Description: Ocean acidification severely affects bivalves, especially their larval stages. Consequently, the fate of this ecologically and economically important group depends on the capacity and rate of evolutionary adaptation to altered ocean carbonate chemistry. We document successful settlement of wild mussel larvae (Mytilus edulis) in a periodically CO2-enriched habitat. The larval fitness of the population originating from the CO2-enriched habitat was compared to the response of a population from a nonenriched habitat in a common garden experiment. The high CO2–adapted population showed higher fitness under elevated Pco2 (partial pressure of CO2) than the non-adapted cohort, demonstrating, for the first time, an evolutionary response of a natural mussel population to ocean acidification. To assess the rate of adaptation, we performed a selection experiment over three generations. CO2 tolerance differed substantially between the families within the F1 generation, and survival was drastically decreased in the highest, yet realistic, Pco2 treatment. Selection of CO2-tolerant F1 animals resulted in higher calcification performance of F2 larvae during early shell formation but did not improve overall survival. Our results thus reveal significant short-term selective responses of traits directly affected by ocean acidification and long-term adaptation potential in a key bivalve species. Because immediate response to selection did not directly translate into increased fitness, multigenerational studies need to take into consideration the multivariate nature of selection acting in natural habitats. Combinations of short-term selection with long-term adaptation in populations from CO2-enriched versus nonenriched natural habitats represent promising approaches for estimating adaptive potential of organisms facing global change.

Description: It is expected that the calcification of foraminifera will be negatively affected by the ongoing acidification of the oceans. Compared to the open oceans, these organisms are subjected to much more adverse carbonate system conditions in coastal and estuarine environments such as the southwestern Baltic Sea, where benthic foraminifera are abundant. This study documents the seasonal changes of carbonate chemistry and the ensuing response of the foraminiferal community with bi-monthly resolution in Flensburg Fjord. In comparison to the surface pCO2, which is close to equilibrium with the atmosphere, we observed large seasonal fluctuations of pCO2 in the bottom and sediment pore waters. The sediment pore water pCO2 was constantly high during the entire year ranging from 1244 to 3324 μatm. Nevertheless, in contrast to the bottom water, sediment pore water was slightly supersaturated with respect to calcite as consequence of higher alkalinity (AT) for the most time of the year. Foraminiferal assemblages were dominated by two calcareous species, Ammonia aomoriensis and Elphidium incertum, and the agglutinated Ammotium cassis. The one year-cycle was characterized by seasonal community shifts. Our results revealed that there is no dynamic response of foraminiferal population density and diversity to elevated sediment pore water pCO2. Surprisingly, the fluctuations of sediment pore water undersaturation (Ωcalc) co-vary with the population densities of living Ammonia aomoriensis. Further, we observed that most of the tests of living calcifying specimens were intact. Only Ammonia aomorienis showed dissolution and recalcification structures on the tests, especially at undersaturated conditions. Therefore, the benthic community is subjected to constantly high pCO2 and tolerates elevated levels as long as sediment pore water remains supersaturated. Model calculations inferred that increasing atmospheric CO2 concentrations will finally lead to a perennial undersaturation in sediment pore waters. Whereas benthic foraminifera indeed may cope with a high sediment pore water pCO2, the steady undersaturation of sediment pore waters would likely cause a significant higher mortality of the dominating Ammonia aomoriensis. This shift may eventually lead to changes in the benthic foraminiferal communities in Flensburg Fjord, as well as in other regions experiencing naturally undersaturated Ωcalc levels.

