responses and these genes are evolutionarily conserved in various marine invertebrates in response to OA. These results suggest that oysters with prior larval exposure history to OA had the ability to trigger rapid local adaptive responses via epigenetic modification to cope with multiple stressors in the field.

Research progress in relationships between freshwater bivalves and algae

Eutrophication in freshwater has become increasingly severe around the world, resulting in phytoplankton overgrowth and benthic algae reduction. Bivalves can change the density, dominant species and community structure of phytoplankton, increase available light levels, and provide physical habitats and growth conditions for benthic algae. The nutritional composition, density, community structure, and toxin of algae affect the growth, feeding, digestion, metabolism, immunity of bivalves in return. Interactions of bivalves and algae and effects of environmental factors on these interactions need a synthesis of studies, when using bivalves as a biomanipulation tool to control eutrophication. Whether bivalves can effectively suppress phytoplankton and promote benthic algae is related to the collective filtration and excretion capacity determined by size, species, population densities of bivalves, the quantity and quality of algae, and environmental factors such as temperature, dissolved oxygen, pH, and hydrodynamic. Small scale bivalve biomanipulation experiments are mostly conducted in lakes, urban ponds, and reservoirs with some success, applying in the whole ecosystem should consider more questions such as natural conditions, selection and death or reproduction of bivalves, and ecological disturbances. This review provides new considerations for technical issues such as the sustainable cultivation of bivalves and the implementation of biomanipulation in eutrophic waters.

The dynamic of the potential pathogenic bacteria, antibiotic-resistant bacteria, and antibiotic resistance genes in the water at different growth stages of grass carp pond

Pond aquaculture has become the most important and broadest breeding model in China, and an extremely important source of aquatic products, but the potential hazard factors of potential pathogenic bacteria (PPB), antibiotic resistance bacteria (ARB), and antibiotic resistance genes (ARGs) in aquaculture environment are largely invisible. In the present study, the bacterial communities in the larvae, juvenile, rearing, and harvesting culture stages of great grass carp (Ctenopharyngodon idellus) ponds were investigated and the structure of microbial flora analysis showed that the larvae culture stage has the highest abundance and the most dominant phyla were Proteobacteria (27.8%). A total of 123 significant Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway annotations and the relative abundance of nine bacterial phenotypes implied that the larvae culture stage had the most abundance of pathogenic potential and mobile elements. The correlation analyses of environmental factors showed that temperature, stocking density, pH, and transparency showed the significant impacts on both the distribution of microbiome and the PPB. More importantly, a total of 40 ARB were identified, and 16 ARGs have the detection rates of 100%, which revealed that they are widely distributed and highly enriched in the aquaculture production. Notably, this is the first robust report to analyze and understand the PPB, ARB, and ARGs characteristics and dynamic changes in the pond aquaculture.

Taxonomic and functional responses of macrofaunal assemblage provide insight into ecological impacts of bottom-based Manila clam aquaculture

Understanding ecological impacts of bottom-based clam aquaculture can improve its management. In this study, taxonomic and functional macrofaunal assemblage were evaluated for two clam farms located in Laizhou Bay, China. Beta diversity and factors potentially regulating the dissimilarity of macrofauna were estimated. Both taxonomic and functional composition of macrofauna showed large differences between the clam farm and the control area. Functional dissimilarity within the clam farms was found to be nestedness and negatively correlated to local clam abundance. Additionally, the cultured clam enhanced the functional richness but made the macrofaunal assemblage more fragile against species or function loss. This effect would increase with clam abundance, which highlights the importance of identifying optimal clam culture intensity in developing a bottom-based clam aquaculture program.

