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Contaminants and Pollution: Ocean Acidification

• Matozzo, V., Chinellato, A., Munari, M., Finos, L., Bressan, M., and Marin, M.G.  First evidence of immunomodulation in bivalves under seawater acidification and increased temperature.  PLoS ONE 7(3): art. e33820, 2012.
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  Water acidification, temperature increases and changes in seawater salinity are predicted to occur in the near future. In such a global climate change (GCC) scenario, there is growing concern for the health status of both wild and farmed organisms. Bivalve molluscs, an important component of coastal marine ecosystems, are at risk. At the immunological level, the ability of an organism to maintain its immunosurveillance unaltered under adverse environmental conditions may enhance its survival capability. To our knowledge, only a few studies have investigated the effects of changing environmental parameters (as predicted in a GCC scenario) on the immune responses of bivalves. In the present study, the effects of both decreased pH values and increased temperature on the important immune parameters of two bivalve species were evaluated for the first time. The clam Chamelea gallina and the mussel Mytilus galloprovincialis, widespread along the coast of the Northwestern Adriatic Sea, were chosen as model organisms. Bivalves were exposed for 7 days to three pH values (8.1, 7.7 and 7.4) at two temperatures (22 and 28°C). Three independent experiments were carried out at salinities of 28, 34 and 40 PSU. The total haemocyte count, Neutral Red uptake, haemolymph lysozyme activity and total protein levels were measured. The results obtained demonstrated that tested experimental conditions affected significantly most of the immune parameters measured in bivalves, even if the variation pattern of haemocyte responses was not always linear. Between the two species, C. gallina appeared more vulnerable to changing pH and temperature than M. galloprovincialis. Overall, this study demonstrated that climate changes can strongly affect haemocyte functionality in bivalves. However, further studies are needed to clarify better the mechanisms of action of changing environmental parameters, both individually and in combination, on bivalve haemocytes.

• Arnold, T., Mealey, C., Leahey, H., Miller, A.W., Hall-Spencer, J.M., Milazzo, M., and Maers, K.  Ocean acidification and the loss of phenolic substances in marine plants.  PLoS ONE 7(4): art. e35107, 2012.
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  Rising atmospheric CO₂ often triggers the production of plant phenolics, including many that serve as herbivore deterrents, digestion reducers, antimicrobials, or ultraviolet sunscreens. Such responses are predicted by popular models of plant defense, especially resource availability models which link carbon availability to phenolic biosynthesis. CO₂ availability is also increasing in the oceans, where anthropogenic emissions cause ocean acidification, decreasing seawater pH and shifting the carbonate system towards further CO₂ enrichment. Such conditions tend to increase seagrass productivity but may also increase rates of grazing on these marine plants. Here we show that high CO₂ / low pH conditions of OA decrease, rather than increase, concentrations of phenolic protective substances in seagrasses and eurysaline marine plants. We observed a loss of simple and polymeric phenolics in the seagrass Cymodocea nodosa near a volcanic CO₂ vent on the Island of Vulcano, Italy, where pH values decreased from 8.1 to 7.3 and pCO₂ concentrations increased ten-fold. We observed similar responses in two estuarine species, Ruppia maritima and Potamogeton perfoliatus, in in situ Free-Ocean-Carbon-Enrichment experiments conducted in tributaries of the Chesapeake Bay, USA. These responses are strikingly different than those exhibited by terrestrial plants. The loss of phenolic substances may explain the higher-than-usual rates of grazing observed near undersea CO₂ vents and suggests that ocean acidification may alter coastal carbon fluxes by affecting rates of decomposition, grazing, and disease. Our observations temper recent predictions that seagrasses would necessarily be "winners" in a high CO₂ world.

• Seibel, B.A., Maas, A.E., and Dierssen, H.M.  Energetic plasticity underlies a variable response to ocean acidification in the pteropod, Limacina helicina antarctica.  PLoS ONE 7(4): art.e30464, 2012.
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  Ocean acidification, caused by elevated seawater carbon dioxide levels, may have a deleterious impact on energetic processes in animals. Here we show that high PCO₂ can suppress metabolism, measured as oxygen consumption, in the pteropod, L. helicina forma antarctica, by ~20%. The rates measured at 180–380 µatm (MO₂ = 1.25 M^−0.25, p = 0.007) were significantly higher (ANCOVA, p = 0.004) than those measured at elevated target CO₂ levels in 2007 (789–1000 µatm, = 0.78 M^−0.32, p = 0.0008). However, we further demonstrate metabolic plasticity in response to regional phytoplankton concentration and that the response to CO₂ is dependent on the baseline level of metabolism. We hypothesize that reduced regional Chl a levels in 2008 suppressed metabolism and masked the effect of ocean acidification. This effect of food limitation was not, we postulate, merely a result of gut clearance and specific dynamic action, but rather represents a sustained metabolic response to regional conditions. Thus, pteropod populations may be compromised by climate change, both directly via CO₂-induced metabolic suppression, and indirectly via quantitative and qualitative changes to the phytoplankton community. Without the context provided by long-term observations (four seasons) and a multi-faceted laboratory analysis of the parameters affecting energetics, the complex response of polar pteropods to ocean acidification may be masked or misinterpreted.