Description: Calcifying foraminifera are expected to be endangered by ocean acidification, However, the response of a complete community kept in natural sediment and over multiple generations under controlled laboratory conditions has not been constrained to date. During 5 six month incubation, foraminiferal assemblages were treated with pCO2 enriched seawater of 430, 907, 1865 and 3247 μatm pCO2. The fauna was dominated by Ammonia aomoriensis and Elphidium species, whereas agglutinated species were rare. After 6 months incubation, pore water alkalinity was much higher in comparison to the overlying seawater. Consequently, the saturation state of Òcalc was much higher in the sedi10 ment than in the water column in all pCO2 treatments and remained close to saturation. As a result, the life cycle of living assemblages was largely unaffected by the tested pCO2 treatments. Growth rates, reproduction and mortality, and therefore population densities and size-frequency distribution of Ammonia aomoriensis varied markedly during the experimental period. Growth rates varied between 25 and 50 μm per month, 15 which corresponds to an addition of 1 or 2 new chambers per month. According to the size-frequency distribution, foraminifera start reproduction at a diameter of 250 μm. Mortality of large foraminifera was recognized, commencing at a test size of 285 μm at a pCO2 ranging from 430 to 1865 μatm, and of 258 μm at 3247 μatm. The total organic content of living Ammonia aomoriensis has been determined to be 4.3% of dry 20 weight. Living individuals had a calcium carbonate production rate of 0.47 gm−2 yr−1, whereas dead empty tests accumulated at a rate of 0.27 gm−2a−1. Although Òcalc was close to 1, some empty tests of Ammonia aomoriensis showed dissolution features at the end of incubation. In contrast, tests of the subdominant species, Elphidium incertum, stayed intact. This species specific response could be explained by differences in 25 the elemental test composition, in particular the higher Mg-concentrations in Ammonia aomoriensis tests. Our results emphasize that the sensitivity to ocean acidification of endobenthic foraminifera in their natural sediment habitat is much lower compared to the experimental response of specimens isolated from the sediment.

Description: Bivalve calcification, particularly of the early larval stages, is highly sensitive to the change in ocean carbonate chemistry resulting from atmospheric CO2 uptake. Earlier studies suggested that declining seawater [CO32−] and thereby lowered carbonate saturation affect shell production. However, disturbances of physiological processes such as acid-base regulation by adverse seawater pCO2 and pH can affect calcification in a secondary fashion. In order to determine the exact carbonate system component by which growth and calcification are affected it is necessary to utilize more complex carbonate chemistry manipulations. As single factors, pCO2 had no effects and [HCO3-] and pH had only limited effects on shell growth, while lowered [CO32−] strongly impacted calcification. Dissolved inorganic carbon (CT) limiting conditions led to strong reductions in calcification, despite high [CO32−], indicating that [HCO3-] rather than [CO32−] is the inorganic carbon source utilized for calcification by mytilid mussels. However, as the ratio [HCO3-] / [H+] is linearly correlated with [CO32−] it is not possible to differentiate between these under natural seawater conditions. An equivalent of about 80 μmol kg−1 [CO32−] is required to saturate inorganic carbon supply for calcification in bivalves. Below this threshold biomineralization rates rapidly decline. A comparison of literature data available for larvae and juvenile mussels and oysters originating from habitats differing substantially with respect to prevailing carbonate chemistry conditions revealed similar response curves. This suggests that the mechanisms which determine sensitivity of calcification in this group are highly conserved. The higher sensitivity of larval calcification seems to primarily result from the much higher relative calcification rates in early life stages. In order to reveal and understand the mechanisms that limit or facilitate adaptation to future ocean acidification, it is necessary to better understand the physiological processes and their underlying genetics that govern inorganic carbon assimilation for calcification.

Description: The present study investigated the combined effects of ocean acidification, temperature, and salinity on growth and test degradation of Ammonia aomoriensis. This species is one of the dominant benthic foraminifera in near-coastal habitats of the southwestern Baltic Sea that can be particularly sensitive to changes in seawater carbonate chemistry. To assess potential responses to ocean acidification and climate change, we performed a fully crossed experiment involving three temperatures (8, 13, and 18uC), three salinities (15, 20, and 25) and four pCO2 levels (566, 1195, 2108, and 3843 matm) for six weeks. Our results highlight a sensitive response of A. aomoriensis to undersaturated seawater with respect to calcite. The specimens continued to grow and increase their test diameter in treatments with pCO2 ,1200 matm, when Vcalc .1. Growth rates declined when pCO2 exceeded 1200 matm (Vcalc ,1). A significant reduction in test diameter and number of tests due to dissolution was observed below a critical Vcalc of 0.5. Elevated temperature (18uC) led to increased Vcalc, larger test diameter, and lower test degradation. Maximal growth was observed at 18uC. No significant relationship was observed between salinity and test growth. Lowered and undersaturated Vcalc, which results from increasing pCO2 in bottom waters, may cause a significant future decline of the population density of A. aomoriensis in its natural environment. At the same time, this effect might be partially compensated by temperature rise due to global warming.