Modeling salinity drop in estuarine areas under extreme precipitation events within a context of climate change: Effect on bivalve mortality in Galician Rías Baixas

The mortality of infaunal bivalves (Venerupis corrugata, Cerastoderma edule, Ruditapes decussatus and Ruditapes philippinarum) due to a drop in salinity caused by extreme precipitation events in estuarine areas has been analyzed within a context of climate change. The Rías Baixas (NW Iberian Peninsula) were selected as a representative area of the estuarine environments where bivalve gathering is performed. Bivalve mortality under extreme precipitation events was analyzed both for historical (1990-2019) and future (2070-2099) periods. Precipitation data were retrieved from the Coordinated Regional Climate Downscaling Experiment (CORDEX) project under the Representative Concentration Pathway (RCP) 8.5 scenario and were converted into river discharges using the HEC-HMS hydrological model. The calculated river discharges were introduced into the Delft3D hydrodynamic model and simulations were performed in order to calculate transport conditions in the Rías Baixas. Salinity data were analyzed to estimate the mortality of the species due to salinity drops. In general, future conditions of moderate and severe mortality may be worse than historically observed, being more intense and covering larger areas. This is mainly observed under neap tides due to less dilution of freshwater plumes when compared with spring tides. Although all the Rías Baixas may be potentially affected, the impact will differ for each ria, being Arousa, where the highest discharges occur, the most affected. The differences among rias, especially those with a similar discharge pattern as Pontevedra and Vigo, suggest that bathymetric features also play a key role in the extent of the area affected by mortality.

Tolerance of northern Gulf of Mexico eastern oysters to chronic warming at extreme salinities

The eastern oyster, Crassostrea virginica, provides critical ecosystem services and supports valuable fishery and aquaculture industries in northern Gulf of Mexico (nGoM) subtropical estuaries where it is grown subtidally. Its upper critical thermal limit is not well defined, especially when combined with extreme salinities. The cumulative mortalities of the progenies of wild C. virginica from four nGoM estuaries differing in mean annual salinity, acclimated to low (4.0), moderate (20.0), and high (36.0) salinities at 28.9 °C (84 °F) and exposed to increasing target temperatures of 33.3 °C (92 °F), 35.6 °C (96 °F) or 37.8 °C (100 °F), were measured over a three-week period. Oysters of all stocks were the most sensitive to increasing temperatures at low salinity, dying quicker (i.e., lower median lethal time, LT₅₀) than at the moderate and high salinities and resulting in high cumulative mortalities at all target temperatures. Oysters of all stocks at moderate salinity died the slowest with high cumulative mortalities only at the two highest temperatures. The F1 oysters from the more southern and hypersaline Upper Laguna Madre estuary were generally more tolerant to prolonged higher temperatures (higher LT₅₀) than stocks originating from lower salinity estuaries, most notably at the highest salinity. Using the measured temperatures oysters were exposed to, 3-day median lethal Celsius degrees (LD₅₀) were estimated for each stock at each salinity. The lowest 3-day LD₅₀ (35.1-36.0 °C) for all stocks was calculated at a salinity of 4.0, while the highest 3-day LD₅₀ (40.1-44.0 °C) was calculated at a salinity of 20.0.

Modeling the impact of climate change on mussel aquaculture in a coastal upwelling system: A critical assessment

Forecasting of climate change impacts on marine aquaculture production has become a major research task, which requires taking into account the biases and uncertainties arising from ocean climate models in coastal areas, as well as considering culture management strategies. Focusing on the suspended mussel culture in the NW Iberian coastal upwelling system, we simulated current and future mussel growth by means of a multistructural net production Dynamic Energy Budget (DEB) model. We considered two scenarios and three ocean climate models to account for climate uncertainty, and applied a bias correction to the climate models in coastal areas. Our results show that the predicted impact of climate change on mussel growth is low compared with the role of the seeding time. However, the response of mussels varied across climate models, ranging from a minor growth decline to a moderate growth increase. Therefore, this work confirms that an accurate forecasting of climate change impacts on shellfish aquaculture should take into account the variability linked to both management strategies and climate uncertainty.