• Kaniewska, P., Campbell, P.R., Kline, D.I., Rodriguez-Lanetty, M., Miller, D.J., Dove, S., and Hoegh-Guldberg, O.  Major cellular and physiological impacts of ocean acidification on a reef building coral.  PLoS ONE 7(4): art. e34659, 2012.
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  As atmospheric levels of CO₂ increase, reef-building corals are under greater stress from both increased sea surface temperatures and declining sea water pH. To date, most studies have focused on either coral bleaching due to warming oceans or declining calcification due to decreasing oceanic carbonate ion concentrations. Here, through the use of physiology measurements and cDNA microarrays, we show that changes in pH and ocean chemistry consistent with two scenarios put forward by the Intergovernmental Panel on Climate Change (IPCC) drive major changes in gene expression, respiration, photosynthesis and symbiosis of the coral, Acropora millepora, before affects on biomineralisation are apparent at the phenotype level. Under high CO₂ conditions corals at the phenotype level lost over half their Symbiodinium populations, and had a decrease in both photosynthesis and respiration. Changes in gene expression were consistent with metabolic suppression, an increase in oxidative stress, apoptosis and symbiont loss. Other expression patterns demonstrate upregulation of membrane transporters, as well as the regulation of genes involved in membrane cytoskeletal interactions and cytoskeletal remodeling. These widespread changes in gene expression emphasize the need to expand future studies of ocean acidification to include a wider spectrum of cellular processes, many of which may occur before impacts on calcification.

• Cooley, S.R., Lucey, N., Kite-Powell, Hauke, and Doney, S.C.  Nutrition and income from molluscs today imply vulnerability to ocean acidification tomorrow.  Fish and Fisheries 13(2): 182-215, 2012.
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  Atmospheric carbon dioxide (CO₂) emissions from human industrial activities are causing a progressive alteration of seawater chemistry, termed ocean acidification, which has decreased seawater pH and carbonate ion concentration markedly since the Industrial Revolution. Many marine organisms, like molluscs and corals, build hard shells and skeletons using carbonate ions, and they exhibit negative overall responses to ocean acidification. This adds to other chronic and acute environmental pressures and promotes shifts away from calcifier-rich communities. In this study, we examine the possible implications of ocean acidification on mollusc harvests worldwide by examining present production, consumption and export and by relating those data to present and future surface ocean chemistry forecast by a coupled climate–ocean model (Community Climate System 3.1; CCSM3). We identify the 'transition decade' when future ocean chemistry will distinctly differ from that of today (2010), and when mollusc harvest levels similar to those of the present cannot be guaranteed if present ocean chemistry is a significant determinant of today's mollusc production. We assess nations' vulnerability to ocean acidification-driven decreases in mollusc harvests by comparing nutritional and economic dependences on mollusc harvests, overall societal adaptability, and the amount of time until the transition decade. Projected transition decades for individual countries will occur 10–50 years after 2010. Countries with low adaptability, high nutritional or economic dependence on molluscs, rapidly approaching transition decades or rapidly growing populations will therefore be most vulnerable to ocean acidification-driven mollusc harvest decreases. These transition decades suggest how soon nations should implement strategies, such as increased aquaculture of resilient species, to help maintain current per capita mollusc harvests.

• Narita, D., Rehdanz, K., and Tol, R.  Economic costs of ocean acidification: a look into the impacts on global shellfish production.  Climatic Change 113(3): 1049-1063, 2012.
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  Ocean acidification is increasingly recognized as a major global problem. Yet economic assessments of its effects are currently almost absent. Unlike most other marine organisms, mollusks, which have significant commercial value worldwide, have relatively solid scientific evidence of biological impact of acidification and allow us to make such an economic evaluation. By performing a partial-equilibrium analysis, we estimate global and regional economic costs of production loss of mollusks due to ocean acidification. Our results show that the costs for the world as a whole could be over 100 billion USD with an assumption of increasing demand of mollusks with expected income growths combined with a business-as-usual emission trend towards the year 2100. The major determinants of cost levels are the impacts on the Chinese production, which is dominant in the world, and the expected demand increase of mollusks in today's developing countries, which include China, in accordance with their future income rise. Our results have direct implications for climate policy. Because the ocean acidifies faster than the atmosphere warms, the acidification effects on mollusks would raise the social cost of carbon more strongly than the estimated damage adds to the damage costs of climate change.

• Chen, S. and Gao, K.  Solar ultraviolet radiation and CO₂-induced ocean acidification interacts to influence the photosynthetic performance of the red tide alga Phaeocystis globosa (Prymnesiophyceae).  Hydrobiologia 675(1): 105-117, 2011.
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  Future CO₂-induced ocean acidification may interact with solar UV radiation to affect physiological performance of microalgae. Therefore, CO₂/pH perturbation experiments were carried out under solar radiation with or without UV radiation (295–400 nm) to evaluate the combined effects of seawater acidification (pH 7.7 at 101.3 Pa CO₂) and UV on Phaeocystis globosa that forms harmful algal blooms. Under high levels of solar radiation, the acidification reduced the growth rate and photochemical efficiency either under PAR alone or with the presence of UVR radiation. Under reduced levels of solar radiation (cloudy days), however, the CO₂-enrichment and UVA acted synergistically to stimulate the photochemical yield and enhanced the growth rate. That is, the effects of CO₂-induced acidification were reversed from the negative (sunny days) to positive (cloudy days). CO₂ concentrating mechanism f P. globosa was not affected by the elevated pCO₂ in view of unchanged photosynthetic affinity for CO₂ and stable activity of both intracellular and extracellular carbonic anhydrase. The increased acidity induced higher UVB-related photoinhibition of growth and non-photochemical quenching, and increased the dark respiration and the contents of Chl a, Chl c, and carotenoids, causing the cells to increase their energy demand against the combined stress. Overall, the findings imply that net or balanced effects of ocean acidification on phytoplankton would depend on the depth or mixing that alters the exposures of the cells in water columns to solar radiation.

