Effects of Co-Varying Diel-Cycling Hypoxia and pH on Growth in the Juvenile Eastern Oyster, Crassostrea virginica

Environmental correlates of oyster farming in an upwelling system: Implication upon growth, biomass production, shell strength and organic composition

Comprehending the potential effects of environmental variability on bivalves aquaculture becomes crucial for its sustainability under climate change scenarios, specially in the Humboldt Current System (HCS) where upwelling intensification leading to frequent hypoxia and acidification is expected. In a year-long study, Pacific oysters (Magallana gigas) were monitored at two depths (1.5m, 6.5m) in a bay affected by coastal upwelling. Surface waters exhibited warmer, well-oxygenated conditions and higher chlorophyll-a concentrations, while at depth greater hypoxia and acidification events occur, especially during upwelling. Surface cultured oysters exhibited 60 % larger size and 35% greater weight due to faster growth rate during the initial month of cultivation. The condition index (CI) increases in surface oysters after 10 months, whereas those at the bottom maintain a lower index. Food availability, temperature, and oxygen, correlates with higher growth rates, while pH associates with morphometric variables, indicating that larger oysters tend to develop under higher pH. Increased upwelling generally raises CI, but bottom oysters face stressful conditions such as hypoxia and acidification, resulting in lower performance. However, they acclimate by changing the organic composition of their shells and making them stronger. This study suggests that under intensified upwelling scenario, oysters would grow slowly, resulting in smaller sizes and lower performance, but the challenges may be confronted through complex compensation mechanisms among biomass production and maintenance of the shell structure and function. This poses a significant challenge for the sustainability of the aquaculture industry, emphasizing the need for adaptive strategies to mitigate the effects of climate change.

Oyster reefs' control of carbonate chemistry-Implications for oyster reef restoration in estuaries subject to coastal ocean acidification

Globally, oyster reef restoration is one of the most widely applied coastal restoration interventions. While reefs are focal points of processes tightly linked to the carbonate system such as shell formation and respiration, how these processes alter reef carbonate chemistry relative to the surrounding seawater is unclear. Moreover, coastal systems are increasingly impacted by coastal acidification, which may affect reef carbonate chemistry. Here, we characterized the growth of multiple constructed reefs as well as summer variations in pH and carbonate chemistry of reef-influenced seawater (in the middle of reefs) and ambient seawater (at locations ~50 m outside of reefs) to determine how reef chemistry was altered by the reef community and, in turn, impacts resident oysters. High frequency monitoring across three subtidal constructed reefs revealed reductions of daily mean and minimum pH (by 0.05-0.07 and 0.07-0.12 units, respectively) in seawater overlying reefs relative to ambient seawater (p < .0001). The proportion of pH measurements below 7.5, a threshold shown to negatively impact post-larval oysters, were 1.8×-5.2× higher in reef seawater relative to ambient seawater. Most reef seawater samples (83%) were reduced in total alkalinity relative to ambient seawater samples, suggesting community calcification was a key driver of modified carbonate chemistry. The net metabolic influence of the reef community resulted in reductions of CaCO3 saturation state in 78% of discrete samples, and juvenile oysters placed on reefs exhibited slower shell growth (p < .05) compared to oysters placed outside of reefs. While differences in survival were not detected, reef oysters may benefit from enhanced survival or recruitment at the cost of slowed growth rates. Nevertheless, subtidal restored reef communities modified seawater carbonate chemistry in ways that likely increased oyster vulnerability to acidification, suggesting that carbonate chemistry dynamics warrant consideration when determining site suitability for oyster restoration, particularly under continued climate change.

Pacific oysters do not compensate growth retardation following extreme acidification events

Ocean acidification caused by anthropogenic carbon dioxide emissions alters the growth of marine calcifiers. Although the immediate effects of acidification from global ocean models have been well studied on calcifiers, their recovery capacity over a wide range of pH has never been evaluated. This aspect is crucial because acidification events that arise in coastal areas can far exceed global ocean predictions. However, such acidification events could occur transiently, allowing for recovery periods during which the effects on growth would be compensated, maintained or amplified. Here we evaluated the recovery capacity of a model calcifier, the Pacific oyster Crassostrea gigas. We exposed juveniles to 15 pH conditions between 6.4 and 7.8 for 14 days. Oyster growth was retarded below pH 7.1 while shells were corroded at pH 6.5. We then placed the oysters under ambient pH > 7.8 for 42 days. Growth retardation persisted at pH levels below pH 7.1 even after the stress was removed. However, despite persistent retardation, growth has resumed rapidly suggesting that the oysters can recover from extreme acidification. Yet we found that the differences in individual weight between pH conditions below 7.1 increased over time, and thus the growth retardation cannot be compensated and may affect the fitness of the bivalves.

