Predictable ecological response to rising CO2 of a community of marine phytoplankton

Rising atmospheric CO₂ and ocean acidification are fundamentally altering conditions for life of all marine organisms, including phytoplankton. Differences in CO₂ related physiology between major phytoplankton taxa lead to differences in their ability to take up and utilize CO₂. These differences may cause predictable shifts in the composition of marine phytoplankton communities in response to rising atmospheric CO₂. We report an experiment in which seven species of marine phytoplankton, belonging to four major taxonomic groups (cyanobacteria, chlorophytes, diatoms, and coccolithophores), were grown at both ambient (500 μatm) and future (1,000 μatm) CO₂ levels. These phytoplankton were grown as individual species, as cultures of pairs of species and as a community assemblage of all seven species in two culture regimes (high‐nitrogen batch cultures and lower‐nitrogen semicontinuous cultures, although not under nitrogen limitation). All phytoplankton species tested in this study increased their growth rates under elevated CO₂ independent of the culture regime. We also find that, despite species‐specific variation in growth response to high CO₂, the identity of major taxonomic groups provides a good prediction of changes in population growth and competitive ability under high CO₂. The CO₂‐induced growth response is a good predictor of CO₂‐induced changes in competition (R² > .93) and community composition (R² > .73). This study suggests that it may be possible to infer how marine phytoplankton communities respond to rising CO₂ levels from the knowledge of the physiology of major taxonomic groups, but that these predictions may require further characterization of these traits across a diversity of growth conditions. These findings must be validated in the context of limitation by other nutrients. Also, in natural communities of phytoplankton, numerous other factors that may all respond to changes in CO2, including nitrogen fixation, grazing, and variation in the limiting resource will likely complicate this prediction.     

Adaptive evolution in the coccolithophore Gephyrocapsa oceanica following 1,000 generations of selection under elevated CO2

Coccolithophores are important oceanic primary producers not only in terms of photosynthesis but also because they produce calcite plates called coccoliths. Ongoing ocean acidification associated with changing seawater carbonate chemistry may impair calcification and other metabolic functions in coccolithophores. While short‐term ocean acidification effects on calcification and other properties have been examined in a variety of coccolithophore species, long‐term adaptive responses have scarcely been documented, other than for the single species Emiliania huxleyi. Here, we investigated the effects of ocean acidification on another ecologically important coccolithophore species, Gephyrocapsa oceanica, following 1,000 generations of growth under elevated CO₂ conditions (1,000 μatm). High CO₂‐selected populations exhibited reduced growth rates and enhanced particulate organic carbon (POC) and nitrogen (PON) production, relative to populations selected under ambient CO₂ (400 μatm). Particulate inorganic carbon (PIC) and PIC/POC ratios decreased progressively throughout the selection period in high CO₂‐selected cell lines. All of these trait changes persisted when high CO₂‐grown populations were moved back to ambient CO₂ conditions for about 10 generations. The results suggest that the calcification of some coccolithophores may be more heavily impaired by ocean acidification than previously predicted based on short‐term studies, with potentially large implications for the ocean's carbon cycle under accelerating anthropogenic influences.     

Elevated CO2 affects embryonic development and larval phototaxis in a temperate marine fish

As an effect of anthropogenic CO₂ emissions, the chemistry of the world's oceans is changing. Understanding how this will affect marine organisms and ecosystems are critical in predicting the impacts of this ongoing ocean acidification. Work on coral reef fishes has revealed dramatic effects of elevated oceanic CO₂ on sensory responses and behavior. Such effects may be widespread but have almost exclusively been tested on tropical reef fishes. Here we test the effects elevated CO₂ has on the reproduction and early life history stages of a temperate coastal goby with paternal care by allowing goby pairs to reproduce naturally in an aquarium with either elevated (ca 1400 μatm) CO₂ or control seawater (ca 370 μatm CO₂). Elevated CO₂ did not affect the occurrence of spawning nor clutch size, but increased embryonic abnormalities and egg loss. Moreover, we found that elevated CO₂ significantly affected the phototactic response of newly hatched larvae. Phototaxis is a vision‐related fundamental behavior of many marine fishes, but has never before been tested in the context of ocean acidification. Our findings suggest that ocean acidification affects embryonic development and sensory responses in temperate fishes, with potentially important implications for fish recruitment.     