• Price, N.N., Hamilton, S.L., Tootell, J.S., and Smith, J.E.  Species-specific consequences of ocean acidification for the calcareous tropical green algae Halimeda.  Marine Ecology Progress Series 440: 67-78, 2011.
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  Ocean acidification (OA), resulting from increasing dissolved carbon dioxide (CO₂) in surface waters, is likely to affect many marine organisms, particularly those that calcify. Recent OA studies have demonstrated negative and/or differential effects of reduced pH on growth, development, calcification and physiology, but most of these have focused on taxa other than calcareous benthic macroalgae. Here we investigate the potential effects of OA on one of the most common coral reef macroalgal genera, Halimeda. Species of Halimeda produce a large proportion of the sand in the tropics and are a major contributor to framework development on reefs because of their rapid calcium carbonate production and high turnover rates. On Palmyra Atoll in the central Pacific, we conducted a manipulative bubbling experiment to investigate the potential effects of OA on growth, calcification and photophysiology of 2 species of Halimeda. Our results suggest that Halimeda is highly susceptible to reduced pH and aragonite saturation state but the magnitude of these effects is species specific. H. opuntia suffered net dissolution and 15% reduction in photosynthetic capacity, while H. taenicola did not calcify but did not alter photophysiology in experimental treatments. The disparate responses of these species to elevated CO₂ partial pressure (pCO₂) may be due to anatomical and physiological differences and could represent a shift in their relative dominance in the face of OA. The ability for a species to exert biological control over calcification and the species specific role of the carbonate skeleton may have important implications for the potential effects of OA on ecological function in the future.

• Büdenbender, J., Riebesell, U., and Form, A.  Calcification of the Arctic coralline red algae Lithothamnion glaciale in response to elevated CO₂.  Marine Ecology Progress Series 441: 79-87, 2011.
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  Rising atmospheric CO₂ concentrations could cause a calcium carbonate subsaturation of Arctic surface waters in the next 20 yr, making these waters corrosive for calcareous organisms. It is presently unknown what effects this will have on Arctic calcifying organisms and the ecosystems of which they are integral components. So far, acidification effects on crustose coralline red algae (CCA) have only been studied in tropical and Mediterranean species. In this work, we investigated calcification rates of the CCA Lithothamnion glaciale collected in northwest Svalbard in laboratory experiments under future atmospheric CO₂ concentrations. The algae were exposed to simulated Arctic summer and winter light conditions in 2 separate experiments at optimum growth temperatures. We found a significant negative effect of increased CO₂ levels on the net calcification rates of L. glaciale in both experiments. Annual mean net dissolution of L. glaciale was estimated to start at an aragonite saturation state between 1.1 and 0.9 which is projected to occur in parts of the Arctic surface ocean between 2030 and 2050 if emissions follow 'business as usual' scenarios (SRES A2; IPCC 2007). The massive skeleton of CCA, which consist of more than 80% calcium carbonate, is considered crucial to withstanding natural stresses such as water movement, overgrowth or grazing. The observed strong negative response of this Arctic CCA to increased CO₂ levels suggests severe threats of the projected ocean acidification for an important habitat provider in the Arctic coastal ocean.

• Thresher, R.E., Tilbrook, B., Fallon, S., Wilson, N.C., and Adkins, J.  Effects of chronic low carbonate saturation levels on the distribution, growth and skeletal chemistry of deep-sea corals and other seamount megabenthos.  Marine Ecology Progress Series 442: 87-99, 2011.
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  Ocean acidification has been predicted to reduce the ability of marine organisms to produce carbonate skeletons, threatening their long-term viability and severely impacting marine ecosystems. Corals, as ecosystem engineers, have been identified as particularly vulnerable and important. To determine the sensitivity of corals and allied taxa to long-term exposure to very low carbonate concentrations, we examined the distribution and skeletal characteristics of coral taxa along a natural deep-sea concentration gradient on seamounts of SW Australia. Carbonate under-saturation had little evident effect on the depth distribution, growth or skeletal composition of live scleractinians or gorgonians, with corals growing, often abundantly, in waters as much as 20 to 30% under-saturated. Developmental anomalies in the deepest skeleton-forming anthozoan collected (an isidid gorgonian, at nearly 4 km depth) suggest an absolute low tolerance limit of about 40% under-saturation. Evidence for an effect of acidification on the accumulation of reef structure is ambiguous, with clear indications of dissolution of high-magnesium calcite (HMC) gorgonian skeletons at depths below 2300 m, but also abundant, old scleractinian skeletons well below the aragonite saturation horizon. The latter might be the result of ferromanganese deposition on exposed skeletons, which, however, may render them inhospitable for benthic organisms.

• Landes, A. and Zimmer, M.  Acidification and warming affect both a calcifying predator and prey, but not their interaction.  Marine Ecology Progress Series 450: 1-10, 2012.
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  Both ocean warming and acidification have been demonstrated to affect the growth, performance and reproductive success of calcifying invertebrates. However, relatively little is known regarding how such environmental change may affect interspecific interactions. We separately treated green crabs Carcinus maenas and periwinkles Littorina littorea under conditions that mimicked either ambient conditions (control) or warming and acidification, both separately and in combination, for 5 mo. After 5 mo, the predators, prey and predator-prey interactions were screened for changes in response to environmental change. Acidification negatively affected the closer-muscle length of the crusher chela and correspondingly the claw-strength increment in C. maenas. The effects of warming and/or acidification on L. littorea were less consistent but indicated weaker shells in response to acidification. On the community level, however, we found no evidence that predator-prey interactions will change in the future. Further experiments exploring the impacts of warming and acidification on key ecological interactions are needed instead of basing predictions of ecosystem change solely on species-specific responses to environmental change.

