Calcifying invertebrates succeed in a naturally CO2-rich coastal habitat but are threatened by high levels of future acidification

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 pCO(2) in Kiel Fjord is elevated for large parts of the year due to upwelling of CO2 rich waters. Peak pCO(2) values of >230 Pa (>2300 mu atm) and pH(NBS) values of <7.5 are encountered during summer and autumn, average pCO(2) values are similar to 70 Pa (similar to 700 mu atm). 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 pCO(2) of 142 Pa (1400 mu atm, pH(NBS) = 7.7). Juvenile mussel recruitment peaks during the summer months, when high water pCO(2) values of similar to 100 Pa (similar to 1000 mu atm) 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 mu atm). These changes will most likely affect calcification and recruitment, and increase external shell dissolution.