Long-term and trans-life-cycle effects of exposure to ocean acidification in the green sea urchin Strongylocentrotus droebachiensis

Anthropogenic CO₂ emissions are acidifying the world’s oceans. A growing body of evidence demonstrates that ocean acidification can impact survival, growth, development and physiology of marine invertebrates. Here, we tested the impact of long-term (up to 16 months) and trans-life-cycle (adult, embryo/larvae and juvenile) exposure to elevated pCO₂ (1,200 μatm, compared to control 400 μatm) on the green sea urchin Strongylocentrotus droebachiensis. Female fecundity was decreased 4.5-fold when acclimated to elevated pCO₂ for 4 months during reproductive conditioning, while no difference was observed in females acclimated for 16 months. Moreover, adult pre-exposure for 4 months to elevated pCO₂ had a direct negative impact on subsequent larval settlement success. Five to nine times fewer offspring reached the juvenile stage in cultures using gametes collected from adults previously acclimated to high pCO₂ for 4 months. However, no difference in larval survival was observed when adults were pre-exposed for 16 months to elevated pCO₂. pCO₂ had no direct negative impact on juvenile survival except when both larvae and juveniles were raised in elevated pCO₂. These negative effects on settlement success and juvenile survival can be attributed to carry-over effects from adults to larvae and from larvae to juveniles. Our results support the contention that adult sea urchins can acclimate to moderately elevated pCO₂ in a matter of a few months and that carry-over effects can exacerbate the negative impact of ocean acidification on larvae and juveniles.     

The swimming kinematics of larval Atlantic cod, Gadus morhua L., are resilient to elevated seawater pCO2

Kinematics of swimming behavior of larval Atlantic cod, aged 12 and 27 days post-hatch (dph) and cultured under three pCO₂ conditions (control-370, medium-1800, and high-4200 μatm) from March to May 2010, were extracted from swim path recordings obtained using silhouette video photography. The swim paths were analyzed for swim duration, distance and speed, stop duration, and horizontal and vertical turn angles to determine whether elevated seawater pCO₂—at beyond near-future ocean acidification levels—affects the swimming kinematics of Atlantic cod larvae. There were no significant differences in most of the variables tested: the swimming kinematics of Atlantic cod larvae at 12 and 27 dph were highly resilient to extremely elevated pCO₂ levels. Nonetheless, cod larvae cultured at the highest pCO₂ concentration displayed vertical turn angles that were more restricted (median turn angle, 15°) than larvae in the control (19°) and medium (19°) treatments at 12 dph (but not at 27 dph). Significant reduction in the stop duration of cod larvae from the high treatment (median stop duration, 0.28 s) was also observed compared to the larvae from the control group (0.32 s) at 27 dph (but not at 12 dph). The functional and ecological significance of these subtle differences are unclear and, therefore, require further investigation in order to determine whether they are ecologically relevant or spurious.     

Within- and transgenerational effects of ocean acidification on life history of marine three-spined stickleback (Gasterosteus aculeatus)

Some studies have demonstrated that elevated CO₂ concentrations in ocean waters negatively impact metabolism and development of marine fish. Particularly, early developmental stages are probably more susceptible to ocean acidification due to insufficient regulations of their acid-base balance. Transgenerational acclimation can be an important mechanism to mediate impacts of increased CO₂ on marine species, yet very little is known about the potential of parental effects in teleosts. Therefore, transgenerational effects were investigated on life history in juvenile three-spined sticklebacks Gasterosteus aculeatus by acclimating parents (collected in April 2012, 55°03′N, 8°44′E) and offspring to ambient (~400 µatm) and elevated (~1,000 µatm) CO₂ levels and measured parental fecundity as well as offspring survival, growth and otolith characteristics. Exposure to elevated CO₂ concentrations led to an increase in clutch size in adults as well as increased juvenile survival and growth rates between 60 and 90 days post-hatch and enlarged otolith areas compared with fish from ambient CO₂ concentrations. Moreover, transgenerational effects were observed in reduced survival and body size 30 days post-hatch as well as in enlarged otoliths at the end of the experiment, when fathers or both parents were acclimated to the high-CO₂ environment. These results may suggest that elevated CO₂ concentrations had rather positive effects on life-history traits of three-spined sticklebacks, but that parental acclimation can modify these effects without improving offspring fitness. Although the mechanistic basis of such transgenerational acclimation remains unclear, selective gradients within generations seem to determine the direction of transgenerational effects.     