• Comeau, S., Alliouane, S., and Gattuso, J.P.  Effects of ocean acidification on overwintering juvenile Arctic pteropods Limacina helicina.  Marine Ecology Progress Series 456: 279-284, 2012.
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  Pteropods are planktonic mollusks that play an important role in the food web of various ecosystems, particularly at high latitudes. Because they produce an aragonitic shell, pteropods are expected to be very sensitive to ocean acidification driven by anthropogenic CO₂ emissions. The effect of ocean acidification was investigated using juveniles of the Arctic pteropod Limacina helicina from the Canada Basin of the Arctic Ocean. The animals were maintained in 3 controlled pH conditions (total scale pH [pH_T] ≈8.05, 7.90 or 7.75) for 8 d, and their mortality and the linear extension of their shell were monitored. The pH did not impact the mortality rate, but the linear extension of the shell decreased as a function of declining pH. Surprisingly, the pteropods were still able to extend their shell at an aragonite saturation state as low as 0.6. Nevertheless, dissolution marks were visible on the whole shell, indicating that calcium carbonate dissolution had also occurred, casting doubts on the ability of the pteropods to maintain a positive balance between precipitation and dissolution of calcium carbonate under corrosive conditions.

• Appelhans, Y.S., Thomsen, J., Pansch, C., Melzner, F., and Wahl, M.  Sour times: seawater acidification effects on growth, feeding behaviour and acid–base status of Asterias rubens and Carcinus maenas.  Marine Ecology Progress Series 459: 85-98, 2012.
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  The impact of seawater acidification on calcifying organisms varies at the species level. If the impact differs between predator and prey in strength and/or sign, trophic interactions may be altered. In the present study, we investigated the impact of 3 different seawater pCO₂ levels (650, 1250 and 3500 µatm) on the acid-base status or the growth of 2 predatory species, the common sea star Asterias rubens and the shore crab Carcinus maenas, and tested whether the quantity or size of prey consumed is affected. We exposed both the predators and their prey, the blue mussel Mytilus edulis, over a time span of 10 wk and subsequently performed feeding experiments. Intermediate acidification levels had no significant effect on growth or consumption in either predator species. The highest acidification level reduced feeding and growth rates in sea stars by 56%, while in crabs a 41% decrease in consumption rates of mussels could be demonstrated over the 10 wk experimental period but not in the subsequent shorter feeding assays. Because only a few crabs moulted in the experiment, acidification effects on crab growth could not be investigated. Active extracellular pH compensation by means of bicarbonate accumulation was observed in C. maenas, whereas the coelomic fluid pH in A. rubens remained uncompensated. Acidification did not provoke a measurable shift in prey size preferred by either predator. Mussels exposed to elevated pCO₂ were preferred by previously untreated A. rubens but not by C. maenas. The observed effects on species interactions were weak even at the high acidification levels expected in the future in marginal marine habitats such as the Baltic Sea. Our results indicate that when stress effects are similar (and weak) on interacting species, biotic interactions may remain unaffected.

• Vogel, N. and Uthicke, S.  Calcification and photobiology in symbiont-bearing benthic foraminifera and responses to a high CO₂ environment.  Journal of Experimental Marine Biology and Ecology 424-425: 15-24, 2012.
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  The present study investigates impacts of ocean acidification on calcification rates and light responses of large benthic foraminifera (LBF). Studies were conducted on diatom-bearing Amphistegina radiata and Heterostegina depressa and dinoflagellate-bearing Marginopora vertebralis in controls and manipulated seawater pCO₂ conditions (467–1925 μatm pCO₂). In a six week experiment, calcification and photobiology were investigated for all three species. Additionally, short-term experiments were carried out on H. depressa and M. vertebralis to determine photosynthetic rates in several pCO₂ environments and impacts of elevated pCO₂ in increasing light intensities (photosynthesis irradiance "P-I" curves) on M. vertebralis. In the long-term experiment, positive growth (inferred through cross-sectional surface area) was measured in all control and acidification conditions but growth rates of A. radiata and H. depressa were not affected by increased pCO₂ (linear models, p > 0.05). However, M. vertebralis displayed significantly (planned comparison t = 2.61, p < 0.05) increased calcification rates (63%) in elevated pCO₂ regimes. Increased pCO₂ did not affect maximum quantum yield (measured by pulse amplitude modulation "PAM" fluorometry) and chlorophyll a content in any species investigated. Photosynthetic measurements (oxygen evolution) on H. depressa and M. vertebralis revealed positive net production under experimental light conditions (10 and 29 μmol photons m^-2 s^-1, respectively), however no significant effect of elevated pCO₂ on net production and dark respiration after both long- and short-term exposure was observed. M. vertebralis measured under nine different light conditions displayed typical P-I curves with light saturation points of app. 500 μmol photons m^-2 s^-1. However, Pmax, α and Ek did not vary under different pCO₂ conditions (496 and 1925 μatm). Thus, foraminiferal species investigated in the present study did not show negative effects in exposures up to 1925 μatm pCO₂. However, previous field studies from natural CO₂ vents showed that LBF disappear at pCO₂ conditions predicted for the near future (pHTotal < 7.9). This indicates that the short term ability of the holobiont or symbiont to cope or even benefit from elevated p CO₂ is no guarantee for their survival in the long-term.

