Abstract. CO 2 emissions are leading to an acidification of the oceans. Predicting marine community vulnerability towards acidification is difficult, as adaptation processes cannot be accounted for in most experimental studies. Naturally CO 2 enriched sites thus can serve as valuable proxies for future changes in community structure. Here we describe a natural analogue site in the Western Baltic Sea. Seawater pCO 2 in Kiel Fjord is elevated for large parts of the year due to upwelling of CO 2 rich waters. Peak pCO 2 values of >230 Pa (>2300 µatm) and pH NBS values of <7.5 are encountered during summer and autumn, average pCO 2 values are ∼70 Pa (∼700 µatm). In contrast to previously described naturally CO 2 enriched sites that have suggested a progressive displacement of calcifying auto-and heterotrophic species, the macrobenthic community in Kiel Fjord is dominated by calcifying invertebrates. We show that blue mussels from Kiel Fjord can maintain control rates of somatic and shell growth at a pCO 2 of 142 Pa (1400 µatm, pH NBS = 7.7). Juvenile mussel recruitment peaks during the summer months, when high water pCO 2 values of ∼100 Pa (∼1000 µatm) prevail. Our findings indicate that calcifying keystone species may be able to cope with surface ocean pH NBS values projected for the end of this century when food supply is sufficient. However, owing to non-linear synergistic effects of future acidification and upwelling of corrosive water, peak seawater pCO 2 in Kiel Fjord and many other productive estuarine habitats could increase to values >400 Pa (>4000 µatm). These changes will most likely affect calcification and recruitment, and increase external shell dissolution.

CO₂ emissions are leading to an acidification of the oceans. Predicting marine community vulnerability towards acidification is difficult, as adaptation processes cannot be accounted for in most experimental studies. Naturally CO₂ enriched sites thus can serve as valuable proxies for future changes in community structure. Here we describe a natural analogue site in the Western Baltic Sea. Seawater pCO₂ in Kiel Fjord is elevated for large parts of the year due to upwelling of CO₂ rich waters. Peak pCO₂ values of >230 Pa (>2300 μatm) and pH values of <7.5 are encountered during summer and autumn, average pCO₂ values are ~70 Pa (~700 μatm). In contrast to previously described naturally CO₂ enriched sites that have suggested a progressive displacement of calcifying auto- and heterotrophic species, the macrobenthic community in Kiel Fjord is dominated by calcifying invertebrates. We show that blue mussels from Kiel Fjord can maintain control rates of somatic and shell growth at a pCO₂ of 142 Pa (1400 μatm, pH=7.7). Juvenile mussel recruitment peaks during the summer months, when high water pCO₂ values of ~100 Pa (~1000 μatm) prevail. Our findings indicate that calcifying keystone species may be able to cope with surface ocean pH values projected for the end of this century. However, owing to non-linear synergistic effects of future acidification and upwelling of corrosive water, peak seawater pCO₂ in Kiel Fjord and many other productive estuarine habitats could increase to values >400 Pa (>4000 μatm). These changes will most likely affect calcification and recruitment, and increase external shell dissolution

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[1] Mytilus edulis were cultured for 3 months under six different seawater pCO 2 levels ranging from 380 to 4000 matm. Specimen were taken from Kiel Fjord (Western Baltic Sea, Germany) which is a habitat with high and variable seawater pCO 2 and related shifts in carbonate system speciation (e.g., low pH and low CaCO 3 saturation state). Hemolymph (HL) and extrapallial fluid (EPF) samples were analyzed for pH and total dissolved inorganic carbon (C T ) to calculate pCO 2 and [HCO 3 − ]. A second experiment was conducted for 2 months with three different pCO 2 levels (380, 1400 and 4000 matm). Boron isotopes (d 11 B) were investigated by LA-MC-ICP-MS (Laser Ablation-Multicollector-Inductively Coupled Plasma-Mass Spectrometry) in shell portions precipitated during experimental treatment time. Additionally, elemental ratios (B/Ca, Mg/Ca and Sr/Ca) in the EPF of specimen from the second experiment were measured via ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry). Extracellular pH was not significantly different in HL and EPF but systematically lower than ambient water pH. This is due to high extracellular pCO 2 values, a prerequisite for metabolic CO 2 excretion. No accumulation of extracellular [HCO 3 − ] was measured. Elemental ratios (B/Ca, Mg/Ca and Sr/Ca) in the EPF increased slightly with pH which is in accordance with increasing growth and calcification rates at higher seawater pH values. Boron isotope ratios were highly variable between different individuals but also within single shells. This corresponds to a high individual variability in fluid B/Ca ratios and may be due to high boron concentrations in the organic parts of the shell. The mean d 11 B value shows no trend with pH but appears to represent internal pH (EPF) rather than ambient water pH.

[1] In this study we present the first combined investigation into the composition of the major matrices involved in calcification processes (surrounding water, extrapallial fluid, aragonite, and calcite) of Mytilus edulis with respect to their calcium isotope (d 44/40 Ca) and elemental compositions (Sr/Ca and Mg/Ca). Our aim was to examine the suitability of Mytilus edulis as a proxy archive and to contribute to the understanding of the process of biomineralization. Mytilus edulis specimens were live collected from the Schwentine Estuary, Kiel Fjord, and North Sea (Sylt). d 44/40 Ca was determined by thermal ionization mass spectrometry (TIMS) accompanied by measurements of Mg/Ca and Sr/Ca using inductively coupled plasma-optical emission spectroscopy (ICP-OES). The elemental and isotopic compositions of the investigated matrices showed systematic offsets. The carbonates are strongly depleted in their magnesium and strontium concentrations and fractionated toward lighter calcium isotope compositions relative to the surrounding Schwentine Estuary water. The opposite is observed for the extrapallial fluid (EPF). Our findings extend the results of previous studies reporting a strong biological control and the interaction of different environmental conditions influencing biomineralization. Future studies should focus on the temporal development of the interrelation between the different matrices.

[1] Blue mussel individuals (Mytilus edulis) were cultured at four different salinities (17, 20, 29, and 34). During the course of the experiment, temperature was gradually increased from 6°C to 14°C. Mg/Ca and Sr/Ca ratios of the shell calcite portions produced during the 9 weeks of experimental treatment as well parts that were precipitated before the treatment phase were measured by laser ablation-multicollectorinductively coupled plasma-mass spectrometry. Mg/Ca ratios show a positive correlation with temperature for individuals cultured at salinity 29 and 34 (Mg/Ca (mmol/mol) ∼ (0.2-0.3)*T (°C)), while for individuals cultured at low salinities (17, 20) no trend was observed. Sr/Ca ratios were not affected by temperature but strongly by salinity. The data show very strong biological influence ("individual differences" and "physiological variability") on elemental ratios (79% on Mg/Ca and 41% on Sr/Ca) in M. edulis calcite. The results challenge the use of blue mussel shell data as environmental proxies.

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