Abstract
Abstract. 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 [HCO3-] and [Ca2+] also decrease with salinity, challenging calcifying organisms through CaCO3 undersaturation (Ω≤1) and unfavourable ratios of calcification substrates ([Ca2+] and [HCO3-]) 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+], [HCO3-] 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), HCO3- concentrations (300–2100 µmol kg−1) and Ca2+ concentrations (0.5–4 mmol kg−1) 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 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.
Highlights
1.1 Baltic Sea hydrochemistryThe Baltic Sea is a semi-enclosed brackish water body with a decreasing salinity (S) gradient from > 25 in the Kattegat to salinities < 3 in the Gulf of Bothnia and the Gulf of Finland (Meier, 2006; Neumann, 2010; Fig. 1)
Calculated mean [HCO−3 ] at the three field sites (Table 1) were significantly higher in Kiel compared to Ahrenshoop and Usedom (ANOVA, F(2, 55) = 38.80, P < 0.001, Table S6) with values never dropping below 1600 μmol kg−1 at any of the three sites during the monitoring period (Fig. 3c)
Mean extended substrate–inhibitor ratio (ESIR) values were below the threshold of 0.7 proposed by Thomsen et al (2018) in Ahrenshoop and Usedom, periods above and below this threshold were observed at all sites (Fig. 3g)
Summary
1.1 Baltic Sea hydrochemistryThe Baltic Sea is a semi-enclosed brackish water body with a decreasing salinity (S) gradient from > 25 in the Kattegat to salinities < 3 in the Gulf of Bothnia and the Gulf of Finland (Meier, 2006; Neumann, 2010; Fig. 1). Salinity and seawater carbonate chemistry are highly variable on both spatial and temporal scales with extreme fluctuations resulting from occasional inflow events of highly saline North Sea water, wind-driven upwelling of hypoxic and hypercapnic deep water, lateral transport of water masses, and pronounced regional salinity gradients (Thomas and Schneider, 1999; Melzner et al, 2013; Saderne et al, 2013; Mohrholz et al, 2015). Along the Baltic Sea salinity gradient, seawater total alkalinity (AT) decreases linearly from. Along with CT, seawater [Ca2+] decreases linearly with salinity from the North Sea to the Baltic Proper (Kremling and Wilhelm, 1997). This decreasing availability of seawater Ca2+ and CT puts pressure on calcifying organisms that extract these 2 substrates (Ca2+ and CT) from seawater for calcification
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