Culturing experiments were performed with the benthic foraminifer Ammonia aomoriensis from Flensburg Fjord, western Baltic Sea. The experiments simulated a projected rise in atmospheric CO 2 concentrations. We exposed specimens to 5 seawater pCO 2 levels ranging from 618 µatm (pH 7.9) to 3130 µatm (pH 7.2) for 6 wk. Growth rates and mortality differed significantly among pCO 2 treatments. The highest increase of mean test diameter (19%) was observed at 618 µatm. At partial pressures >1829 µatm, the mean test diameter was observed to decrease, by up to 22% at 3130 µatm. At pCO 2 levels of 618 and 751 µatm, A. aomoriensis tests were found intact after the experiment. The outer chambers of specimens incubated at 929 and 1829 µatm were severely damaged by corrosion. Visual inspection of specimens incubated at 3130 µatm revealed wall dissolution of all outer chambers, only their inner organic lining stayed intact. Our results demonstrate that pCO 2 values of ≥929 µatm in Baltic Sea waters cause reduced growth of A. aomoriensis and lead to shell dissolution. The bottom waters in Flensburg Fjord and adjacent areas regularly experience pCO 2 levels in this range during summer and fall. Increasing atmospheric CO 2 concentrations are likely to extend and intensify these periods of undersaturation. This may eventually slow down calcification in A. aomoriensis to the extent that net carbonate precipitation terminates. The possible disappearance of this species from the Baltic Sea and other areas prone to seasonal undersaturation would likely cause significant shifts in shallow-water benthic ecosystems in the near future. KEY WORDS: Benthic foraminifera · Biometry · Test dissolution · Carbon dioxide · Baltic Sea Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 432: 53-67, 2011 54 to about one-third of the production by planktonic foraminifers (Schiebel 2002).In addition to CO 2 -induced ocean acidification, anthropogenic eutrophication by river and groundwater discharge and by atmospheric deposition can lead to changes in carbonate chemistry, especially in coas tal marine environments such as the Baltic Sea (Rosenberg 1985, Conley et al. 2007, Levin et al. 2009, Borges & Gypens 2010, Cossellu & Nordberg 2010, Zhang et al. 2010. In comparison to the open ocean, the Baltic Sea exhibits lower salinities, lower [CO 3 2-] and consequently lower calcium carbonate saturation states (Ω). In the western Baltic Sea, seasonal effects are super imposed (Hansen et al. 1999). Vertical stratification, enhanced microbial activity and the ensuing consumption of dissolved oxygen by the decay of particulate organic matter causes hypoxic conditions in the bottom water and therefore strong seasonally varying pCO 2 values over the year (Diaz & Rosenberg 2008, Conley et al. 2009. In response to low Ω and seasonal acidification, a reduced calcification of foraminifera is expected in Flensburg Fjord (Polovodova et al. 2009).An increasing number of field and laboratory studies have show...

Abstract. Living benthic foraminifera of Flensburg Fjord were surveyed in June 2006. The muddy and organic-rich sediments of the inner fjord were dominated by Elphidium incertum. E. incertum and E. excavatum were frequent in muds and sandy muds of the fjord loop around Holnis Peninsula and in the outer part. Gelting Bay yielded a different biofacies, indicating a brackish and sandy habitat, poor in food supply and with microfauna dominated by Ammonia beccarii and E. albiumbilicatum. The central fjord and nearshore zones of the loop were characterized by sandy muds, relatively poor in food and occupied by A. beccarii, E. incertum and E. excavatum subspecies. High abundances of E. excavatum were encountered in the innermost fjord, with fine-grained and organic-rich muddy sediments.A comparison with previous studies revealed the profound changes in species composition in the outer Flensburg Fjord since the 1970s. A decline in numbers of Ammotium cassis and flourishing of Ammonia beccarii in Gelting Bay were recognized. These changes are most likely associated with decreased intensity and frequency of salt-water inflows into the Baltic Sea since the 1960s. It is inferred that the decline of A. cassis is similar to that of Eggerelloides scaber, which currently is found only in depressions of Kiel Bight with higher salinity.

Ammonia and Elphidium collected in the Kiel Fjord for the present study were first identified on morphological bases as Ammonia beccarii (Linné, 1758) and Elphidium excavatum (Terquem, 1876). Phylogenetic analyses based on partial SSU rDNA and LSU rDNA sequences show that Ammonia specimens sampled in the Kiel Fjord belong to the phylotype T6, which has a disjunct distribution (Wadden and Baltic Seas/China and Japan) and has been identified as Ammonia aomoriensis (Asano, 1951). Partial SSU rDNA sequence analyses indicate that Elphidium specimens from the Kiel Fjord belong to the clade E. excavatum, confirming the morphological identification. This clade can be further divided in three subclades. Kiel Fjord Elphidium belong to two of these subclades and were identified morphologically as the subspecies E. excavatum excavatum (Terquem, 1876) and E. e. clavatum Cushman, 1930.