Ocean acidification is expected to decrease calcification rates of bivalves. Nevertheless, in many coastal areas high pCO 2 variability is encountered already today. Kiel Fjord (Western Baltic Sea) is a brackish (12-20 g kg À1 ) and CO 2 enriched habitat, but the blue mussel Mytilus edulis dominates the benthic community. In a coupled field and laboratory study we examined the annual pCO 2 variability in this habitat and the combined effects of elevated pCO 2 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 pCO 2 up to 3350 latm. Kiel Fjord was characterized by strong seasonal pCO 2 variability. During summer, maximal pCO 2 values of 2500 latm were observed at the surface and >3000 latm at the bottom. However, the field growth experiment revealed seven times higher growth and calcification rates of M. edulis at a high pCO 2 inner fjord field station (mean pCO 2 ca. 1000 latm) in comparison to a low pCO 2 outer fjord station (ca. 600 latm). In addition, mussels were able to out-compete the barnacle Amphibalanus improvisus at the high pCO 2 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 pCO 2 . 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 pCO 2 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.

Progressive ocean acidification due to anthropogenic CO2 emissions will alter marine ecosytem processes. Calcifying organisms might be particularly vulnerable to these alterations in the speciation of the marine carbonate system. While previous research efforts have mainly focused on external dissolution of shells in seawater under saturated with respect to calcium carbonate, the internal shell interface might be more vulnerable to acidification. In the case of the blue mussel Mytilus edulis, high body fluid pCO2 causes low pH and low carbonate concentrations in the extrapallial fluid, which is in direct contact with the inner shell surface. In order to test whether elevated seawater pCO2 impacts calcification and inner shell surface integrity we exposed Baltic M. edulis to four different seawater pCO2 (39, 142, 240, 405 Pa) and two food algae (310–350 cells mL−1 vs. 1600–2000 cells mL−1) concentrations for a period of seven weeks during winter (5°C). We found that low food algae concentrations and high pCO2 values each significantly decreased shell length growth. Internal shell surface corrosion of nacreous ( = aragonite) layers was documented via stereomicroscopy and SEM at the two highest pCO2 treatments in the high food group, while it was found in all treatments in the low food group. Both factors, food and pCO2, significantly influenced the magnitude of inner shell surface dissolution. Our findings illustrate for the first time that integrity of inner shell surfaces is tightly coupled to the animals' energy budget under conditions of CO2 stress. It is likely that under food limited conditions, energy is allocated to more vital processes (e.g. somatic mass maintenance) instead of shell conservation. It is evident from our results that mussels exert significant biological control over the structural integrity of their inner shell surfaces.

Larval stages are considered as the weakest link when a species is exposed to challenging environmental changes 1,2 . Reduced rates of growth and development in larval stages of calcifying invertebrates in response to ocean acidification might be caused by energetic limitations 3 . So far no information exists on how ocean acidification affects digestive processes in marine larval stages. Here we reveal alkaline (∼pH 9.5) conditions in the stomach of sea urchin larvae. Larvae exposed to decreased seawater pH suffer from a drop in gastric pH, which directly translates into decreased digestive efficiencies and triggers compensatory feeding. These results suggest that larval digestion represents a critical process in the context of ocean acidification, which has been overlooked so far.Ocean acidification as it is projected for the next century can affect vital functions of marine organisms. Larval stages are often particularly sensitive to ocean acidification. Decreased survival of larvae can directly affect population stability and could lead to decreased ecosystem integrity. As observed in several species, disturbances of extra-or intracellular acid-base homeostasis were correlated with energy budget reallocation and decreased scope for somatic growth and development 4,5 . However, little attention has been placed on whether digestive processes are impacted by decreased seawater pH, particularly in larval stages of marine invertebrates.According to the preferred sources of nutrients and the necessary catabolic enzymes, digestive systems with distinct pH environments have evolved. For example, stomachs of most vertebrates operate at an acidic pH of ∼2, corresponding to maximum activity of most gastric enzymes at low pH (ref. 6). On the other hand, the midgut of larvae of several insect species operates at a strongly alkaline pH of ∼11 for the benefit of digestive enzymes (proteases, phosphatases) with a highly alkaline pH optimum 7 . To maintain high enzyme activities, digestive system pH is regulated by active ion transport processes through the net export or import of acid equivalents 8 .Here we investigated the effects of seawater acidification on digestive processes in green sea urchin pluteus larvae (Strongylocentrotus droebachiensis), which are keystone species in temperate and subpolar kelp ecosystems 9 . Owing to the fact that pluteus larvae cannot regulate the extracellular pH in their primary body cavity 3 the larval digestive system is directly exposed to changes in seawater pH. We reasoned that changes in seawater pH will directly influence the larval physiology by reducing stomach pH, digestive enzyme activity, and thus food assimilation and/or by challenging the acid-base regulatory machinery responsible for stomach pH maintenance. In terms of larval energy budgets, such challenges may be a critical reason for the reported reductions in growth and development of echinoid larvae in response to acidified sea water.Using ion-selective micro-electrodes we found that the stomach pH of sea urchin pluteus l...

