Calcification in a marginal sea – influence of seawater [Ca²⁺] and carbonate chemistry on bivalve shell formation

In estuarine coastal systems such as the Baltic Sea, mussels suffer from low salinity which limits their distribution. Anthropogenic climate change is expected to cause further desalination which will lead to local extinctions of mussels in the low saline areas. It is commonly accepted that mussel distribution is limited by osmotic stress. However, along the salinity gradient, environmental conditions for biomineralization are successively becoming more adverse as a result of reduced [Ca²⁺] and dissolved inorganic carbon (<i>C</i>_T) availability. In larvae, calcification is an essential process starting during early development with formation of the prodissoconch I (PD I) shell, which is completed under optimal conditions within 2 days.<br><br> Experimental manipulations of seawater [Ca²⁺] start to impair PD I formation in <i>Mytilus </i>larvae at concentrations below 3 mM, which corresponds to conditions present in the Baltic at salinities below 8 g kg⁻¹. In addition, lowering dissolved inorganic carbon to critical concentrations (&lt; 1 mM) similarly affected PD I size, which was well correlated with calculated Ω_Aragonite and [Ca²⁺][HCO₃⁻] ∕ [H⁺] in all treatments. Comparing results for larvae from the western Baltic with a population from the central Baltic revealed a significantly higher tolerance of PD I formation to lowered [Ca²⁺] and [Ca²⁺][HCO₃⁻] ∕ [H⁺] in the low saline adapted population. This may result from genetic adaptation to the more adverse environmental conditions prevailing in the low saline areas of the Baltic.<br><br> The combined effects of lowered [Ca²⁺] and adverse carbonate chemistry represent major limiting factors for bivalve calcification and can thereby contribute to distribution limits of mussels in the Baltic Sea