The synergistic effects of increasing temperature and CO2 levels on activity capacity and acid–base balance in the spider crab, Hyas araneus

With global climate change, ocean warming and acidification occur concomitantly. In this study, we tested the hypothesis that increasing CO₂ levels affect the acid–base balance and reduce the activity capacity of the Arctic spider crab Hyas araneus, especially at the limits of thermal tolerance. Crabs were acclimated to projected oceanic CO₂ levels for 12 days (today: 380, towards the year 2100: 750 and 1,120 and beyond: 3,000 μatm) and at two temperatures (1 and 4 °C). Effects of these treatments on the righting response (RR) were determined (1) at acclimation temperatures followed by (2) righting when exposed to an additional acute (15 min) heat stress at 12 °C. Prior to (resting) and after the consecutive stresses of combined righting activity and heat exposure, acid–base status and lactate contents were measured in the haemolymph. Under resting conditions, CO₂ caused a decrease in haemolymph pH and an increase in oxygen partial pressure. Despite some buffering via an accumulation of bicarbonate, the extracellular acidosis remained uncompensated at 1 °C, a trend exacerbated when animals were acclimated to 4 °C. The additional combined exposure to activity and heat had only a slight effect on blood gas and acid–base status. Righting activity in all crabs incubated at 1 and 4 °C was unaffected by elevated CO₂ levels or acute heat stress but was significantly reduced when both stressors acted synergistically. This impact was much stronger in the group acclimated at 1 °C where some individuals acclimated to high CO₂ levels stopped responding. Lactate only accumulated in the haemolymph after combined righting and heat stress. In the group acclimated to 1 °C, lactate content was highest under normocapnia and lowest at the highest CO₂ level in line with the finding that RR was largely reduced. In crabs acclimated to 4 °C, the RR was less affected by CO₂ such that activity caused lactate to increase with rising CO₂ levels. In line with the concept of oxygen and capacity limited thermal tolerance, all animals exposed to temperature extremes displayed a reduction in scope for performance, a trend exacerbated by increasing CO₂ levels. Additionally, the differences seen between cold- and warm-acclimated H. araneus after heat stress indicate that a small shift to higher acclimation temperatures also alleviates the response to temperature extremes, indicating a shift in the thermal tolerance window which reduces susceptibility to additional CO₂ exposure.     

Fertilisation, embryogenesis and larval development in the tropical intertidal sand dollar Arachnoides placenta in response to reduced seawater pH

We examined the response of the tropical sand dollar Arachnoides placenta to reduced seawater pH in experiments spanning ca. 50 % of the planktonic larval duration. A. placenta inhabits intertidal sandy beaches where we observed a minimum in situ pH range 0.06 pH units (pH 8.10–8.16). The responses of gametes and larvae to seawater pH were tested in vitro in ambient (pH 8.14, pCO₂ = 525.7 μatm, total alkalinity = 2,651 μmol kg soln^?1) and three reduced pH seawater treatments (7.8–7.0). Percentage fertilisation decreased significantly with decreasing pH across a range of sperm/egg ratios (4:1 up to 4,000:1). A. placenta reached the advanced pluteus stage in 4 days, and during this time, we saw no difference in survival rate of larvae between the ambient (67 %) and pH 7.79 (72 %) treatments. Four-day survival was, however, reduced to 44 and 11 % in the pH 7.65 and 7.12 treatments, respectively. Larval development and morphometrics varied among pH treatments. Embryos reared in pH 7.12 exhibited arrested development. Larvae reared at pH 7.65 showed delayed development and greater mortality compared with those reared at pH 7.79 and 8.14. When larval morphometrics are compared among larvae of the same size, differences in larval width and total arm length between pH treatments disappear. These results suggest that variation in larval size among the three highest pH treatments at a given time are likely the result of slower development and apparent shrinkage of surviving larvae and not direct changes in larval shape. There were no differences in the percentage inorganic content (a proxy for calcification) in larvae reared in either an ambient or a pH 7.7 treatment. The responses of fertilisation and development to decreased pH/increased pCO₂ in A. placenta are within the range of those reported for other intertidal and subtidal echinoid species from colder latitudes.     