Decreased photosynthesis and growth with reduced respiration in the model diatom Phaeodactylum tricornutum grown under elevated CO2 over 1800 generations

Studies on the long‐term responses of marine phytoplankton to ongoing ocean acidification (OA) are appearing rapidly in the literature. However, only a few of these have investigated diatoms, which is disproportionate to their contribution to global primary production. Here we show that a population of the model diatom Phaeodactylum tricornutum, after growing under elevated CO₂ (1000 μatm, HCL, pH_T: 7.70) for 1860 generations, showed significant differences in photosynthesis and growth from a population maintained in ambient CO₂ and then transferred to elevated CO₂ for 20 generations (HC). The HCL population had lower mitochondrial respiration, than did the control population maintained in ambient CO₂ (400 μatm, LCL, pH_T: 8.02) for 1860 generations. Although the cells had higher respiratory carbon loss within 20 generations under the elevated CO₂, being consistent to previous findings, they downregulated their respiration to sustain their growth in longer duration under the OA condition. Responses of phytoplankton to OA may depend on the timescale for which they are exposed due to fluctuations in physiological traits over time. This study provides the first evidence that populations of the model species, P. tricornutum, differ phenotypically from each other after having been grown for differing spans of time under OA conditions, suggesting that long‐term changes should be measured to understand responses of primary producers to OA, especially in waters with diatom‐dominated phytoplankton assemblages.     

Calcification is not the Achilles’ heel of cold‐water corals in an acidifying ocean

Ocean acidification is thought to be a major threat to coral reefs: laboratory evidence and CO₂ seep research has shown adverse effects on many coral species, although a few are resilient. There are concerns that cold‐water corals are even more vulnerable as they live in areas where aragonite saturation (Ω_ara) is lower than in the tropics and is falling rapidly due to CO₂ emissions. Here, we provide laboratory evidence that net (gross calcification minus dissolution) and gross calcification rates of three common cold‐water corals, Caryophyllia smithii, Dendrophyllia cornigera, and Desmophyllum dianthus, are not affected by pCO₂ levels expected for 2100 (pCO₂ 1058 μatm, Ω_ara 1.29), and nor are the rates of skeletal dissolution in D. dianthus. We transplanted D. dianthus to 350 m depth (pH_T 8.02; pCO₂ 448 μatm, Ω_ara 2.58) and to a 3 m depth CO₂ seep in oligotrophic waters (pH_T 7.35; pCO₂ 2879 μatm, Ω_ara 0.76) and found that the transplants calcified at the same rates regardless of the pCO₂ confirming their resilience to acidification, but at significantly lower rates than corals that were fed in aquaria. Our combination of field and laboratory evidence suggests that ocean acidification will not disrupt cold‐water coral calcification although falling aragonite levels may affect other organismal physiological and/or reef community processes.     