Legacy of past exposure to hypoxia and warming regulates an ecosystem service provided by oysters

Climate change is having substantial impacts on organism fitness and ability to deliver critical ecosystem services, but these effects are often examined only in response to current environments. Past exposure to stress can also affect individuals via carryover effects, and whether these effects scale from individuals to influence ecosystem function and services is unknown. We explored within-generation carryover effects of two coastal climate change stressors-hypoxia and warming-on oyster (Crassostrea virginica) growth and nitrogen bioassimilation, an important ecosystem service. Oysters were exposed to a factorial combination of two temperature and two diel-cycling dissolved oxygen treatments at 3-months-old and again 1 year later. Carryover effects of hypoxia and warming influenced oyster growth and nitrogen storage in complex and context-dependent ways. When operating, carryover effects of single stressors generally reduced oyster nitrogen bioassimilation and relative investment in tissue versus shell growth, particularly in warm environments, while early life exposure to multiple stressors generally allowed oysters to perform as well as control oysters. When extrapolated to the reef scale, carryover effects decreased nitrogen stored by modeled oyster reefs in most conditions, with reductions as large as 41%, a substantial decline in a critical ecosystem service. In some scenarios, however, carryover effects increased nitrogen storage by modeled oyster reefs, again highlighting the complexity of these effects. Hence, even brief exposure to climate change stressors early in life may have persistent effects on an ecosystem service 1 year later. Our results show for the first time that within-generation carryover effects on individual phenotypes can impact processes at the ecosystem scale and may therefore be an overlooked factor determining ecosystem service delivery in response to anthropogenic change.

Dissolved oxygen gradient on three dimensionally printed microfluidic platform for studying its effect on fish at three levels: cell, embryo, and larva

A simple and low-cost dissolved oxygen gradient platform of three dimensionally (3D) printed microfluidic chip was developed for cultivating cells, embryos, and larvae of fish. "Christmas tree" structure channel networks generated a dissolved oxygen gradient out of two fluids fed to the device. Polydimethylsiloxane (PDMS) membrane with high biocompatibility was used as the substrate for cell culture in the 3D-printed microfluidic chip, which made the cell analysis easy. The embryos and larvae of fish could be cultured directly in the chip, and their development can be observed in real time with a microscope. Using zebrafish as a model, we assessed the effect of different dissolved oxygen on its cells, embryos, and larvae. Hypoxia induced production of reactive oxygen species (ROS) in zebrafish cells, embryos, and larvae, eventually leading to cell apoptosis and developmental impairment. Hypoxia also increased nitric oxide content in zebrafish cells, which might be a defensive strategy to overcome the adverse effect of hypoxia in fish cells. This is the first platform that could comprehensively investigate the effects of different dissolved oxygen on fish at the cell, embryo, and larva levels, which has great potential in studying the responses of aquatic organisms under different oxygen concentrations.

Seasonal changes of trace elements, nutrients, dissolved organic matter, and coastal acidification over the largest oyster reef in the Western Mississippi Sound, USA

Seasonal changes of trace elements, nutrients, dissolved organic matter (DOM), and carbonate system parameters were evaluated over the largest deteriorating oyster reef in the Western Mississippi Sound using data collected during spring, summer, and winter of 2018, and summer of 2019. Higher concentrations of Pb (224%), Cu (211%), Zn (2400%), and Ca (240%) were observed during winter of 2018 compared to summer 2019. Phosphate and ammonia concentrations were higher (> 800%) during both summers of 2018 and 2019 than winter of 2018. Among the three distinct DOM components identified, two terrestrial humic-like components were more abundant during both spring (12% and 36%) and summer (11% and 33%) of 2018 than winter of 2018, implying a relatively lesser supply of humic-like components from terrestrial sources during winter. On the other hand, the protein-like component was more abundant during summer of 2019 compared to rest of the study period, suggesting a higher rate of autochthonous production during summer 2019. In addition, to their significant depth-wise variation, ocean acidification parameters including pH, pCO₂, CO₃^2-, and carbonate saturation states were all higher during both summers of 2018 and 2019. The measured variables such as trace elements, organic carbon, suspended particulates, and acidification parameters exhibited conservative mixing behavior against salinity. These observations have strong implications for the health of the oyster reefs, which provides ecologically important habitats and supports the economy of the Gulf Coast.