• Dupont, S. and Thorndyke, M.  Relationship between CO₂-driven changes in extracellular acid-base balance and cellular immune response in two polar echinoderm species.  Journal of Experimental Marine Biology and Ecology 424-425: 32-37, 2012.
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  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. However, little is known on the impact of elevated pCO₂ on immune-response. Here we investigate the impact of short-term (5–7 days) exposure to elevated pCO₂ (1275 μatm compared to 350 μatm in the control) on extracellular pH (pHe) and cellular immune response in two polar echinoderm species, the green sea urchin Strongylocentrotus droebachiensis and the seastar Leptasterias polaris. Both species experienced extracellular acidosis following short term exposure to elevated pCO₂. While this acidosis remained uncompensated within 7 days for L. polaris, pHe was fully compensated after 5 days for S. droebachiensis. For both species, coelomic fluid acidosis was associated with an increase in total coelomocyte number and a reduction in vibratile cells in S. droebachiensis. A relationship between pHe and phagocyte numbers was observed in S. droebachiensis suggesting a direct link between pHe and cellular immune-response. Further studies would require the coordinated effort of ecologists and immunologists to understand the role of elevated pCO₂ on the host-pathogen interactions that are involved in the stability of ecosystems.

• Range, P., Piló, D., Ben Hamadou, R., Chícharo, M.A., Matias, D., Joaquim, S., Oliveira, A.P., and Chícharo, L.  Seawater acidification by CO₂ in a coastal lagoon environment: effects on life history traits of juvenile mussels Mytilus galloprovincialis.  Journal of Experimental Marine Biology and Ecology 424-425: 89-98, 2012. .
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  The carbonate chemistry of seawater from the Ria Formosa lagoon was experimentally manipulated, by diffusing pure CO₂, to attain two reduced pH levels, by -0.3 and -0.6 pH units, relative to unmanipulated seawater. After 84 days of exposure, no differences were detected in terms of growth (somatic or shell) or mortality of juvenile mussels Mytilus galloprovincialis. The naturally elevated total alkalinity of the seawater (≈3550 µmol kg^-1) prevented under-saturation of CaCO₃, even under pCO₂ values exceeding 4000 µatm, attenuating the detrimental effects on the carbonate supply-side. Even so, variations in shell weight showed that net calcification was reduced under elevated CO₂ and reduced pH, although the magnitude and significance of this effect varied among size-classes. Most of the loss of shell material probably occurred as post-deposition dissolution in the internal aragonitic nacre layer. Our results show that, even when reared under extreme levels of CO₂-induced acidification, juvenile M. galloprovincialis can continue to calcify and grow in this coastal lagoon environment. The complex responses of bivalves to ocean acidification suggest a large degree of interspecific and intraspecific variability in their sensitivity to this type of perturbation. Further research is needed to assess the generality of these patterns and to disentangle the relative contributions of acclimation to local variations in seawater chemistry and genetic adaptation.

• Albright, R., Bland, C., Gillette, P., Serafy, J.E., Langdon, C., and Capo, T.R.  Juvenile growth of the tropical sea urchin Lytechinus variegatus exposed to near-future ocean acidification scenarios.  Journal of Experimental Marine Biology and Ecology 426-427: 12-17, 2012.
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  To evaluate the effect of elevated pCO₂ exposure on the juvenile growth of the sea urchin Lytechinus variegatus, we reared individuals for 3 months in one of three target pCO₂ levels: ambient seawater (380 µatm) and two scenarios that are projected to occur by the middle (560 µatm) and end (800 µatm) of this century. At the end of 89 days, urchins reared at ambient pCO₂ weighed 12% more than those reared at 560 µatm and 28% more than those reared at 800 µatm. Skeletons were analyzed using scanning electron microscopy, revealing degradation of spines in urchins reared at elevated pCO₂ (800 µatm). Our results indicate that elevated pCO₂ levels projected to occur this century may adversely affect the development of juvenile sea urchins. Acidification-induced changes to juvenile urchin development would likely impair performance and functioning of juvenile stages with implications for adult populations.

• Weydmann, A., Søreide, J.E., Kwasniewski, S., and Widdicombe, S.  Influence of CO₂-induced acidification on the reproduction of a key Arctic copepod Calanus glacialis.  Journal of Experimental Marine Biology and Ecology 428: 39-42, 2012.
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  The Arctic Ocean is facing rapid changes in seawater carbonate chemistry due to the uptake of atmospheric carbon dioxide (CO₂). In the current study, the effects of different seawater pH levels (8.2, 7.6 and 6.9) on the reproduction of Calanus glacialis, an Arctic shelf-water copepod, have been quantified. Results indicated that CO₂-induced seawater acidification had no significant effect on C. glacialis egg production. However, a reduction in pH to 6.9 significantly delayed hatching and possibly reduced overall hatching success. The results of the current study are in agreement with previous studies on other copepod species and would indicate that copepods, as a group, may be well equipped to deal with the chemical changes associated with ocean acidification. However, all previous studies have been over relatively short exposure periods and most have only considered the isolated impacts of elevated CO₂. Long-term exposures examining the synergistic effects of ocean acidification with other climate stressors, particularly warming on population viability and success, have yet to be conducted.