Increased CO2 stimulates reproduction in a coral reef fish

Ocean acidification is predicted to negatively impact the reproduction of many marine species, either by reducing fertilization success or diverting energy from reproductive effort. While recent studies have demonstrated how ocean acidification will affect larval and juvenile fishes, little is known about how increasing partial pressure of carbon dioxide (pCO₂) and decreasing pH might affect reproduction in adult fishes. We investigated the effects of near‐future levels of pCO₂ on the reproductive performance of the cinnamon anemonefish, Amphiprion melanopus, from the Great Barrier Reef, Australia. Breeding pairs were held under three CO₂ treatments [Current‐day Control (430 μatm), Moderate (584 μatm) and High (1032 μatm)] for a 9‐month period that included the summer breeding season. Unexpectedly, increased CO₂ dramatically stimulated breeding activity in this species of fish. Over twice as many pairs bred in the Moderate (67% of pairs) and High (55%) compared to the Control (27%) CO₂ treatment. Pairs in the High CO₂ group produced double the number of clutches per pair and 67% more eggs per clutch compared to the Moderate and Control groups. As a result, reproductive output in the High group was 82% higher than that in the Control group and 50% higher than that in the Moderate group. Despite the increase in reproductive activity, there was no difference in adult body condition among the three treatment groups. There was no significant difference in hatchling length between the treatment groups, but larvae from the High CO₂ group had smaller yolks than Controls. This study provides the first evidence of the potential effects of ocean acidification on key reproductive attributes of marine fishes and, contrary to expectations, demonstrates an initially stimulatory (hormetic) effect in response to increased pCO₂. However, any long‐term consequences of increased reproductive effort on individuals or populations remain to be determined.     

Ocean acidification and temperature increase impact mussel shell shape and thickness: problematic for protection?

Ocean acidification threatens organisms that produce calcium carbonate shells by potentially generating an under‐saturated carbonate environment. Resultant reduced calcification and growth, and subsequent dissolution of exoskeletons, would raise concerns over the ability of the shell to provide protection for the marine organism under ocean acidification and increased temperatures. We examined the impact of combined ocean acidification and temperature increase on shell formation of the economically important edible mussel Mytilus edulis. Shell growth and thickness along with a shell thickness index and shape analysis were determined. The ability of M. edulis to produce a functional protective shell after 9 months of experimental culture under ocean acidification and increasing temperatures (380, 550, 750, 1000 μatm pCO ₂, and 750, 1000 μatm pCO ₂ + 2°C) was assessed. Mussel shells grown under ocean acidification conditions displayed significant reductions in shell aragonite thickness, shell thickness index, and changes to shell shape (750, 1000 μatm pCO ₂) compared to those shells grown under ambient conditions (380 μatm pCO ₂). Ocean acidification resulted in rounder, flatter mussel shells with thinner aragonite layers likely to be more vulnerable to fracture under changing environments and predation. The changes in shape presented here could present a compensatory mechanism to enhance protection against predators and changing environments under ocean acidification when mussels are unable to grow thicker shells. Here, we present the first assessment of mussel shell shape to determine implications for functional protection under ocean acidification.     

Transgenerational effects alleviate severe fecundity loss during ocean acidification in a ubiquitous planktonic copepod

Ocean acidification (OA) caused by anthropogenic CO₂ emission is projected for thousands of years to come, and significant effects are predicted for many marine organisms. While significant evolutionary responses are expected during such persistent environmental change, most studies consider only short‐term effects. Little is known about the transgenerational effects of parental environments or natural selection on the capacity of populations to counter detrimental OA effects. In this study, six laboratory populations of the calanoid copepod Pseudocalanus acuspes were established at three different CO₂ partial pressures (pCO₂ of 400, 900 and 1550 μatm) and grown for two generations at these conditions. Our results show evidence of alleviation of OA effects as a result of transgenerational effects in P. acuspes. Second generation adults showed a 29% decrease in fecundity at 900 μatm CO₂ compared to 400 μatm CO₂. This was accompanied by a 10% increase in metabolic rate indicative of metabolic stress. Reciprocal transplant tests demonstrated that this effect was reversible and the expression of phenotypic plasticity. Furthermore, these tests showed that at a pCO₂ exceeding the natural range experienced by P. acuspes (1550 μatm), fecundity would have decreased by as much as 67% compared to at 400 μatm CO₂ as a result of this plasticity. However, transgenerational effects partly reduced OA effects so that the loss of fecundity remained at a level comparable to that at 900 μatm CO₂. This also relieved the copepods from metabolic stress, and respiration rates were lower than at 900 μatm CO₂. These results highlight the importance of tests for transgenerational effects to avoid overestimation of the effects of OA.     