The physiological and molecular responses of larvae from the reef-building coral Pocillopora damicornis exposed to near-future increases in temperature and pCO2

Given the threats of greenhouse gas emissions and a changing climate to marine ecosystems, there is an urgent need to better understand the response of not only adult corals, which are particularly sensitive to environmental changes, but also their larvae, whose mechanisms of acclimation to both temperature increases and ocean acidification are not well understood. Brooded larvae from the reef coral Pocillopora damicornis collected from Nanwan Bay, Southern Taiwan, were exposed to ambient or elevated temperature (25 or 29 °C) and pCO₂ (415 or 635 μatm) in a factorial experiment for 9 days, and a variety of physiological and molecular parameters were measured. Respiration and rubisco protein expression decreased in larvae exposed to elevated temperature, while those incubated at high pCO₂ were larger in size. Collectively, these findings highlight the complex metabolic and molecular responses of this life history stage and the need to integrate our understanding across multiple levels of biological organization. Our results also suggest that for this pocilloporid larval life stage, the impacts of elevated temperature are likely a greater threat under near-future predictions for climate change than ocean acidification.     

Appropriate pCO2 treatments in ocean acidification experiments

Experiments in which organisms are reared in treatments simulating current and future pCO₂ concentrations are critical for ocean acidification (OA) research. The majority of OA exposure experiments use average atmospheric pCO₂ levels as a baseline treatment. We conducted an ecoregion-scale analysis of global carbon chemistry datasets. For many locales, atmospheric pCO₂ levels are not an appropriate characterization of marine carbon chemistry. We argue that atmospheric pCO₂ should be disregarded when setting baseline treatment conditions and experimental design should rely on measurements of carbon chemistry in a study subject’s habitat. As carbon chemistry conditions vary with space and time, we suggest using a range of pCO₂ values as a control rather than a single value. We illustrate this issue with data on the habitat of Euphausia pacifica, which currently lives in waters with a pCO₂ around 900 μatm, a concentration much higher than the current global atmospheric mean.     

Effects of pCO2 on physiology and skeletal mineralogy in a tidal pool coralline alga Corallina elongata

Marine organisms inhabiting environments where pCO₂/pH varies naturally are suggested to be relatively resilient to future ocean acidification. To test this hypothesis, the effect of elevated pCO₂ was investigated in the articulated coralline red alga Corallina elongata from an intertidal rock pool on the north coast of Brittany (France), where pCO₂ naturally varied daily between 70 and 1000 μatm. Metabolism was measured on algae in the laboratory after they had been grown for 3 weeks at pCO₂ concentrations of 380, 550, 750 and 1000 μatm. Net and gross primary production, respiration and calcification rates were assessed by measurements of oxygen and total alkalinity fluxes using incubation chambers in the light and dark. Calcite mol % Mg/Ca (mMg/Ca) was analysed in the tips, branches and basal parts of the fronds, as well as in new skeletal structures produced by the algae in the different pCO₂ treatments. Respiration, gross primary production and calcification in light and dark were not significantly affected by increased pCO₂. Algae grown under elevated pCO₂ (550, 750 and 1000 μatm) formed fewer new structures and produced calcite with a lower mMg/Ca ratio relative to those grown under 380 μatm. This study supports the assumption that C. elongata from a tidal pool, where pCO₂ fluctuates over diel and seasonal cycles, is relatively robust to elevated pCO₂ compared to other recently investigated coralline algae.     