Oxygen Variability in the Offshore Northern Benguela Upwelling System From Glider Data

Despite their role in modulating the marine ecosystem, variability and drivers of low-oxygen events in the offshore northern Benguela Upwelling System (BenUS) have been rarely investigated due to the events' episodicity which is difficult to resolve using shipboard measurements. We address this issue using 4 months of high-resolution glider data collected between February and June 2018, 100 km offshore at 18°S. We find that oxygen (O2) concentrations in the offshore northern Benguela are determined by the subsurface alternation of low-oxygen Angola-derived water and oxygenated water from the south at 100-500 m depth. We observe intermittent hypoxia (O2 < 60 μmol kg-1) which occurs on average for ∼30% of the 4 months deployment and is driven by the time-varying subsurface pulses of Angola-derived tropical water. Hypoxic events are rather persistent at depths of 300-450 m, while they are more sporadic and have weekly duration at shallower depths (100-300 m). We find extreme values of hypoxia, with O2 minima of 16 μmol kg-1, associated with an anticyclonic eddy spinning from the undercurrent flowing on the BenUS shelf and showing no surface signature. Fine-scale patchiness and water mass mixing are associated with cross-frontal stirring by a large anticyclone recirculating tropical water into the northern BenUS. The dominance of physical drivers and their high variability on short time scales reveal a dynamic coupling between Angola and Benguela, calling for long-term and high-resolution measurements and studies focusing on future changes of both tropical O2 minima and lateral fluxes in this region.

Antioxidant response of the oyster Crassostrea hongkongensis exposed to diel-cycling hypoxia under different salinities

Intertidal and estuarine bivalves are adapted to fluctuating environmental conditions but the cellular adaptive mechanisms under combined stress scenarios are not well understood. The Hong Kong oysters Crassostrea hongkongensis experience periodic hypoxia/reoxygenation and salinity fluctuations during tidal cycles and extreme weather, which can negatively affect the respiratory organs (gills) involved in oxygen uptake and transport. We determined the effects of periodic hypoxia under different salinities on the oxidative stress response in Hong Kong oysters. Oxidative stress parameters (activities of superoxide dismutase (SOD), and catalase (CAT), tissue levels of malondialdehyde (MDA) and protein carbonyl content (PCC)) were determined in the gills of oysters exposed to diel-cycling hypoxia (hypoxia at night: 12h at 2 mg/L, reoxygenation: 12h at 6 mg/L) and normal dissolved oxygen (DO) (6 mg/L) under three salinities (10, 25, and 35‰) for 28 days. Oxygen regime in combination with salinity changes had significant interactive effects on all studied parameters except SOD. Salinity, DO and their interactions increased PCC after 14 and 28 days of exposure, and the combination of hypoxia/reoxygenation and decreased salinity showed the most severe effect. MDA content of the gills increased only after the long-term (28 days) exposure in decreased or increased salinity under normal DO treatments, showing PCC was more sensitive than MDA as biomarker of oxidative stress. Low salinity suppressed SOD activity regardless of the DO, whereas hypoxia induced SOD responses. CAT activities decreased significantly under high salinity with hypoxia/reoxygenation conditions. Our findings highlighted that periodic hypoxia/reoxygenation with salinity change induced antioxidant responses, which can impact the health of Hong Kong oyster C. hongkongensis and prolonged salinity stress may be one reason for the mortality during its aquaculture process.

Variation in Survival and Gut Microbiome Composition of Hatchery-Grown Native Oysters at Various Locations within the Puget Sound

The Olympia oyster (Ostrea lurida) of the Puget Sound suffered a dramatic population crash, but restoration efforts hope to revive this native species. One overlooked variable in the process of assessing ecosystem health is association of bacteria with marine organisms and the environments they occupy. Oyster microbiomes are known to differ significantly between species, tissue type, and the habitat in which they are found. The goals of this study were to determine the impact of field site and habitat on the oyster microbiome and to identify core oyster-associated bacteria in the Puget Sound. Olympia oysters from one parental family were deployed at four sites in the Puget Sound both inside and outside of eelgrass (Zostera marina) beds. Using 16S rRNA gene amplicon sequencing of the oyster gut, shell, and surrounding seawater and sediment, we demonstrate that gut-associated bacteria are distinct from the surrounding environment and vary by field site. Furthermore, regional differences in the gut microbiota are associated with the survival rates of oysters at each site after 2 months of field exposure. However, habitat type had no influence on microbiome diversity. Further work is needed to identify the specific bacterial dynamics that are associated with oyster physiology and survival rates.

