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 biomineralized skeletons is well understood, but few studies have investigated the corrosive potential of significant freshwater input into marine habitats. To examine the susceptibility of invertebrate biocomposites to low salinity, we exposed blocks containing polished surfaces of pristine 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 across microstructures. Overall, shell dissolution at low salinities is significant and comparable to dissolution predicted under end-of-century ocean acidification scenarios. The compromised integrity of marine invertebrate biomineralized skeletons because of ocean freshening is of critical concern with future predicted increases in precipitation and sea ice melt.