Supplementary material to "Decoupling salinity and carbonate chemistry: Low calcium ion concentration rather than salinity limits calcification in Baltic Sea mussels"

The Baltic Sea has a salinity gradient decreasing from fully marine (> 25) in the West to below 7 in the Central Baltic Proper. Reef forming mytilid mussels exhibit decreasing growth when salinity < 11, however the 15 mechanisms underlying reduced calcification rates in dilute seawater are not fully understood. In fact, both [HCO3 ] and [Ca] also decrease with salinity, challenging calcifying organisms through CaCO3 undersaturation (Ω ≤ 1) and unfavourable ratios of calcification substrate (Ca and HCO3) to inhibitor (H). In this study we assessed the impact of isolated individual factors (salinity, [Ca], [HCO3] and pH) on calcification and growth of mytilid mussel populations along the Baltic salinity gradient. Laboratory experiments rearing juvenile Baltic Mytilus at a 20 range of salinities (6, 11 and 16), HCO3 concentrations (300-2100 μmol kg) and Ca concentrations (0.5-4 mmol kg) were coupled with field monitoring in three Baltic mussel reefs. Results reveal that as individual factors, low [HCO3], pH and salinity cannot explain low calcification rates in the Baltic Sea. Calcification rates are impeded when Ωaragonite ≤ 1 or the substrate inhibitor ratio ≤ 0.7, primarily due to [Ca] limitation which corresponds to a salinity of ca. 11. Increased food availability may be able to mask these negative impacts, but 25 not when seawater conditions are permanently adverse, as observed in two Baltic reefs at salinities < 11. Future climatic models predict rapid desalination of the southwest and Central Baltic and potentially a reduction in [Ca] which may lead to a westward distribution shift of marine calcifiers. It is therefore vital to understand the mechanisms by which the ionic composition of seawater impacts bivalve calcification for better predicting the future of benthic Baltic ecosystems. 30