IMPORTANCE This is the first exploration of the microbial colonizers of the Olympia oyster, a native oyster species to the West Coast, which is a focus of restoration efforts. The patterns of differential microbial colonization by location reveal microscale characteristics of potential restoration sites which are not typically considered. These microbial dynamics can provide a more holistic perspective on the factors that may influence oyster performance.

Effects of Biophysical Processes on Diel-Cycling Hypoxia in a Subtropical Estuary

In shallow estuaries, fluctuations in bottom dissolved oxygen (DO) at diel (24 h) timescales are commonly attributed to cycles of net production and respiration. However, bottom DO can also be modulated by physical processes, such as tides and wind, that vary at or near diel timescales. Here, we examine processes affecting spatiotemporal variations in diel-cycling DO in Escambia Bay, a shallow estuary along the Gulf of Mexico. We collected continuous water quality measurements in the upper and middle reaches of the Bay following relatively high (> 850 m³ s^-1) and low (< 175 m³ s^-1) springtime freshwater discharge. Variations in diel-cycling amplitude over time were estimated using the continuous wavelet transform, and correlations between DO and biophysical processes at diel timescales were examined using wavelet coherence. Our results reveal that freshwater discharge modulated inter-annual variations in the spatial extent and duration of summertime hypoxia through its effect on vertical density stratification. In the absence of strong stratification (> 15 kg m^-3), vertical mixing by tropic tides and sea breeze enhanced diel fluctuations in deeper areas near the channel, while in shallower areas the largest fluctuations were associated with irradiance. Our findings suggest that processes affecting diel-cycling DO in the bottom layer can vary over a relatively short spatial extent less than 2 km and with relatively small changes in bottom elevation of 1 m or less. Implications for water quality monitoring were illustrated by subsampling DO timeseries, which demonstrates how low-frequency measurements may misrepresent water quality in estuaries where diel-cycling DO is common. In these systems, adequate assessment of hypoxia and its aquatic life impacts requires continuous measurements that capture the variation in DO at diel timescales.

Context-dependent carryover effects of hypoxia and warming in a coastal ecosystem engineer

Organisms are increasingly likely to be exposed to multiple stressors repeatedly across ontogeny as climate change and other anthropogenic stressors intensify. Early life stages can be particularly sensitive to environmental stress, such that experiences early in life can "carry over" to have long-term effects on organism fitness. Despite the potential importance of these within-generation carryover effects, we have little understanding of how they vary across ecological contexts, particularly when organisms are re-exposed to the same stressors later in life. In coastal marine systems, anthropogenic nutrients and warming water temperatures are reducing average dissolved oxygen (DO) concentrations while also increasing the severity of naturally occurring daily fluctuations in DO. Combined effects of warming and diel-cycling DO can strongly affect the fitness and survival of coastal organisms, including the eastern oyster (Crassostrea virginica), a critical ecosystem engineer and fishery species. However, whether early life exposure to hypoxia and warming affects oysters' subsequent response to these stressors is unknown. Using a multiphase laboratory experiment, we explored how early life exposure to diel-cycling hypoxia and warming affected oyster growth when oysters were exposed to these same stressors 8 weeks later. We found strong, interactive effects of early life exposure to diel-cycling hypoxia and warming on oyster tissue : shell growth, and these effects were context-dependent, only manifesting when oysters were exposed to these stressors again two months later. This change in energy allocation based on early life stress exposure may have important impacts on oyster fitness. Exposure to hypoxia and warming also influenced oyster tissue and shell growth, but only later in life. Our results show that organisms' responses to current stress can be strongly shaped by their previous stress exposure, and that context-dependent carryover effects may influence the fitness, production, and restoration of species of management concern, particularly for sessile species such as oysters.