Contrasting responsiveness of four ecologically and economically important bivalves to simulated heat waves

Heat waves are expected to increase in duration and frequency, impacting coastal ecosystems, especially intertidal organisms living near their thermal tolerance limits. Sedentary infaunal species are limited to some extent in escapes from sudden temperature changes, rather modifications to their physiology and behaviour are expected. This may lead to strong ecological and economic impacts on commercial bivalve species, such as Venerupis corrugata, Ruditapes decussatus, the introduced Ruditapes philippinarum and Cerastoderma edule, the most relevant in NW Spain. We investigated lethal and sublethal effects of heat during low tide on these species in the laboratory. Summer temperatures experienced within field, shallow sediments at approximately 2 cm depth i.e. 20 °C (control), 27 °C, 32 °C, and 37 °C, were replicated during four consecutive days and the diffusion of heat at the burrowing depth of each species was estimated; temperature exposure was expressed as degree hours above 22 °C. After two days of tidal exposure, C. edule and V. corrugata suffered significant mortalities, and also the most dramatic decrease in scope for growth (SFG) as well as reduction in burrowing activity. After four days under stress, all species had negative SFG. On recovery, species showed compensation at longer exposures, particularly C. edule. These effects of temperature on mortality, growth potential and burrowing ability may increase the time to achieve commercial size and exposure to predation. Particularly, V. corrugata, with a center of distribution lower in the intertidal and subtidal, and C. edule, shallower in the sediment, may be the most affected. Clearly the intensity and frequency of heat waves will affect these key species in the intertidal sediment flats changing ecosystem functioning and fisheries management strategies.

Effects of climate change on coastal ecosystem food webs: Implications for aquaculture

Coastal ecosystems provide important ecosystem services for millions of people. Climate change is modifying coastal ecosystem food web structure and function and threatens these essential ecosystem services. We used a combination of two new and one existing ecosystem food web models and altered scenarios that are possible with climate change to quantify the impacts of climate change on ecosystem stability in three coastal bays in Maine, United States. We also examined the impact of climate change on bivalve fisheries and aquaculture. Our modeled scenarios explicitly considered the predicted effects of future climatic change and human intervention and included: 1) the influence of increased terrestrial dissolved organic carbon loading on phytoplankton biomass; 2) benthic community change driven by synergisms between climate change, historical overfishing, and increased species invasion; and 3) altered trophic level energy transfer driven by ocean warming and acidification. The effects of climate change strongly negatively influenced ecosystem energy flow and ecosystem stability and negatively affected modeled bivalve carrying capacity in each of our models along the Maine coast of the eastern United States. Our results suggest that the interconnected nature of ecosystem food webs make them extremely vulnerable to synergistic effects of climate change. To better inform fisheries and aquaculture management, the effects of climate change must be explicitly incorporated.

DNA damage and oxidative stress responses of mussels Mytilus galloprovincialis to paralytic shellfish toxins under warming and acidification conditions - Elucidation on the organ-specificity

Commonly affected by changes in climate and environmental conditions, coastal areas are very dynamic environments where shellfish play an important ecological role. In this study, the oxidative stress and genotoxic responses of mussels (Mytilus galloprovincialis) exposed to paralytic shellfish toxin (PST) - producing dinoflagellates Gymnodinium catenatum were evaluated under i) current conditions (CC: 19 °C; pH 8.0), ii) warming (W: 24 °C; pH 8.0), iii) acidification (A:19 °C; pH 7.6) and iv) combined effect of warming and acidification (WA: 24 °C; pH 7.6). Mussels were fed with G. catenatum for 5 days, and to a non-toxic diet during the following 10 days. A battery of oxidative stress biomarkers and comet assay was performed at the peak of toxin accumulation and at the end of the post-exposure phase. Under CC, gills and hepatopancreas displayed different responses/vulnerabilities and mechanisms to cope with PST. While gills presented a tendency for lipid peroxidation (LPO) and genetic damage (expressed by the Genetic Damage Indicator - GDI), hepatopancreas seems to better cope with the toxins, as no LPO was observed. However, the mechanisms involved in hepatopancreas protection were not enough to maintain DNA integrity. The absence of LPO, and the antioxidant system low responsiveness, suggests DNA damage was not oxidative. When exposed to toxic algae under W, toxin-modulated antioxidant responses were observed in both gills and hepatopancreas. Simultaneous exposure to the stressors highlighted gills susceptibility with a synergistic interaction increasing DNA damage. Exposure to toxic algae under A led to genotoxicity potentiation in both organs. The combined effect of WA did not cause relevant interactions in gills antioxidant responses, but stressors interactions impacted LPO and GDI. Antioxidant responses and LPO pointed out to be modulated by the environmental conditions in hepatopancreas, while GDI results support the dominance of toxin-triggered process. Overall, these results reveal that simultaneous exposure to warming, acidification and PSTs impairs mussel DNA integrity, compromising the genetic information due to the synergetic effects. Finally, this study highlights the increasing ecological risk of harmful algal blooms to Mytilus galloprovinciallis populations.