• Unsworth, R.K.F., Collier, C.J., Henderson, G.M., and McKenzie, L.J.  Tropical seagrass meadows modify seawater carbon chemistry: implications for coral reefs impacted by ocean acidification.  Environmental Research Letters 7(2): art. 024026, 2012.
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  Highly productive tropical seagrasses often live adjacent to or among coral reefs and utilize large amounts of inorganic carbon. In this study, the effect of seagrass productivity on seawater carbonate chemistry and coral calcification was modelled on the basis of an analysis of published data. Published data (11 studies, 64 records) reveal that seagrass meadows in the Indo-Pacific have an 83% chance of being net autotrophic, resulting in an average net sink of 155 gC m^-2 yr^-1. The capacities for seagrass productivity were analysed using an empirical model to examine the effect on seawater carbonate chemistry. Our analyses indicate that increases in pH of up to 0.38 units, and Ω_arag increases of 2.9 are possible in the presence of seagrass meadows (compared to their absence) with the precise values of these increases dependent on water residence time (tidal flushing) and water depth. In shallow water reef environments, Scleractinian coral calcification downstream of seagrass has the potential to be ≈18% greater than in an environment without seagrass. If this potential benefit to reef calcifiers is supported by further study it offers a potential tool in marine park management at a local scale. The applicability of this will depend upon local physical conditions as well as the spatial configuration of habitats, and the factors that influence their productivity. This novel study suggests that, in addition to their importance to fisheries, sediment stabilization and primary production, seagrass meadows may enhance coral reef resilience to future ocean acidification.

• Feely, R.A., Sabine, C.L., Byrne, R.H., Millero, F.J., Dickson, A.G., Wanninkhof, R., Murata, A., Miller, L.A., and Greeley, D.  Decadal changes in the aragonite and calcite saturation state of the Pacific Ocean.  Global Biogeochemical Cycles 26(3): GB3001, 2012.
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  Based on measurements from the WOCE/JGOFS global CO₂ survey, the CLIVAR/CO₂ Repeat Hydrography Program and the Canadian Line P survey, we have observed an average decrease of 0.34% yr^-1 in the saturation state of surface seawater in the Pacific Ocean with respect to aragonite and calcite. The upward migrations of the aragonite and calcite saturation horizons, averaging about 1 to 2 m yr^-1, are the direct result of the uptake of anthropogenic CO₂ by the oceans and regional changes in circulation and biogeochemical processes. The shoaling of the saturation horizon is regionally variable, with more rapid shoaling in the South Pacific where there is a larger uptake of anthropogenic CO₂. In some locations, particularly in the North Pacific Subtropical Gyre and in the California Current, the decadal changes in circulation can be the dominant factor in controlling the migration of the saturation horizon. If CO₂ emissions continue as projected over the rest of this century, the resulting changes in the marine carbonate system would mean that many coral reef systems in the Pacific would no longer be able to sustain a sufficiently high rate of calcification to maintain the viability of these ecosystems as a whole, and these changes perhaps could seriously impact the thousands of marine species that depend on them for survival.

• Murata, A. and Saito, S.  Decadal changes in the CaCO₃ saturation state along 179ºE in the Pacific Ocean.  Geophysical Research Letters 39(12): L12604, 2012.
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  To assess degrees of ocean acidification, we mainly investigated decadal changes in the saturation state of seawater with respect to aragonite (Ω_arg), which is a more vulnerable mineral form of CaCO₃, along the 179°E meridian (WOCE P14N) in the Pacific Ocean. We found a maximum decrease of Ω_arg of -0.48 (-0.034 a^-1) at 200–300 dbar (isopycnal surfaces of 24.0–25.8 kg m^-3) at 20°N. Between 1993 and 2007, the saturation horizon rose by 17 dbar (1.2 dbar a^-1) at latitudes 10°N-50°N. Although ΔΩ_arg mostly reflected changes in normalized dissolved inorganic carbon (ΔnCT), it was larger than could be explained by anthropogenic CO₂ storage alone. Decomposition of ΔnCT revealed that ΔΩ_arg was enhanced by approximately 50% by a non-anthropogenic CO₂ contribution represented by changes in apparent oxygen utilization. Our results suggest that ocean acidification can be temporarily accelerated by temporal changes in oceanic conditions.

• Maas, A.E., Wishner, K.F., and Seibel, B.A.  The metabolic response of pteropods to acidification reflects natural CO₂-exposure in oxygen minimum zones.  Biogeosciences 9(2): 747-757, 2012.
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  Shelled pteropods (Thecosomata) are a group of holoplanktonic mollusks that are believed to be especially sensitive to ocean acidification because their aragonitic shells are highly soluble. Despite this concern, there is very little known about the physiological response of these animals to conditions of elevated carbon dioxide. This study examines the oxygen consumption and ammonia excretion of five pteropod species, collected from tropical regions of the Pacific Ocean, to elevated levels of carbon dioxide (0.10%, 1000 ppm). Our results show that pteropods that naturally migrate into oxygen minimum zones, such as Hyalocylis striata, Clio pyramidata, Cavolinia longirostris and Creseis virgula, were not affected by carbon dioxide at the levels and duration tested. Diacria quadridentata, which does not migrate, responds to high carbon dioxide conditions with reduced oxygen consumption and ammonia excretion. This indicates that the natural chemical environment of individual species may influence their resilience to ocean acidification.

• Andersson, A.J. and Mackenzie, F.T.  Revisiting four scientific debates in ocean acidification research.  Biogeosciences 9(3): 893-905, 2012.
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  In recent years, ocean acidification has gained continuously increasing attention from scientists and a number of stakeholders and has raised serious concerns about its effects on marine organisms and ecosystems. With the increase in interest, funding resources, and the number of scientific investigations focusing on this environmental problem, increasing amounts of data and results have been produced, and a progressively growing and more rigorous understanding of this problem has begun to develop. Nevertheless, there are still a number of scientific debates, and in some cases misconceptions, that keep reoccurring at a number of forums in various contexts. In this article, we revisit four of these topics that we think require further thoughtful consideration including: (1) surface seawater CO₂ chemistry in shallow water coastal areas, (2) experimental manipulation of marine systems using CO₂ gas or by acid addition, (3) net versus gross calcification and dissolution, and (4) CaCO₃ mineral dissolution and seawater buffering. As a summation of these topics, we emphasize that: (1) many coastal environments experience seawater pCO₂ that is significantly higher than expected from equilibrium with the atmosphere and is strongly linked to biological processes; (2) addition of acid, base or CO₂ gas to seawater can all be useful techniques to manipulate seawater chemistry in ocean acidification experiments; (3) estimates of calcification or CaCO₃ dissolution based on present techniques are measuring the net of gross calcification and dissolution; and (4) dissolution of metastable carbonate mineral phases will not produce sufficient alkalinity to buffer the pH and carbonate saturation state of shallow water environments on timescales of decades to hundreds of years to the extent that any potential negative effects on marine calcifiers will be avoided.