Context-dependent carryover effects of hypoxia and warming in a coastal ecosystem engineer

Organisms are increasingly likely to be exposed to multiple stressors repeatedly across ontogeny as climate change and other anthropogenic stressors intensify. Early life stages can be particularly sensitive to environmental stress, such that experiences early in life can "carry over" to have long-term effects on organism fitness. Despite the potential importance of these within-generation carryover effects, we have little understanding of how they vary across ecological contexts, particularly when organisms are re-exposed to the same stressors later in life. In coastal marine systems, anthropogenic nutrients and warming water temperatures are reducing average dissolved oxygen (DO) concentrations while also increasing the severity of naturally occurring daily fluctuations in DO. Combined effects of warming and diel-cycling DO can strongly affect the fitness and survival of coastal organisms, including the eastern oyster (Crassostrea virginica), a critical ecosystem engineer and fishery species. However, whether early life exposure to hypoxia and warming affects oysters' subsequent response to these stressors is unknown. Using a multiphase laboratory experiment, we explored how early life exposure to diel-cycling hypoxia and warming affected oyster growth when oysters were exposed to these same stressors 8 weeks later. We found strong, interactive effects of early life exposure to diel-cycling hypoxia and warming on oyster tissue : shell growth, and these effects were context-dependent, only manifesting when oysters were exposed to these stressors again two months later. This change in energy allocation based on early life stress exposure may have important impacts on oyster fitness. Exposure to hypoxia and warming also influenced oyster tissue and shell growth, but only later in life. Our results show that organisms' responses to current stress can be strongly shaped by their previous stress exposure, and that context-dependent carryover effects may influence the fitness, production, and restoration of species of management concern, particularly for sessile species such as oysters.

Energetic response of Atlantic surfclam Spisula solidissima to ocean acidification

In this study, we assessed the Atlantic surfclam (Spisula solidissima) energy budget under different ocean acidification conditions (OA). During 12 weeks, 126 individuals were maintained at three different ρCO₂ concentrations. Every two weeks, individuals were sampled for physiological measurements and scope for growth (SFG). In the high ρCO₂ treatment, clearance rate decreased and excretion rate increased relative to the low ρCO₂ treatment, resulting in reduced SFG. Moreover, oxygen:nitrogen (O:N) excretion ratio dropped, suggesting that a switch in metabolic strategy occurred. The medium ρCO₂ treatment had no significant effects upon SFG; however, metabolic loss increased, suggesting a rise in energy expenditure. In addition, a significant increase in food selection efficiency was observed in the medium treatment, which could be a compensatory reaction to the metabolic over-costs. Results showed that surfclams are particularly sensitive to OA; however, the different compensatory mechanisms observed indicate that they are capable of some temporary resilience.

Combined and independent effects of hypoxia and tributyltin on mRNA expression and physiology of the Eastern oyster (Crassostrea virginica)

Oyster reefs are vital to estuarine health, but they experience multiple stressors and globally declining populations. This study examined effects of hypoxia and tributyltin (TBT) on adult Eastern oysters (Crassostrea virginica) exposed either in the laboratory or the field following a natural hypoxic event. In the laboratory, oysters were exposed to either hypoxia followed by a recovery period, or to hypoxia combined with TBT. mRNA expression of HIF1-α and Tβ-4 along with hemocyte counts, biomarkers of hypoxic stress and immune health, respectively, were measured. In field-deployed oysters, HIF1-α and Tβ-4 expression increased, while no effect on hemocytes was observed. In contrast, after 6 and 8 days of laboratory-based hypoxia exposure, both Tβ-4 expression and hemocyte counts declined. After 8 days of exposure to hypoxia + TBT, oysters substantially up-regulated HIF1-α and down-regulated Tβ-4, although hemocyte counts were unaffected. Results suggest that hypoxic exposure induces immunosuppression which could increase vulnerability to pathogens.