End of the Century pCO2 Levels Do Not Impact Calcification in Mediterranean Cold-Water Corals

Ocean acidification caused by anthropogenic uptake of CO₂ is perceived to be a major threat to calcifying organisms. Cold-water corals were thought to be strongly affected by a decrease in ocean pH due to their abundance in deep and cold waters which, in contrast to tropical coral reef waters, will soon become corrosive to calcium carbonate. Calcification rates of two Mediterranean cold-water coral species, Lophelia pertusa and Madrepora oculata, were measured under variable partial pressure of CO₂ (pCO₂) that ranged between 380 µatm for present-day conditions and 930 µatm for the end of the century. The present study addressed both short- and long-term responses by repeatedly determining calcification rates on the same specimens over a period of 9 months. Besides studying the direct, short-term response to elevated pCO₂ levels, the study aimed to elucidate the potential for acclimation of calcification of cold-water corals to ocean acidification. Net calcification of both species was unaffected by the levels of pCO₂ investigated and revealed no short-term shock and, therefore, no long-term acclimation in calcification to changes in the carbonate chemistry. There was an effect of time during repeated experiments with increasing net calcification rates for both species, however, as this pattern was found in all treatments, there is no indication that acclimation of calcification to ocean acidification occurred. The use of controls (initial and ambient net calcification rates) indicated that this increase was not caused by acclimation in calcification response to higher pCO₂. An extrapolation of these data suggests that calcification of these two cold-water corals will not be affected by the pCO₂ level projected at the end of the century.     

Synergistic effects of ocean acidification and warming on overwintering pteropods in the Arctic

Ocean acidification and warming will be most pronounced in the Arctic Ocean. Aragonite shell‐bearing pteropods in the Arctic are expected to be among the first species to suffer from ocean acidification. Carbonate undersaturation in the Arctic will first occur in winter and because this period is also characterized by low food availability, the overwintering stages of polar pteropods may develop into a bottleneck in their life cycle. The impacts of ocean acidification and warming on growth, shell degradation (dissolution), and mortality of two thecosome pteropods, the polar Limacina helicina and the boreal L. retroversa, were studied for the first time during the Arctic winter in the Kongsfjord (Svalbard). The abundance of L. helicina and L. retroversa varied from 23.5 to 120 ind m^?2 and 12 to 38 ind m^?2, and the mean shell size ranged from 920 to 981 μm and 810 to 823 μm, respectively. Seawater was aragonite‐undersaturated at the overwintering depths of pteropods on two out of ten days of our observations. A 7‐day experiment [temperature levels: 2 and 7 °C, pCO₂ levels: 350, 650 (only for L. helicina) and 880 μatm] revealed a significant pCO₂ effect on shell degradation in both species, and synergistic effects between temperature and pCO₂ for L. helicina. A comparison of live and dead specimens kept under the same experimental conditions indicated that both species were capable of actively reducing the impacts of acidification on shell dissolution. A higher vulnerability to increasing pCO₂ and temperature during the winter season is indicated compared with a similar study from fall 2009. Considering the species winter phenology and the seasonal changes in carbonate chemistry in Arctic waters, negative climate change effects on Arctic thecosomes are likely to show up first during winter, possibly well before ocean acidification effects become detectable during the summer season.     

Odor tracking in sharks is reduced under future ocean acidification conditions

Recent studies show that ocean acidification impairs sensory functions and alters the behavior of teleost fishes. If sharks and other elasmobranchs are similarly affected, this could have significant consequences for marine ecosystems globally. Here, we show that projected future CO₂ levels impair odor tracking behavior of the smooth dogfish (Mustelus canis). Adult M. canis were held for 5 days in a current‐day control (405 ± 26 μatm) and mid (741 ± 22 μatm) or high CO₂ (1064 ± 17 μatm) treatments consistent with the projections for the year 2100 on a ‘business as usual’ scenario. Both control and mid CO₂‐treated individuals maintained normal odor tracking behavior, whereas high CO₂‐treated sharks significantly avoided the odor cues indicative of food. Control sharks spent >60% of their time in the water stream containing the food stimulus, but this value fell below 15% in high CO₂‐treated sharks. In addition, sharks treated under mid and high CO₂ conditions reduced attack behavior compared to the control individuals. Our findings show that shark feeding could be affected by changes in seawater chemistry projected for the end of this century. Understanding the effects of ocean acidification on critical behaviors, such as prey tracking in large predators, can help determine the potential impacts of future ocean acidification on ecosystem function.     