• Wooldridge, S.A.  A hypothesis linking sub-optimal seawater pCO₂ conditions for cnidarian-Symbiodinium symbioses with the exceedence of the interglacial threshold (>260 ppmv).  Biogeosciences 9(5): 1709-1723, 2012.
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  Most scleractinian corals and many other cnidarians host intracellular photosynthetic dinoflagellate symbionts ("zooxanthellae"). The zooxanthellae contribute to host metabolism and skeletogenesis to such an extent that this symbiosis is well recognised for its contribution in creating the coral reef ecosystem. The stable functioning of cnidarian symbioses is however dependent upon the host's ability to maintain demographic control of its algal partner. In this review, I explain how the modern envelope of seawater conditions found within many coral reef ecosystems (characterised by elevated temperatures, rising pCO₂, and enriched nutrient levels) are antagonistic toward the dominant host processes that restrict excessive symbiont proliferation. Moreover, I outline a new hypothesis and initial evidence base, which support the suggestion that the additional "excess" zooxanthellae fraction permitted by seawater pCO₂ levels beyond 260 ppmv significantly increases the propensity for symbiosis breakdown ("bleaching") in response to temperature and irradiance extremes. The relevance of this biological threshold is discussed in terms of historical reef extinction events, glacial-interglacial climate cycles and the modern decline of coral reef ecosystems.

• Yamamoto, A., Kawamiya, M., Ishida, A., Yamanaka, Y., and Watanabe, S.  Impact of rapid sea-ice reduction in the Arctic Ocean on the rate of ocean acidification.  Biogeosciences 9(6): 2365-2375, 2012.
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  The largest pH decline and widespread undersaturation with respect to aragonite in this century due to uptake of anthropogenic carbon dioxide in the Arctic Ocean have been projected. The reductions in pH and aragonite saturation state in the Arctic Ocean have been caused by the melting of sea ice as well as by an increase in the concentration of atmospheric carbon dioxide. Therefore, future projections of pH and aragonite saturation in the Arctic Ocean will be affected by how rapidly the reduction in sea ice occurs. The observed recent Arctic sea-ice loss has been more rapid than projected by many of the climate models that contributed to the Intergovernmental Panel on Climate Change Fourth Assessment Report. In this study, the impact of sea-ice reduction rate on projected pH and aragonite saturation state in the Arctic surface waters was investigated. Reductions in pH and aragonite saturation were calculated from the outputs of two versions of an Earth system model with different sea-ice reduction rates under similar CO₂ emission scenarios. The newer model version projects that Arctic summer ice-free condition will be achieved by the year 2040, and the older version predicts ice-free condition by 2090. The Arctic surface water was projected to be undersaturated with respect to aragonite in the annual mean when atmospheric CO₂ concentration reaches 513 (606) ppm in year 2046 (2056) in new (old) version. At an atmospheric CO₂ concentration of 520 ppm, the maximum differences in pH and aragonite saturation state between the two versions were 0.1 and 0.21 respectively. The analysis showed that the decreases in pH and aragonite saturation state due to rapid sea-ice reduction were caused by increases in both CO₂ uptake and freshwater input. Thus, the reductions in pH and aragonite saturation state in the Arctic surface waters are significantly affected by the difference in future projections for sea-ice reduction rate. Our results suggest that the future reductions in pH and aragonite saturation state could be significantly faster than previously projected if the sea-ice reduction in the Arctic Ocean keeps its present pace.

• Hoppe, C.J.M., Langer, G., Rokitta, S.D., Wolf-Gladrow, D.A., and Rost, B.  Implications of observed inconsistencies in carbonate chemistry measurements for ocean acidification studies.  Biogeosciences 9(7): 2401-2405, 2012.
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  The growing field of ocean acidification research is concerned with the investigation of organism responses to increasing pCO₂ values. One important approach in this context is culture work using seawater with adjusted CO₂ levels. As aqueous pCO₂ is difficult to measure directly in small-scale experiments, it is generally calculated from two other measured parameters of the carbonate system (often A_T, C_T or pH). Unfortunately, the overall uncertainties of measured and subsequently calculated values are often unknown. Especially under high pCO₂, this can become a severe problem with respect to the interpretation of physiological and ecological data. In the few datasets from ocean acidification research where all three of these parameters were measured, pCO₂ values calculated from A_T and C_T are typically about 30% lower (i.e. ~300 μatm at a target pCO₂ of 1000 μatm) than those calculated from A_T and pH or C_T and pH. This study presents and discusses these discrepancies as well as likely consequences for the ocean acidification community. Until this problem is solved, one has to consider that calculated parameters of the carbonate system (e.g. pCO₂, calcite saturation state) may not be comparable between studies, and that this may have important implications for the interpretation of CO₂ perturbation experiments.