Tolerance of Hyas araneus zoea I larvae to elevated seawater PCO2 despite elevated metabolic costs

Early life stages of marine crustaceans respond sensitively to elevated seawater PCO₂. However, the underlying physiological mechanisms have not been studied well. We therefore investigated the effects of elevated seawater PCO₂ on oxygen consumption, dry weight, elemental composition, median developmental time (MDT) and mortality in zoea I larvae of the spider crab Hyas araneus (Svalbard 79°N/11°E; collection, May 2009; hatch, December 2009). At the time of moulting, oxygen consumption rate had reached a steady state level under control conditions. In contrast, elevated seawater PCO₂ caused the metabolic rate to rise continuously leading to a maximum 1.5-fold increase beyond control level a few days before moulting into the second stage (zoea II), followed by a pronounced decrease. Dry weight of larvae reared under high CO₂ conditions was lower than in control larvae at the beginning of the moult cycle, yet this difference had disappeared at the time of moulting. MDT of zoea I varied between 45 ± 1 days under control conditions and 42 ± 2 days under the highest seawater CO₂ concentration. The present study indicates that larval development under elevated seawater PCO₂ levels results in higher metabolic costs during premoulting events in zoea I. However, H. araneus zoea I larvae seem to be able to compensate for higher metabolic costs as larval MDT and survival was not affected by elevated PCO₂ levels.     

Brooded coral larvae differ in their response to high temperature and elevated pCO2 depending on the day of release

To evaluate the effects of temperature and pCO₂ on coral larvae, brooded larvae of Pocillopora damicornis from Nanwan Bay, Taiwan (21°56.179′N, 120°44.85′E), were exposed to ambient (419–470 μatm) and high (604–742 μatm) pCO₂ at ~25 and ~29 °C in two experiments conducted in March 2010 and March 2012. Larvae were sampled from four consecutive lunar days (LD) synchronized with spawning following the new moon, incubated in treatments for 24 h, and measured for respiration, maximum photochemical efficiency of PSII (F _v/F _m), and mortality. The most striking outcome was a strong effect of time (i.e., LD) on larvae performance: respiration was affected by an LD × temperature interaction in 2010 and 2012, as well as an LD × pCO₂ × temperature interaction in 2012; F _v/F _m was affected by LD in 2010 (but not 2012); and mortality was affected by an LD × pCO₂ interaction in 2010, and an LD × temperature interaction in 2012. There were no main effects of pCO₂ in 2010, but in 2012, high pCO₂ depressed metabolic rate and reduced mortality. Therefore, differences in larval performance depended on day of release and resulted in varying susceptibility to future predicted environmental conditions. These results underscore the importance of considering larval brood variation across days when designing experiments. Subtle differences in experimental outcomes between years suggest that transgenerational plasticity in combination with unique histories of exposure to physical conditions can modulate the response of brooded coral larvae to climate change and ocean acidification.     

Effects of ocean acidification on the early developmental stages of the horned turban,Turbo cornutus

To estimate the impact of CO₂-driven ocean acidification on the early life stages of gastropods, the effects of increased partial pressure of seawater carbon dioxide (pCO₂) (800–2,000 μatm) on the early developmental stages and larval shell length of the commercially important gastropod, the horned turban snail, Turbo cornutus were investigated. Increase in experimental seawater pCO₂ had an increasingly negative impact on the early developmental rate; the proportion of embryos or larvae displaying retarded development increased at higher pCO₂. The proportion of embryos that developed to the 4-cell stage at 2 h after fertilization decreased linearly with increasing pCO₂. At ~1,000 μatm pCO₂, retarded development was observed in ~50 % of larvae. No embryos developed to the 4-cell stage at 2,000 μatm pCO₂ within 2 h of fertilization. A similar trend continued until 24–26 h after fertilization; the proportion of larvae attaining veliger stage by 24–26 h also decreased with increasing pCO₂. The shell length of T. cornutus veligers decreased gradually as seawater pCO₂ increased, but markedly decreased in seawater under nearly unsaturated and unsaturated conditions (≤1.04) of the aragonite saturation state (Ω _aragonite). The results indicate that increased pCO₂ seawater has a progressive and acute effect on embryonic and larval T. cornutus, and imply that the extended early developmental period and/or the downsized larval shell produced by ocean acidification will have a negative impact on survival, settlement and recruitment well into the future.     

