Description: Numerous studies have been conducted on the effect of ocean acidification on calcifiers inhabiting nearshore benthic habitats, such as the blue mussel Mytilus edulis. The majority of these experiments was performed under stable CO2 partial pressure (pCO2), carbonate chemistry and oxygen (O2) levels, reflecting present or expected future open ocean conditions. Consequently, levels and variations occurring in coastal habitats, due to biotic and abiotic processes, were mostly neglected, even though these variations largely override global long-term trends. To highlight this hiatus and guide future research, state-of-the-art technologies were deployed to obtain high-resolution time series of pCO2 and [O2] on a mussel patch within a Zostera marina seagrass bed, in Kiel Bay (western Baltic Sea) in August and September 2013. Combining the in situ data with results of discrete sample measurements, a full seawater carbonate chemistry was derived using statistical models. An average pCO2 more than 50 % (~ 640 µatm) higher than current atmospheric levels was found right above the mussel patch. Diel amplitudes of pCO2 were large: 765 ± 310 (mean ± SD). Corrosive conditions for calcium carbonates (Ωarag and Ωcalc 〈 1) centered on sunrise were found, but the investigated habitat never experienced hypoxia throughout the study period. It is estimated that mussels experience conditions limiting calcification for 12–15 h per day, based on a regional calcium carbonate concentration physiological threshold. Our findings call for more extensive experiments on the impact of fluctuating corrosive conditions on mussels. We also stress the complexity of the interpretation of carbonate chemistry time series data in such dynamic coastal environments.

Description: The Baltic Sea has a salinity gradient decreasing from fully marine (〉 25) in the west to below 7 in the central Baltic Proper. Habitat-forming and ecologically dominant mytilid mussels exhibit decreasing growth when salinity 〈 11; however, the mechanisms underlying reduced calcification rates in dilute seawater are not fully understood. Both [HCO−3] and [Ca2+] also decrease with salinity, challenging calcifying organisms through CaCO3 undersaturation (Ω≤1) and unfavourable ratios of calcification substrates ([Ca2+] and [HCO−3]) to the inhibitor (H+), expressed as the extended substrate–inhibitor ratio (ESIR). This study combined in situ monitoring of three southwest Baltic mussel reefs with two laboratory experiments to assess how various environmental conditions and isolated abiotic factors (salinity, [Ca2+], [HCO−3] and pH) impact calcification in mytilid mussels along the Baltic salinity gradient. Laboratory experiments rearing juvenile Baltic Mytilus at a range of salinities (6, 11 and 16), HCO−3 concentrations (300–2100 µmol kg−1) and Ca2+ concentrations (0.5–4 mmol kg−1) reveal that as individual factors, low [HCO−3], pH and salinity cannot explain low calcification rates in the Baltic Sea. Calcification rates are impeded when Ωaragonite ≤ 1 or ESIR ≤ 0.7 primarily due to [Ca2+] limitation which becomes relevant at a salinity of ca. 11 in the Baltic Sea. Field monitoring of carbonate chemistry and calcification rates suggest increased food availability may be able to mask the negative impacts of periodic sub-optimal carbonate chemistry, but not when seawater conditions are permanently adverse, as observed in two Baltic reefs at salinities 〈 11. Regional climate models predict a rapid desalination of the southwest and central Baltic over the next century and potentially a reduction in [Ca2+] which may shift the distribution of marine calcifiers westward. 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.