• Bates, N.R., Best, M.H.P., Neely, K., Garley, R., Dickson, A.G., and Johnson, R.J.  Detecting anthropogenic carbon dioxide uptake and ocean acidification in the North Atlantic Ocean.  Biogeosciences 9(7): 2509-2522, 2012.
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  Fossil fuel use, cement manufacture and land-use changes are the primary sources of anthropogenic carbon dioxide (CO₂) to the atmosphere, with the ocean absorbing approximately 30% (Sabine et al., 2004). Ocean uptake and chemical equilibration of anthropogenic CO₂ with seawater results in a gradual reduction in seawater pH and saturation states (Ω) for calcium carbonate (CaCO₃) minerals in a process termed ocean acidification. Assessing the present and future impact of ocean acidification on marine ecosystems requires detection of the multi-decadal rate of change across ocean basins and at ocean time-series sites. Here, we show the longest continuous record of ocean CO₂ changes and ocean acidification in the North Atlantic subtropical gyre near Bermuda from 1983–2011. Dissolved inorganic carbon (DIC) and partial pressure of CO₂ (pCO₂) increased in surface seawater by ~40 μmol kg^-1 and ~50 μatm (~20%), respectively. Increasing Revelle factor (β) values imply that the capacity of North Atlantic surface waters to absorb CO₂ has also diminished. As indicators of ocean acidification, seawater pH decreased by ~0.05 (0.0017 yr^-1) and ω values by ~7–8%. Such data provide critically needed multi-decadal information for assessing the North Atlantic Ocean CO₂ sink and the pH changes that determine marine ecosystem responses to ocean acidification.

• Briffa, M., de la Haye, K., and Munday, P.L.  High CO₂ and marine animal behaviour: potential mechanisms and ecological consequences.  Marine Pollution Bulletin 64(8): 1519-1528, 2012.
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  Exposure to pollution and environmental change can alter the behaviour of aquatic animals and here we review recent evidence that exposure to elevated CO₂ and reduced sea water pH alters the behaviour of tropical reef fish and hermit crabs. Three main routes through which behaviour might be altered are discussed; elevated metabolic load, 'info-disruption' and avoidance behaviour away from polluted locations. There is clear experimental evidence that exposure to high CO₂ disrupts the ability to find settlement sites and shelters, the ability to detect predators and the ability to detect prey and food. In marine vertebrates and marine crustaceans behavioural change appears to occur via info-disruption. In hermit crabs and other crustaceans impairment of performance capacities might also play a role. We discuss the implications for such behavioural changes in terms of potential impacts at the levels of population health and ecosystem services, and consider future directions for research.

• Yang, G. and Gao, K.  Physiological responses of the marine diatom Thalassiosira pseudonana to increased pCO₂ and seawater acidity.  Marine Environmental Research 79: 142-151, 2012.
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  We studied the effects of elevated CO₂ concentration and seawater acidity on inorganic carbon acquisition, photoinhibition and photoprotection as well as growth and respiration in the marine diatom Thalassiosira pseudonana. After having grown under the elevated CO₂ level (1000 μatm, pH 7.83) at sub-saturating photosynthetically active radiation (PAR, 75 μmol photons m^-2 s^-1) for 20 generations, photosynthesis and dark respiration of the alga increased by 25% (14.69 ± 2.55 fmolC cell^-1 h^-1) and by 35% (4.42 ± 0.98 fmol O₂ cell^-1 h^-1), respectively, compared to that grown under the ambient CO₂ level (390 μatm, pH 8.16), leading to insignificant effects on growth (1.09 ± 0.08 d^-1 v 1.04 ± 0.07 d^-1). The photosynthetic affinity for CO₂ was lowered in the high-CO₂ grown cells, reflecting a down-regulation of the CO₂ concentrating mechanism (CCM). When exposed to an excessively high level of PAR, photochemical and non-photochemical quenching responded similarly in the low- and high-CO₂ grown cells, reflecting that photoinhibition was not influenced by the enriched level of CO₂. In T.pseudonana, it appeared that the energy saved due to the down-regulated CCM did not contribute to any additional light stress as previously found in another diatom Phaeodactylum tricornutum, indicating differential physiological responses to ocean acidification. between these two diatom species.

• Smith, H.E.K. et al.  Predominance of heavily calcified coccolithophores at low CaCO₃ saturation during winter in the Bay of Biscay.  Proceedings of the National Academy of Sciences [USA] 109(23): 8845-8849, 2012.
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  Coccolithophores are an important component of the Earth system, and, as calcifiers, their possible susceptibility to ocean acidification is of major concern. Laboratory studies at enhanced pCO₂ levels have produced divergent results without overall consensus. However, it has been predicted from these studies that, although calcification may not be depressed in all species, acidification will produce "a transition in dominance from more to less heavily calcified coccolithophores" [Ridgwell A, et al., (2009) Biogeosciences 6:2611-2623]. A recent observational study [Beaufort L, et al., (2011) Nature 476:80-83] also suggested that coccolithophores are less calcified in more acidic conditions. We present the results of a large observational study of coccolithophore morphology in the Bay of Biscay. Samples were collected once a month for over a year, along a 1,000-km-long transect. Our data clearly show that there is a pronounced seasonality in the morphotypes of Emiliania huxleyi, the most abundant coccolithophore species. Whereas pH and CaCO₃ saturation are lowest in winter, the E. huxleyi population shifts from <10% (summer) to >90% (winter) of the heavily calcified form. However, it is unlikely that the shifts in carbonate chemistry alone caused the morphotype shift. Our finding that the most heavily calcified morphotype dominates when conditions are most acidic is contrary to the earlier predictions and raises further questions about the fate of coccolithophores in a high-CO₂ world.

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