Commercial Performance of Blue Mussel (Mytilus edulis, L.) Stocks at a Microgeographic Scale

Bivalve aquaculture is an important component of the economy in eastern Canada. Because of current social, environmental, economic, and resource constraints, offshore mussel cultivation seems to be a promising strategy. With the objective of optimizing farming strategies that support the sustainability and development of the mussel industry at a microgeographic scale, we evaluated, after a traditional two year production cycle, the commercial performance of spat from several mussel (Mytilus edulis) stocks originating from sites separated by less than 65 km and cultivated at two different grow-out sites (shallow lagoon and offshore waters). The spatiotemporal variation in spat performance was studied through a multiyear in situ “stock-site” spat transfer design. The spat supply originating from the Bassin du Havre-Aubert lagoon systematically exhibited a larger size at sleeving time when compared to other stocks, and a better productivity level when harvested. Nevertheless, an alternative strategy would be to collect spat from the Havre-aux-Maisons lagoon, mostly because of the important commercial volumes of spat that can be collected there. Commercial performance (net income) was three times higher in the deep offshore grow-out site than in the shallow lagoon site. This better productivity in the open sea confirms the highly valuable strategy of offshore mussel farming in this area, where it was hypothesized that the less stressful environmental conditions positively influence reproduction, survival, and growth trends.

Climate Change and Bivalve Mass Mortality in Temperate Regions

One of the fastest-growing global food sectors is the bivalve aquaculture industry. Bivalves particularly oysters, mussels and clams are important sources of animal protein (Tan and Ransangan 2016a, b). Bivalve aquaculture represents 14-16% of the average per capita animal protein for 1.5 billion people and supports over 200,000 livelihoods, mostly in developing countries (FAO 2018). Most of the bivalves produced around the world (89%) are from aquaculture (FAO 2016). To date, mollusc aquaculture have accounted for 21.42% (17.14 million tonnes) of the total aquaculture production, with Asia being the largest contributor (92.27%) (FAO 2018).

Toxic Algae Silence Physiological Responses to Multiple Climate Drivers in a Tropical Marine Food Chain

Research on the effects of climate change in the marine environment continues to accelerate, yet we know little about the effects of multiple climate drivers in more complex, ecologically relevant settings - especially in sub-tropical and tropical systems. In marine ecosystems, climate change (warming and freshening from land run-off) will increase water column stratification which is favorable for toxin producing dinoflagellates. This can increase the prevalence of toxic microalgal species, leading to bioaccumulation of toxins by filter feeders, such as bivalves, with resultant negative impacts on physiological performance. In this study we manipulated multiple climate drivers (warming, freshening, and acidification), and the availability of toxic microalgae, to determine their impact on the physiological health, and toxin load of the tropical filter-feeding clam, Meretrix meretrix. Using a structural equation modeling (SEM) approach, we found that exposure to projected marine climates resulted in direct negative effects on metabolic and immunological function and, that these effects were often more pronounced in clams exposed to multiple, rather than single climate drivers. Furthermore, our study showed that these physiological responses were modified by indirect effects mediated through the food chain. Specifically, we found that when bivalves were fed with a toxin-producing dinoflagellate (Alexandrium minutum) the physiological responses, and toxin load changed differently and in a non-predictable way compared to clams exposed to projected marine climates only. Specifically, oxygen consumption data revealed that these clams did not respond physiologically to climate warming or the combined effects of warming, freshening and acidification. Our results highlight the importance of quantifying both direct and, indirect food chain effects of climate drivers on a key tropical food species, and have important implications for shellfish production and food safety in tropical regions.