Elevated CO2 affects the behavior of an ecologically and economically important coral reef fish

We tested the effect of near-future CO₂ levels (≈490, 570, 700, and 960 μatm CO₂) on the olfactory responses and activity levels of juvenile coral trout, Plectropomus leopardus, a piscivorous reef fish that is also one of the most important fisheries species on the Great Barrier Reef, Australia. Juvenile coral trout reared for 4 weeks at 570 μatm CO₂ exhibited similar sensory responses and behaviors to juveniles reared at 490 μatm CO₂ (control). In contrast, juveniles reared at 700 and 960 μatm CO₂ exhibited dramatically altered sensory function and behaviors. At these higher CO₂ concentrations, juveniles became attracted to the odor of potential predators, as has been observed in other reef fishes. They were more active, spent less time in shelter, ventured further from shelter, and were bolder than fish reared at 490 or 570 μatm CO₂. These results demonstrate that behavioral impairment of coral trout is unlikely if pCO₂ remains below 600 μatm; however, at higher levels, there are significant impacts on juvenile performance that are likely to affect survival and energy budgets, with consequences for predator–prey interactions and commercial fisheries.     

Ocean acidification induces budding in larval sea urchins

Ocean acidification (OA), the reduction of ocean pH due to hydration of atmospheric CO₂, is known to affect growth and survival of marine invertebrate larvae. Survival and transport of vulnerable planktonic larval stages play important roles in determining population dynamics and community structures in coastal ecosystems. Here, we show that larvae of the purple urchin, Strongylocentrotus purpuratus, underwent high-frequency budding (release of blastula-like particles) when exposed to elevated pCO₂ level (>700 μatm). Budding was observed in >50 % of the population and was synchronized over short periods of time (~24 h), suggesting this phenomenon may be previously overlooked. Although budding can be a mechanism through which larval echinoids asexually reproduce, here, the released buds did not develop into viable clones. OA-induced budding and the associated reduction in larval size suggest new hypotheses regarding physiological and ecological tradeoffs between short-term benefits (e.g. metabolic savings and predation escape) and long-term costs (e.g. tissue loss and delayed development) in the face of climate change.     

Acid–base regulation in the Dungeness crab (Metacarcinus magister)

Homeostatic regulation allows organisms to secure basic physiological processes in a varying environment. To counteract fluctuations in ambient carbonate system speciation due to elevated seawater pCO₂ (hypercapnia), many aquatic crustaceans excrete/accumulate acid–base equivalents through their gills; however, not much is known about the role of ammonia in this response. The present study investigated the effects of hypercapnia on acid–base and ammonia regulation in the Dungeness crab, Metacarcinus magister on the whole animal and isolated gill levels. Hemolymph pCO₂ and [HCO₃ ^?] increased in M. magister acclimated to elevated pCO₂ (330 Pa), while pH remained stable. Additionally, hemolymph [Na⁺], [Ca²⁺], and [SO₄ ^2?] were significantly increased. When challenged with varying pH during gill perfusion, the pH of the artificial hemolymph remained relatively unchanged. Overall, ammonia production and excretion, as well as oxygen consumption, were reduced in crabs acclimated to elevated pCO₂, demonstrating that either (amino acid) oxidation is reduced in response to this particular stress, or nitrogenous wastes are excreted in an alternative form.     