Lessons from two high CO2 worlds – future oceans and intensive aquaculture

Exponentially rising CO₂ (currently ~400 μatm) is driving climate change and causing acidification of both marine and freshwater environments. Physiologists have long known that CO₂ directly affects acid–base and ion regulation, respiratory function and aerobic performance in aquatic animals. More recently, many studies have demonstrated that elevated CO₂ projected for end of this century (e.g. 800–1000 μatm) can also impact physiology, and have substantial effects on behaviours linked to sensory stimuli (smell, hearing and vision) both having negative implications for fitness and survival. In contrast, the aquaculture industry was farming aquatic animals at CO₂ levels that far exceed end‐of‐century climate change projections (sometimes >10 000 μatm) long before the term ‘ocean acidification’ was coined, with limited detrimental effects reported. It is therefore vital to understand the reasons behind this apparent discrepancy. Potential explanations include 1) the use of ‘control’ CO₂ levels in aquaculture studies that go beyond 2100 projections in an ocean acidification context; 2) the relatively benign environment in aquaculture (abundant food, disease protection, absence of predators) compared to the wild; 3) aquaculture species having been chosen due to their natural tolerance to the intensive conditions, including CO₂ levels; or 4) the breeding of species within intensive aquaculture having further selected traits that confer tolerance to elevated CO₂. We highlight this issue and outline the insights that climate change and aquaculture science can offer for both marine and freshwater settings. Integrating these two fields will stimulate discussion on the direction of future cross‐disciplinary research. In doing so, this article aimed to optimize future research efforts and elucidate effective mitigation strategies for managing the negative impacts of elevated CO₂ on future aquatic ecosystems and the sustainability of fish and shellfish aquaculture.     

Seaweed fails to prevent ocean acidification impact on foraminifera along a shallow‐water CO2 gradient

Ocean acidification causes biodiversity loss, alters ecosystems, and may impact food security, as shells of small organisms dissolve easily in corrosive waters. There is a suggestion that photosynthetic organisms could mitigate ocean acidification on a local scale, through seagrass protection or seaweed cultivation, as net ecosystem organic production raises the saturation state of calcium carbonate making seawater less corrosive. Here, we used a natural gradient in calcium carbonate saturation, caused by shallow‐water CO₂ seeps in the Mediterranean Sea, to assess whether seaweed that is resistant to acidification (Padina pavonica) could prevent adverse effects of acidification on epiphytic foraminifera. We found a reduction in the number of species of foraminifera as calcium carbonate saturation state fell and that the assemblage shifted from one dominated by calcareous species at reference sites (pH ~8.19) to one dominated by agglutinated foraminifera at elevated levels of CO₂ (pH ~7.71). It is expected that ocean acidification will result in changes in foraminiferal assemblage composition and agglutinated forms may become more prevalent. Although Padina did not prevent adverse effects of ocean acidification, high biomass stands of seagrass or seaweed farms might be more successful in protecting epiphytic foraminifera.     

Ocean acidification weakens the structural integrity of coralline algae

The uptake of anthropogenic emission of carbon dioxide is resulting in a lowering of the carbonate saturation state and a drop in ocean pH. Understanding how marine calcifying organisms such as coralline algae may acclimatize to ocean acidification is important to understand their survival over the coming century. We present the first long‐term perturbation experiment on the cold‐water coralline algae, which are important marine calcifiers in the benthic ecosystems particularly at the higher latitudes. Lithothamnion glaciale, after three months incubation, continued to calcify even in undersaturated conditions with a significant trend towards lower growth rates with increasing pCO₂. However, the major changes in the ultra‐structure occur by 589 μatm (i.e. in saturated waters). Finite element models of the algae grown at these heightened levels show an increase in the total strain energy of nearly an order of magnitude and an uneven distribution of the stress inside the skeleton when subjected to similar loads as algae grown at ambient levels. This weakening of the structure is likely to reduce the ability of the alga to resist boring by predators and wave energy with severe consequences to the benthic community structure in the immediate future (50 years).     