Description: Increasing atmospheric CO2 concentrations have a strong impact on the marine carbonate chemistry leading to a phenomenon called ocean acidification. Excess CO2 dissolves in the surface water of the ocean, thereby the seawater pCO2 increases, whereas the [CO3 2-] and pH decrease. Reduced CO3 2- concentrations may affect marine, especially calcifying, organisms such as benthic foraminifera, in that their ability to form calcareous tests might be affected. In comparison to open oceans, water pCO2 levels are often not in equilibrium with the atmosphere in coastal regions, which are characterized by high CO2 variability during the seasonal cycle. This has also been observed for the southwestern Baltic, an eutrophic marginal sea, where bacterial degradation of large amounts of organic matter cause O2 depletion and CO2 enrichment in the bottom water. In the frame of this thesis, the impact of elevated pCO2, temperature and salinity changes on the survival and calcification ability of the benthic foraminiferal species Ammonia aomoriensis was investigated in mid-term and long-term laboratory experiments. Under laboratory conditions, foraminifera were either isolated from the sediment or remained in their natural microhabitat. Further, the natural carbonate system variability and its impact on foraminiferal communities were monitored in a one-year field study. Specimens of Ammonia aomoriensis were isolated from their natural sediment. They exhibited reduced survival and growth rates with increasing pCO2 of up to 3130 μatm under laboratory conditions. At pCO2 levels above 1800 μatm, dissolution caused a decrease of test diameter, and at the highest pCO2, only the inner organic lining remained. Testing the combined effects of ocean acidification, temperature and salinity on living Ammonia aomoriensis, a significant reduction of test diameter was observed at a pCO2 〉1200 μatm (Ωcalc〈1). Tests were mainly affected by undersaturation of calcite. This effect was partly compensated by a temperature rise, which increased Ωcalc and led to lower test degradation. In contrast, salinity did not have a significant effect on test growth. These results revealed that Ammonia ammoriensis exhibited a high sensitivity to elevated pCO2 and accompanying calcium carbonate undersaturation when the specimens were kept without their protective sedimentary habitat. During the field survey, large seasonal fluctuations of pCO2 from 465 up to 3429 μatm were encountered in the bottom water of Flensburg Fjord in the southwestern Baltic Sea. The pCO2 in the sediment pore water reached even higher values ranging from 1244 to 3324 μatm. However, and as a consequence of higher alkalinity (AT), the calcium carbonate saturation state of the sediment pore water remained slightly supersaturated with respect to calcite for most of the year. Accordingly, during the monitoring period, no dynamic responses of foraminiferal population density and diversity to elevated sediment pore water pCO2 were recognized. Benthic foraminifera may indeed cope with a high sediment pore water pCO2 as long as the sediment pore water remains calcite supersaturated. This evidence from the field study was also supported by the results of a long-term laboratory experiment, in which a complete foraminiferal fauna in their natural sediment was exposed to elevated pCO2 levels. Similar to field observations, the sediment pore water exhibited higher alkalinity and consequently higher saturation state of Ωcalc in comparison to the overlying seawater. Thereby the sediment chemistry created a microhabitat, which sustained the growth and development of the foraminiferal community even at highly elevated pCO2. The dominant species Ammonia aomoriensis exhibited growth and several reproduction events during the incubation time. Nevertheless, dissolution was observed on dead, empty tests of Ammonia aomoriensis, whereas tests of the second-ranked species Elphidium incertum stayed intact at high pCO2 and Ωcalc〈1. This species-specific response could be due to differences in elemental composition and ultrastructure of the test walls. Benthic foraminifera in their natural, sedimentary habitat tolerate elevated pCO2 under laboratory conditions and the current high sedimentary pore water pCO2, which prevails in the southwestern Baltic Sea. In this environment, organic-rich mud influences the carbonate chemistry, and thereby provides a suitable habitat for benthic foraminifera. Consequently, the calcifying Ammonia aomoriensis plays an important role in benthic carbonate production and accumulation in this area. These results emphasize the importance of understanding the carbonate chemistry in the natural environment of benthic foraminifera, which depends upon sediment composition and remineralization processes. It is expected that enhanced future CO2 uptake in the water column will cause a further rise of sedimentary pore water pCO2 levels. As a consequence, undersaturation with respect to calcite will occur more frequently even in the sediment. This will most probably affect the dominant species Ammonia aomoriensis, which might induce changes in the benthic foraminiferal communities and their carbonate production in the southwestern Baltic Sea.