In situ recovery of bivalve shell characteristics after temporary exposure to elevated pCO2

Ocean uptake of carbon dioxide (CO2) is causing changes in carbonate chemistry that affect calcification in marine organisms. In coastal areas, this CO2-enriched seawater mixes with waters affected by seasonal degradation of organic material loaded externally from watersheds or produced as a response to nutrient enrichment. As a result, coastal bivalves often experience strong seasonal changes in carbonate chemistry. In some cases, these changes may resemble those experienced by aquacultured bivalves during translocation activities. We mimicked these changes by exposing juvenile hard clams (500 μm, Mercenaria mercenaria) to pCO2 in laboratory upwellers at levels resembling those already reported for northeastern US estuaries (mean upweller pCO2 = 773, 1274, and 1838 μatm) and then transplanting to three grow-out sites along an expected nutrient gradient in Narragansett Bay, RI (154 bags of 100 clams). Prior to the field grow-out, clams exposed to elevated pCO2 exhibited larger shells but lower dry weight per unit volume (dw/V). However, percent increase in dw/V was highest for this group during the 27-day field grow-out, suggesting that individuals with low dw/V after the laboratory treatment accelerated accumulation of dw/V when they were transferred to the bay. Treatments also appeared to affect shell mineral structure and condition of digestive diverticula. Although treatment effects diminished during the field grow-out, clams that were pre-exposed for several weeks to high pCO2 would likely have been temporarily vulnerable to predation or other factors that interact with shell integrity. This would be expected to reduce population recovery from short-term exposures to high pCO2.

Effects of coastal acidification on North Atlantic bivalves: interpreting laboratory responses in the context of in situ populations

Experimental exposure of early life stage bivalves has documented negative effects of elevated pCO₂ on survival and growth, but the population consequences of these effects are unknown. Following standard practices from population viability analysis and wildlife risk assessment, we substituted laboratory-derived stress-response relationships into baseline population models of Mercenaria mercenaria and Argopecten irradians. The models were constructed using inverse demographic analyses with time series of size-structured field data in NY, USA, whereas the stress-response relationships were developed using data from a series of previously published laboratory studies. We used stochastic projection methods and diffusion approximations of extinction probability to estimate cumulative risk of 50% population decline during ten-year population projections at 1, 1.5 and 2 times ambient pCO₂ levels. Although the A. irradians population exhibited higher growth in the field data (12% per year) than the declining M. mercenaria population (-8% per year), cumulative risk was high for A. irradians in the first ten years due to high variance in the stochastic growth rate estimate (log λ_s = -0.02, σ² = 0.24). This ten-year cumulative risk increased from 69% to 94% and >99% at 1.5 and 2 times ambient scenarios. For M. mercenaria (log λ_s = -0.09, σ² = 0.01), ten-year risk was 81%, 96% and >99% at 1, 1.5 and 2 times ambient pCO₂, respectively. These estimates of risk could be improved with detailed consideration of harvest effects, disease, restocking, compensatory responses, other ecological complexities, and the nature of interactions between these and other effects that are beyond the scope of available data. However, results clearly indicate that early life stage responses to plausible levels of pCO₂ enrichment have the potential to cause significant increases in risk to these marine bivalve populations.

Individual and combined effects of low dissolved oxygen and low pH on survival of early stage larval blue crabs, Callinectes sapidus

A large number of coastal ecosystems globally are subjected to concurrent hypoxic and acidified conditions that will likely intensify and expand with continued climate change. In temperate regions, the spawning of many important organisms including the Atlantic blue crab Callinectes sapidus occurs during the summer months when the severity of coastal hypoxia and acidification is the greatest. While the blue crab earliest larval stage can be exposed to co-occurring hypoxia and acidification observed in many coastal ecosystems, the effects of these concurrent stressors on larval blue crab survival is unknown. This study investigated the individual and combined consequences of low dissolved oxygen (DO) and low pH on blue crab larvae survival through a series of short-term experiments. During 14-day experiments with moderately hypoxic conditions (117-127 μM O2 or 3.74-4.06 mg L-1) and acidified conditions (pH on total scale of 7.16-7.33), low DO and low pH individually and significantly reduced larval survival by 60% and 49%, respectively, with the combination of stressors reducing survival by 87% compared to the control treatment (210-269 μM O2 or 6.72-8.61 mg L-1, 7.91-7.94 DO and pH, respectively). During 4-day experiments with lower DO levels (68-83 μM O2 or 2.18-2.62 mg L-1) and comparable pH levels of 7.29-7.39, low DO individually reduced survival by >90% compared to the control (261-267 μM O2 or 8.35-8.54 mg L-1, 7.92-7.97 DO and pH, respectively), whereas low pH had no effect and there was no interaction between stressors. Over a 4-day period, the DO threshold at which 50% of the larval blue crab population died (LC50) was 121 μM O2 (3.86 mgL-1). In 14-day experiments, the DO and pH effects were additive, yielding survival rates lower than the individual treatments, and significantly correlated with DO and pH concentrations. Collectively, these findings indicate that blue crab sensitivity to both low DO and low pH are acute within the larval stage, depend on the intensity and duration of exposure, and leads to mortality, thereby potentially contributing to the interannual variability and possible regional declines of this fishery.

