Bethany F. King

Calcifying bivalves account for > 80% of the total macrozoobenthic biomass in the southern Baltic Sea. Salinity declines from around 30 in the western Kattegat to 7 in the central Baltic Proper providing a natural gradient along which to investigate the impact of lowering salinity on shelled organisms. Low salinity conditions constrain biomineralisation with limited calcification substrates and periods of aragonite undersaturation. For species with wholly aragonitic shells like the cockle, Cerastoderma glaucum , shell maintenance and production can be challenging. While studies have investigated the corrosive effects of ocean acidification on calcifying organisms, few have considered the long-term consequences of low salinity. This study investigated the shell characteristics of C. glaucum sampled in 2016 and 2024–25 along the southern Baltic salinity gradient to examine variations in shell thickness, shape and condition. Comparisons were made with populations sampled from coastal lagoons in 2024–25 (UK and France) where seasonal salinity fluctuations are significant (< 20 to > 40). This comparative study revealed Baltic populations of C. glaucum exposed to salinities of < 10 to produce shells that are up to 85.2% thicker than populations in more variable environments. However, their size is limited to lengths of 20 mm, around 58% of the usual length achieved by individuals in salinities of > 10. Integral to survival, the shell offers protection from predation and adverse ambient conditions. Ocean freshening poses significant threats to biomineralisation and understanding the response of calcifying species to low salinity is fundamental for assessing their adaptive capacity to future projections of declining salinity.

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.