Efficient CO2 fixation by surface Prochlorococcus in the Atlantic Ocean

Nearly half of the Earth''s surface is covered by the ocean populated by the most abundant photosynthetic organisms on the planet—Prochlorococcus cyanobacteria. However, in the oligotrophic open ocean, the majority of their cells in the top half of the photic layer have levels of photosynthetic pigmentation barely detectable by flow cytometry, suggesting low efficiency of CO₂ fixation compared with other phytoplankton living in the same waters. To test the latter assumption, CO₂ fixation rates of flow cytometrically sorted ¹⁴C-labelled phytoplankton cells were directly compared in surface waters of the open Atlantic Ocean (30°S to 30°N). CO₂ fixation rates of Prochlorococcus are at least 1.5–2.0 times higher than CO₂ fixation rates of the smallest plastidic protists and Synechococcus cyanobacteria when normalised to photosynthetic pigmentation assessed using cellular red autofluorescence. Therefore, our data indicate that in oligotrophic oceanic surface waters, pigment minimisation allows Prochlorococcus cells to harvest plentiful sunlight more effectively than other phytoplankton.     

Effect of ocean acidification on growth and otolith condition of juvenile scup,Stenotomus chrysops

Increasing amounts of atmospheric carbon dioxide (CO₂) from human industrial activities are causing changes in global ocean carbonate chemistry, resulting in a reduction in pH, a process termed “ocean acidification.” It is important to determine which species are sensitive to elevated levels of CO₂ because of potential impacts to ecosystems, marine resources, biodiversity, food webs, populations, and effects on economies. Previous studies with marine fish have documented that exposure to elevated levels of CO₂ caused increased growth and larger otoliths in some species. This study was conducted to determine whether the elevated partial pressure of CO₂ (pCO₂) would have an effect on growth, otolith (ear bone) condition, survival, or the skeleton of juvenile scup, Stenotomus chrysops, a species that supports both important commercial and recreational fisheries. Elevated levels of pCO₂ (1200–2600 μatm) had no statistically significant effect on growth, survival, or otolith condition after 8 weeks of rearing. Field data show that in Long Island Sound, where scup spawn, in situ levels of pCO₂ are already at levels ranging from 689 to 1828 μatm due to primary productivity, microbial activity, and anthropogenic inputs. These results demonstrate that ocean acidification is not likely to cause adverse effects on the growth and survivability of every species of marine fish. X‐ray analysis of the fish revealed a slightly higher incidence of hyperossification in the vertebrae of a few scup from the highest treatments compared to fish from the control treatments. Our results show that juvenile scup are tolerant to increases in seawater pCO_2, possibly due to conditions this species encounters in their naturally variable environment and their well‐developed pH control mechanisms.     

Food availability outweighs ocean acidification effects in juvenile Mytilus edulis: laboratory and field experiments

