Decoupling salinity and carbonate chemistry: low calcium ion concentration rather than salinity limits calcification in Baltic Sea mussels

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.

Polymorphism ofCaCO3and the variability of elemental composition of the calcareous skeletons secreted by invertebrates along the salinity gradient of the Baltic Sea

“…Although the parameters of the seawater carbonate system such as alkalinity, the concentrations of Ca 2+ or the saturation state of calcite and aragonite were not recorded in this study, it is established that in the Baltic Sea they decrease with the decreasing salinity (Beldowski et al, 2010;Müller et al, 2016;Sanders et al, 2021).…”

“…The present study, focusing on the calcifying organisms from the Baltic Sea, shows that the mineralogical type (calcite, aragonite, or their combination) of the bulk skeleton depends simultaneously on many biological (e.g., evolutionary adaptations, biomineralization strategy, metabolic rate, growth rate, calcification rate), and environmental (e.g., temperature, the availability of elements) factors and the dependence on salinity is not obvious (Tables 2 and S2). Although the parameters of the seawater carbonate system such as alkalinity, the concentrations of Ca 2+ or the saturation state of calcite and aragonite were not recorded in this study, it is established that in the Baltic Sea they decrease with the decreasing salinity (Beldowski et al, 2010; Müller et al, 2016; Sanders et al, 2021). Yet, the Baltic population of calcifying organisms does not seem to reflect this environmental trend in the mineralogical composition (calcite to aragonite ratio) of their skeleton (Tables 2 and S2).…”

“…The bimineralic bryozoan E. immersa collected in the Scottish waters (salinity = 30–35, mean water temperature = 8°C, the North Sea) contained on average 31 wt% (range 25–99 wt%, n = 147) skeletal aragonite, while E. immersa from the Baltic Sea (this study; salinity = 27.2, mean water temperature = 10.7°C) had a lower 24 ± 7.5 wt% (range 14–39 wt%, n = 13) aragonite content (Table 2). Concentrations of Ca 2+ , the saturation state of CaCO 3 , and the ratio of calcification substrates (Ca 2+ and HCO 3− to H + ) decrease along the salinity gradient (Beldowski et al, 2010; Müller et al, 2016; Sanders et al, 2021); therefore, lower aragonite content in the Baltic population could have been caused by the lower concentration of carbonate system parameters in the brackish waters. Along the Baltic Sea salinity gradient, the total alkalinity of seawater decreases linearly from approximately 2300 μmol/kg in the North Sea to approximately 1600 μmol/kg at salinity six in the Baltic Proper (Müller et al, 2016).…”

“…Calcium carbonate (CaCO 3 ) formation strongly depends on the seawater total alkalinity, availability of calcium ions (Ca 2+ ), and dissolved inorganic carbon (DIC), for example, carbonate ions (CO 3 2− ) and bicarbonate ions (HCO 3 − ). There is a salinity‐dependent trend that the concentrations of key carbonate species (e.g., CO 3 2− and HCO 3 − ) decrease proportionally with decreasing salinity (Beldowski et al, 2010; Müller et al, 2016; Sanders et al, 2021). Studying the skeletal variability along a salinity gradient is needful to reveal how calcifying organisms deal with a low CaCO 3 saturation state, that is, the ratio of the concentrations of Ca 2+ and CO 3 2− to the apparent solubility product of CaCO 3, which, in turn, depends on the temperature, pressure, and salinity.…”

“…is a worldwide, coastal, brackish, and estuarine mussel, that is very abundant in the Baltic Sea, acting as a foundation species and playing an important ecological role in benthic nutrient recycling (Kautsky & Wallentinus, 1980; Norling & Kautsky, 2008). Based on the example of the Baltic Mytilus spp., it was shown that the availability of seawater Ca 2+ and DIC for organisms affects calcification (Thomsen et al, 2018), yet the mechanisms underlying decreasing calcification rates along decreasing salinity are not fully understood (Sanders et al, 2021). In the Baltic Sea, alkalinity, the concentrations of Ca 2+ , the saturation state of CaCO 3 , and the ratio of calcification substrates (Ca 2+ and HCO 3− ) to the concentration of protons (H + ) decrease along the salinity gradient (Beldowski et al, 2010; Müller et al, 2016; Sanders et al, 2021) and create changing conditions for the calcification process, making the Baltic Sea an ideal model system to investigate the variability of skeletal mineralogy and geochemistry between calcareous invertebrates.…”