Elevated pCO2 causes developmental delay in early larval Pacific oysters, Crassostrea gigas

Increasing atmospheric CO₂ equilibrates with surface seawater, elevating the concentration of aqueous hydrogen ions. This process, ocean acidification, is a future and contemporary concern for aquatic organisms, causing failures in Pacific oyster (Crassostrea gigas) aquaculture. This experiment determines the effect of elevated pCO₂ on the early development of C. gigas larvae from a wild Pacific Northwest population. Adults were collected from Friday Harbor, Washington, USA (48°31.7′N, 12°1.1′W) and spawned in July 2011. Larvae were exposed to Ambient (400 μatm CO₂), MidCO₂ (700 μatm), or HighCO₂ (1,000 μatm). After 24 h, a greater proportion of larvae in the HighCO₂ treatment were calcified as compared to Ambient. This unexpected observation is attributed to increased metabolic rate coupled with sufficient energy resources. Oyster larvae raised at HighCO₂ showed evidence of a developmental delay by 3 days post-fertilization, which resulted in smaller larvae that were less calcified.     

Tolerance of juvenile barnacles (Amphibalanus improvisus) to warming and elevated pCO2

We investigated the impacts of warming and elevated pCO₂ on newly settled Amphibalanus improvisus from Kiel Fjord, an estuarine ecosystem characterized by significant natural pCO₂ variability. In two experiments, juvenile barnacles were maintained at two temperature and three pCO₂ levels (20/24 °C, 700–2,140 μatm) for 8 weeks in a batch culture and at four pCO₂ levels (20 °C, 620–2,870 μatm) for 12 weeks in a water flow-through system. Warming as well as elevated pCO₂ hardly affected growth or the condition index of barnacles, although some factor combinations led to temporal significances in enhanced or reduced growth with an increase in pCO₂. While warming increased the shell strength of A. improvisus individuals, elevated pCO₂ had only weak effects. We demonstrate a strong tolerance of juvenile A. improvisus to mean acidification levels of about 1,000 μatm pCO₂ as is already naturally experienced by the investigated barnacle population.     

Egg and early larval stages of Baltic cod, Gadus morhua, are robust to high levels of ocean acidification

The accumulation of carbon dioxide in the atmosphere will lower the pH in ocean waters, a process termed ocean acidification (OA). Despite its potentially detrimental effects on calcifying organisms, experimental studies on the possible impacts on fish remain scarce. While adults will most likely remain relatively unaffected by changes in seawater pH, early life-history stages are potentially more sensitive, due to the lack of gills with specialized ion-regulatory mechanisms. We tested the effects of OA on growth and development of embryos and larvae of eastern Baltic cod, the commercially most important fish stock in the Baltic Sea. Cod were reared from newly fertilized eggs to early non-feeding larvae in 5 different experiments looking at a range of response variables to OA, as well as the combined effect of CO₂ and temperature. No effect on hatching, survival, development, and otolith size was found at any stage in the development of Baltic cod. Field data show that in the Bornholm Basin, the main spawning site of eastern Baltic cod, in situ levels of pCO₂ are already at levels of 1,100 μatm with a pH of 7.2, mainly due to high eutrophication supporting microbial activity and permanent stratification with little water exchange. Our data show that the eggs and early larval stages of Baltic cod seem to be robust to even high levels of OA (3,200 μatm), indicating an adaptational response to CO₂.     