Ocean acidification is expected to decrease calcification rates of bivalves. Nevertheless, in many coastal areas high pCO₂ variability is encountered already today. Kiel Fjord (Western Baltic Sea) is a brackish (12–20 g kg^?1) and CO₂ enriched habitat, but the blue mussel Mytilus edulis dominates the benthic community. In a coupled field and laboratory study we examined the annual pCO₂ variability in this habitat and the combined effects of elevated pCO₂ and food availability on juvenile M. edulis growth and calcification. In the laboratory experiment, mussel growth and calcification were found to chiefly depend on food supply, with only minor impacts of pCO₂ up to 3350 μatm. Kiel Fjord was characterized by strong seasonal pCO₂ variability. During summer, maximal pCO₂ values of 2500 μatm were observed at the surface and >3000 μatm at the bottom. However, the field growth experiment revealed seven times higher growth and calcification rates of M. edulis at a high pCO₂ inner fjord field station (mean pCO₂ ca. 1000 μatm) in comparison to a low pCO₂ outer fjord station (ca. 600 μatm). In addition, mussels were able to out‐compete the barnacle Amphibalanus improvisus at the high pCO₂ site. High mussel productivity at the inner fjord site was enabled by higher particulate organic carbon concentrations. Kiel Fjord is highly impacted by eutrophication, which causes bottom water hypoxia and consequently high seawater pCO₂. At the same time, elevated nutrient concentrations increase the energy availability for filter feeding organisms such as mussels. Thus, M. edulis can dominate over a seemingly more acidification resistant species such as A. improvisus. We conclude that benthic stages of M. edulis tolerate high ambient pCO₂ when food supply is abundant and that important habitat characteristics such as species interactions and energy availability need to be considered to predict species vulnerability to ocean acidification.     

Response to ocean acidification in larvae of a large tropical marine fish,Rachycentron canadum

Currently, ocean acidification is occurring at a faster rate than at any time in the last 300 million years, posing an ecological challenge to marine organisms globally. There is a critical need to understand the effects of acidification on the vulnerable larval stages of marine fishes, as there is potential for large ecological and economic impacts on fish populations and the human economies that rely on them. We expand upon the narrow taxonomic scope found in the literature today, which overlooks many life history characteristics of harvested species, by reporting on the larvae of Rachycentron canadum (cobia), a large, highly mobile, pelagic‐spawning, widely distributed species with a life history and fishery value contrasting other species studied to date. We raised larval cobia through the first 3 weeks of ontogeny under conditions of predicted future ocean acidification to determine effects on somatic growth, development, otolith formation, swimming ability, and swimming activity. Cobia exhibited resistance to treatment effects on growth, development, swimming ability, and swimming activity at 800 and 2100 μatm pCO₂. However, these scenarios resulted in a significant increase in otolith size (up to 25% larger area) at the lowest pCO₂ levels reported to date, as well as the first report of significantly wider daily otolith growth increments. When raised under more extreme scenarios of 3500 and 5400 μatm pCO₂, cobia exhibited significantly reduced size‐at‐age (up to 25% smaller) and a 2–3 days developmental delay. The robust nature of cobia may be due to the naturally variable environmental conditions this species currently encounters throughout ontogeny in coastal environments, which may lead to an increased acclimatization ability even during long‐term exposure to stressors.     

The Response of Antarctic Sea Ice Algae to Changes in pH and CO2

Ocean acidification substantially alters ocean carbon chemistry and hence pH but the effects on sea ice formation and the CO₂ concentration in the enclosed brine channels are unknown. Microbial communities inhabiting sea ice ecosystems currently contribute 10–50% of the annual primary production of polar seas, supporting overwintering zooplankton species, especially Antarctic krill, and seeding spring phytoplankton blooms. Ocean acidification is occurring in all surface waters but the strongest effects will be experienced in polar ecosystems with significant effects on all trophic levels. Brine algae collected from McMurdo Sound (Antarctica) sea ice was incubated in situ under various carbonate chemistry conditions. The carbon chemistry was manipulated with acid, bicarbonate and bases to produce a pCO₂ and pH range from 238 to 6066 µatm and 7.19 to 8.66, respectively. Elevated pCO₂ positively affected the growth rate of the brine algal community, dominated by the unique ice dinoflagellate, Polarella glacialis. Growth rates were significantly reduced when pH dropped below 7.6. However, when the pH was held constant and the pCO₂ increased, growth rates of the brine algae increased by more than 20% and showed no decline at pCO₂ values more than five times current ambient levels. We suggest that projected increases in seawater pCO₂, associated with OA, will not adversely impact brine algal communities.