Assessing the sensitivity of bivalve populations to global warming using an individual-based modelling approach

Climate change exposes benthic species populations in coastal ecosystems to a combination of different stressors (e.g., warming, acidification and eutrophication), threatening the sustainability of the ecological functions they provide. Thermal stress appears to be one of the strongest drivers impacting marine ecosystems, acting across a wide range of scales, from individual metabolic performances to geographic distribution of populations. Accounting for and integrating the response of species functional traits to thermal stress is therefore a necessary step in predicting how populations will respond to the warming expected in coming decades. Here, we developed an individual-based population model using a mechanistic formulation of metabolic processes within the framework of the dynamic energy budget theory. Through a large number of simulations, we assessed the sensitivity of population growth potential to thermal stress and food conditions based on a climate projection scenario (Representative Concentration Pathway; RCP8.5: no reduction of greenhouse gas emissions). We focused on three bivalve species with contrasting thermal tolerance ranges and distinct distribution ranges along 5,000 km of coastline in the NE Atlantic: the Pacific oyster (Magallana gigas), and two mussel species: Mytilus edulis and Mytilus galloprovincialis. Our results suggest substantial and contrasting changes within species depending on local temperature and food concentration. Reproductive phenology appeared to be a core process driving the responses of the populations, and these patterns were closely related to species thermal tolerances. The nonlinear relationship we found between individual life-history traits and response at the population level emphasizes the need to consider the interactions resulting from upscaling across different levels of biological organisation. These results underline the importance of a process-based understanding of benthic population response to seawater warming, which will be necessary for forward planning of resource management and strategies for conservation and adaptation to environmental changes.

Combined effects of warming and acidification on accumulation and elimination dynamics of paralytic shellfish toxins in mussels Mytilus galloprovincialis

Harmful algal blooms (HAB) have been increasing in frequency and intensity most likely due to changes on global conditions, which constitute a significant threat to wild shellfish and its commercial farming. This study evaluated the impact of increasing seawater temperature and acidification on the accumulation/elimination dynamics of HAB-toxins in shellfish. Mytilus galloprovincialis were acclimated to four environmental conditions simulating different climate change scenarios: i) current conditions, ii) warming, iii) acidification and iv) interaction of warming with acidification. Once acclimated, mussels were exposed to the paralytic shellfish toxins (PSTs) producing dinoflagellate Gymnodinium catenatum for 5 days and to non-toxic diet during the subsequent 10 days. High toxicity levels (1493 µg STX eq. kg^-1) exceeding the safety limits were determined under current conditions at the end of the uptake period. Significantly lower PSP toxicity levels were registered for warming- and acidification-acclimated mussels (661 and 761 µg STX eq. kg^-1). The combined effect of both warming and acidification resulted in PSP toxicity values slightly higher (856 μg STX eq. kg^-1). A rapid decrease of toxicity was observed in mussels at the current conditions after shifting to a non-toxic diet, which was not noticed under the predicted climate change scenarios. Variability of each PST analogue, measured throughout the experiment, highlighted different mechanisms are associated with changes of each environmental factor, although both resulting in lower toxicity. Warming-acclimated mussels showed lower accumulation/elimination rates, while acidification-acclimated mussels showed higher capability to accumulate toxins, but also a higher elimination rate preventing high toxicity levels. As different mechanisms are triggered by warming and acidification, their combined effect not leads to a synergism of their individual effects. The present work is the first assessing the combined effect of climate change drivers on accumulation/elimination of PSTs, in mussels, indicating that warming and acidification may lead to lower toxicity values but longer toxic episodes. PSTs are responsible for the food poisoning syndrome, paralytic shellfish poisoning (PSP) in humans. This study can be considered as the first step to build models for predicting shellfish toxicity under climate change scenarios.