Mineralogy and organic content are major predictors of shell loss in bivalves under reduced salinity, ocean freshening conditions

Ocean freshening due to increased precipitation and ice melting in a
warming world poses a significant threat to marine calcifiers. The reduced
availability of calcification substrates and an undersaturated calcium
carbonate state challenge shell construction and maintenance. The
corrosive potential of ocean acidification on biomineralised skeletons is
well understood, but few studies have investigated the corrosive potential
of significant and irregular freshwater input into marine habitats. To
examine the susceptibility of invertebrate biocomposites to low salinity,
we exposed blocks containing shell material of six bivalve taxa,
representing different mineralogies and microstructures, to a salinity
gradient (0–45‰) for 180 days. By measuring the loss of shell thickness,
we revealed a significant correlation between dissolution and decreasing
salinity. Significantly different amounts of shell thickness loss were
observed across microstructures, revealing mineralogy and organic content
as important predictors for dissolution. Aragonite layers lost
significantly more shell thickness than calcite, and higher organic
content retarded dissolution in aragonitic microstructures. Overall, shell
dissolution in low salinities is greater than under the acidification
predicted for end-of-century ocean pH conditions. The compromised
integrity of marine invertebrate biomineralised skeletons because of ocean
freshening is of critical concern with future predicted increases in
precipitation and sea ice melt.