Ocean acidification and warming alter photosynthesis and calcification of the symbiont-bearing foraminifera Marginopora vertebralis

The impact of elevated CO₂ and temperature on photosynthesis and calcification in the symbiont-bearing benthic foraminifer Marginopora vertebralis was studied. Individual specimens of M. vertebralis were collected from Heron Island on the southern Great Barrier Reef (Australia). They were maintained for 5 weeks at different temperatures (28, 32 °C) and pCO₂ (400, 1,000 µatm) levels spanning a range of current and future climate-change scenarios. The photosynthetic capacity of M. vertebralis was measured with O₂ microsensors and a pulse-amplitude-modulated chlorophyll (Chl) fluorometer, in combination with estimates of Chl a and Chl c ₂ concentrations and calcification rates. After 5 weeks, control specimens remained unaltered for all parameters. Chlorophyll a concentrations significantly decreased in the specimens at 1,000 µatm CO₂ for both temperatures, while no change in Chl c ₂ concentration was observed. Photoinhibition was observed under elevated CO₂ and temperature, with a 70–80 % decrease in the maximum quantum yield of PSII. There was no net O₂ production at elevated temperatures in both CO₂ treatments as compared to the control temperature, supporting that temperature has more impact on photosynthesis and O₂ flux than changes in ambient CO₂. Photosynthetic pigment loss and a decrease in photochemical efficiency are thus likely to occur with increased temperature. The elevated CO₂ and high temperature treatment also lead to a reduction in calcification rate (from +0.1 to >?0.1 % day^?1). Thus, both calcification and photosynthesis of the major sediment-producing foraminifer M. vertebralis appears highly vulnerable to elevated temperature and ocean acidification scenarios predicted in climate-change models.     

Effects of seawater pCO2 and temperature on shell growth, shell stability, condition and cellular stress of Western Baltic Sea Mytilus edulis (L.) and Arctica islandica (L.)

Acidification of the World’s oceans may directly impact reproduction, performance and shell formation of marine calcifying organisms. In addition, since shell production is costly and stress in general draws on an organism’s energy budget, shell growth and stability of bivalves should indirectly be affected by environmental stress. The aim of this study was to investigate whether a combination of warming and acidification leads to increased physiological stress (lipofuscin accumulation and mortality) and affects the performance [shell growth, shell breaking force, condition index (C_i)] of young Mytilus edulis and Arctica islandica from the Baltic Sea. We cultured the bivalves in a fully-crossed 2-factorial experimental setup (seawater (sw) pCO₂ levels “low”, “medium” and “high” for both species, temperature levels 7.5, 10, 16, 20 and 25 °C for M. edulis and 7.5, 10 and 16 °C for A. islandica) for 13 weeks in summer. Mytilus edulis and A. islandica appeared to tolerate wide ranges of sw temperature and pCO₂. Lipofuscin accumulation of M. edulis increased with temperature while the C_i decreased, but shell growth of the mussels only sharply decreased while its mortality increased between 20 and 25 °C. In A. islandica, lipofuscin accumulation increased with temperature, whereas the C_i, shell growth and shell breaking force decreased. The pCO₂ treatment had only marginal effects on the measured parameters of both bivalve species. Shell growth of both bivalve species was not impaired by under-saturation of the sea water with respect to aragonite and calcite. Furthermore, independently of water temperatures shell breaking force of both species and shell growth of A. islandica remained unaffected by the applied elevated sw pCO₂ for several months. Only at the highest temperature (25 °C), growth arrest of M. edulis was recorded at the high sw pCO₂ treatment and the C_i of M. edulis was slightly higher at the medium sw pCO₂ treatment than at the low and high sw pCO₂ treatments. The only effect of elevated sw pCO₂ on A. islandica was an increase in lipofuscin accumulation at the high sw pCO₂ treatment compared to the medium sw pCO₂ treatment. Our results show that, despite this robustness, growth of both M. edulis and A. islandica can be reduced if sw temperatures remain high for several weeks in summer. As large body size constitutes an escape from crab and sea star predation, this can make bivalves presumably more vulnerable to predation—with possible negative consequences on population growth. In M. edulis, but not in A. islandica, this effect is amplified by elevated sw pCO₂. We follow that combined effects of elevated sw pCO₂ and ocean warming might cause shifts in future Western Baltic Sea community structures and ecosystem services; however, only if predators or other interacting species do not suffer as strong from these stressors.