Recently, several studies indicated that species from the Ponto‐Caspian region may be evolutionarily predisposed to become nonindigenous species (NIS); however, origin of NIS established in different regions has rarely been compared to confirm these statements. More importantly, if species from certain area/s are proven to be better colonizers, management strategies to control transport vectors coming from those areas must be more stringent, as prevention of new introductions is a cheaper and more effective strategy than eradication or control of established NIS populations. To determine whether species evolved in certain areas have inherent advantages over other species in colonizing new habitats, we explored NIS established in the North and Baltic Seas and Great Lakes–St. Lawrence River regions—two areas intensively studied in concern to NIS, highly invaded by Ponto‐Caspian species and with different salinity patterns (marine vs. freshwater). We compared observed numbers of NIS in these two regions to expected numbers of NIS from major donor regions. The expected numbers were calculated based on the available species pool from donor regions, frequency of shipping transit, and an environmental match between donor and recipient regions. A total of 281 NIS established in the North and Baltic Seas and 188 in the Great Lakes–St. Lawrence River. Ponto‐Caspian taxa colonized both types of habitats, saltwater areas of the North and Baltic Seas and freshwater of the Great Lakes–St. Lawrence River, in much higher numbers than expected. Propagule pressure (i.e., number of introduced individuals or introduction effort) is of great importance for establishment success of NIS; however in our study, either shipping vector or environmental match between regions did not clarify the high numbers of Ponto‐Caspian taxa in our study areas. Although we cannot exclude the influence of other transport vectors, our findings suggest that the origin of the species plays an important role for the predisposition of successful invaders.

19We studied the effects of temperature and pH on larval development, settlement and 20 juvenile survival of a Mediterranean population of the sea urchin Arbacia lixula. Three 21 temperatures (16, 17.5 and 19 ºC) were tested at present pH conditions (pH T 8.1). At 19 22 ºC, two pH levels were compared to reflect present average (pH T 8.1) and near-future 23 average conditions (pH T 7.7, expected by 2100). Larvae were reared for 52-days to 24 achieve the full larval development and complete the metamorphosis to the settler stage. 25We analysed larval survival, growth, morphology and settlement success. We also 26 tested the carry-over effect of acidification on juvenile survival after 3 days. Our results 27showed that larval survival and size significantly increased with temperature. 28Acidification resulted in higher survival rates and developmental delay. Larval 29 morphology was significantly altered by low temperatures, which led to narrower larvae 30 with relatively shorter skeletal rods, but larval morphology was only marginally 31 affected by acidification. No carry-over effects between larvae and juveniles were 32 detected in early settler survival, though settlers from larvae reared at pH 7.7 were 33 significantly smaller than their counterparts developed at pH 8.

Seawater acidification due to anthropogenic release of CO 2 as well as the potential leakage of pure CO 2 from sub-seabed carbon capture storage (CCS) sites may impose a serious threat to marine organisms. Although infaunal organisms can be expected to be particularly impacted by decreases in seawater pH, as a result of naturally acidified conditions in benthic habitats, information regarding physiological and behavioral responses is still scarce. Determination of P O2 and P CO2 gradients within burrows of the brittlestar Amphiura filiformis during environmental hypercapnia demonstrated that besides hypoxic conditions, increases of environmental P CO2 are additive to the already high P CO2 (up to 0.08 kPa) within the burrows. In response to up to 4 weeks exposure to pH 7.3 (0.3 kPa P CO2 ) and pH 7.0 (0.6 kPa P CO2 ), metabolic rates of A. filiformis were significantly reduced in pH 7.0 treatments, accompanied by increased ammonium excretion rates. Gene expression analyses demonstrated significant reductions of acid-base (NBCe and AQP9) and metabolic (G6PDH, LDH) genes. Determination of extracellular acid-base status indicated an uncompensated acidosis in CO 2 -treated animals, which could explain the depressed metabolic rates. Metabolic depression is associated with a retraction of filter feeding arms into sediment burrows. Regeneration of lost arm tissues following traumatic amputation is associated with significant increases in metabolic rate, and hypercapnic conditions (pH 7.0, 0.6 kPa) dramatically reduce the metabolic scope for regeneration, reflected in an 80% reduction in regeneration rate. Thus, the present work demonstrates that elevated seawater P CO2 significantly affects the environment and the physiology of infaunal organisms like A. filiformis.

Aim Numerous regions worldwide are highly impacted by anthropogenic activities and globalization, with climate change and species introductions being among the greatest stressors to biodiversity and ecosystems. A main donor region of non‐indigenous species (NIS) for numerous European water bodies, as well as in the North American Great Lakes is the Ponto‐Caspian region (i.e., Black, Azov and Caspian Seas), with some of those species having significant impact on local communities and ecosystem functioning. Location Northern European, Ponto‐Caspian and North American regions. Methods To determine environmental tolerance of native species and related NIS under current and future global warming scenarios of the Baltic Sea, we conducted common garden experiments to test temperature tolerance of three euryhaline gammarid species: one Baltic (Gammarus oceanicus), one Ponto‐Caspian (Pontogammarus maeoticus) and one North American species (Gammarus tigrinus) in two different salinities. Results Our results determined that mortality of P. maeoticus in all temperature treatments (i.e., increased, control, and decreased) at the end of both experiments (i.e., conducted in salinities of 10 and 16 g/kg) was lower when compared to mortality of G. oceanicus and (c) G. tigrinus. The highest mortality was observed for G. oceanicus, reaching 100% in both experiments in the increased temperature treatment. Main conclusions Due to the high environmental tolerance of the Ponto‐Caspian species tested in this study, as well as the fact that Ponto‐Caspian species evolved in environmentally variable habitats and currently inhabit warmer waters than species from North America and Northern Europe, we suggest that species from the Ponto‐Caspian region may benefit from global warming when invading new areas. Those new invasions may, in the best case scenario, increase biodiversity of the Baltic Sea. However, if notorious invaders arrive, they may have a significant impact on local communities and ecosystem functioning.

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