Preprints
https://doi.org/10.5194/bg-2020-382
https://doi.org/10.5194/bg-2020-382

  14 Nov 2020

14 Nov 2020

Review status: this preprint is currently under review for the journal BG.

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

Trystan Sanders1, Jörn Thomsen1, Jens Daniel Müller2, Gregor Rehder3, and Frank Melzner1 Trystan Sanders et al.
  • 1Marine Ecology, Helmholtz Centre for Ocean Research (GEOMAR), Kiel, Germany
  • 2Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland
  • 3Department of Marine Chemistry, Leibniz Institute for Baltic Sea Research, Warnemünde, Germany

Abstract. 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 mechanisms underlying reduced calcification rates in dilute seawater are not fully understood. In fact, both [HCO3] and [Ca2+] also decrease with salinity, challenging calcifying organisms through CaCO3 undersaturation (Ω ≤ 1) and unfavourable ratios of calcification substrate (Ca2+ and HCO3) to inhibitor (H+). In this study we assessed the impact of isolated individual factors (salinity, [Ca2+], [HCO3] and pH) on calcification and growth of mytilid mussel populations 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) 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 [Ca2+] limitation which corresponds to a salinity of ca. 11. Increased food availability may be able to mask these negative impacts, but 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 [Ca2+] 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.

Trystan Sanders et al.

 
Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

Trystan Sanders et al.

Trystan Sanders et al.

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Short summary
Salinity in the Baltic Sea is predicted to decrease over the next century due to climate change. This has implications for habitat forming mussels which utilize dissolved seawater ions to build calcium carbonate shells. Combining laboratory experiments with field monitoring reveals that calcium ion limitation is the primary factor limiting shell growth in mussels at low salinities with potential implications for future Baltic Sea ecosystems.
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