Responses of an oyster host (Crassostrea virginica) and its protozoan parasite (Perkinsus marinus) to increasing air temperature

Background

Changes in climate are predicted to influence parasite and pathogen infection patterns in terrestrial and marine environments. Increases in temperature in particular may greatly alter biological processes, such as host-parasite interactions. For example, parasites could differentially benefit from increased reproduction and transmission or hosts could benefit from elevated immune responses that may mediate or even eliminate infections. In the southeastern United States, the Eastern oyster, Crassostrea virginica, is infected by the lethal protozoan parasite, Perkinsus marinus. Under field conditions, intertidal (air-exposed) oysters have been found to have significantly higher P. marinus infection intensity and marginally higher infection prevalence than subtidal (submerged) oysters. During summer, air temperatures are much warmer than water and this exposure of intertidal oysters to higher temperatures is a suggested mechanism for increased infection intensity.

Methods

We simulated intertidal exposure using controlled laboratory experiments to determine how host traits (survival and immune response) and parasite infection intensity will respond to elevated air temperature ranging from 27 °C to 53 °C during emersion at low tide. In Georgia, where our work was conducted, the average summer water temperature is 29 °C and the average maximum high air temperature in July is 33 °C (though oysters have been shown to survive at much higher air temperatures).

Results

Host survival declined as temperature increased, with a definitive drop-off between 39–43 °C. Negative effects of air temperature on host immune response (phagocytic activity) were detectable only at extremely high temperatures (47–50 °C) when hosts were suffering acute mortality. Parasite infection intensity peaked at 35 °C.

Discussion

Our results suggest that an increase in average summer air temperature to 35 °C or higher could affect oyster survival directly through temperature-related impacts in the short-term and indirectly through increased P. marinus infection intensity over the long-term.

Oysters and eelgrass: potential partners in a high pCO2 ocean

Climate change is affecting the health and physiology of marine organisms and altering species interactions. Ocean acidification (OA) threatens calcifying organisms such as the Pacific oyster, Crassostrea gigas. In contrast, seagrasses, such as the eelgrass Zostera marina, can benefit from the increase in available carbon for photosynthesis found at a lower seawater pH. Seagrasses can remove dissolved inorganic carbon from OA environments, creating local daytime pH refugia. Pacific oysters may improve the health of eelgrass by filtering out pathogens such as Labyrinthula zosterae (LZ), which causes eelgrass wasting disease (EWD). We examined how co-culture of eelgrass ramets and juvenile oysters affected the health and growth of eelgrass and the mass of oysters under different pCO₂ exposures. In Phase I, each species was cultured alone or in co-culture at 12°C across ambient, medium, and high pCO₂ conditions, (656, 1,158 and 1,606 μatm pCO₂ , respectively). Under high pCO₂ , eelgrass grew faster and had less severe EWD (contracted in the field prior to the experiment). Co-culture with oysters also reduced the severity of EWD. While the presence of eelgrass decreased daytime pCO₂ , this reduction was not substantial enough to ameliorate the negative impact of high pCO₂ on oyster mass. In Phase II, eelgrass alone or oysters and eelgrass in co-culture were held at 15°C under ambient and high pCO₂ conditions, (488 and 2,013 μatm pCO₂ , respectively). Half of the replicates were challenged with cultured LZ. Concentrations of defensive compounds in eelgrass (total phenolics and tannins), were altered by LZ exposure and pCO₂ treatments. Greater pathogen loads and increased EWD severity were detected in LZ exposed eelgrass ramets; EWD severity was reduced at high relative to low pCO₂ . Oyster presence did not influence pathogen load or EWD severity; high LZ concentrations in experimental treatments may have masked the effect of this treatment. Collectively, these results indicate that, when exposed to natural concentrations of LZ under high pCO₂ conditions, eelgrass can benefit from co-culture with oysters. Further experimentation is necessary to quantify how oysters may benefit from co-culture with eelgrass, examine these interactions in the field and quantify context-dependency.