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.