Sensitivity of pteropod calcification to multi stressor variability in coastal habitats.

Comprehensive understanding of environmental multiple stressors on calcification in marine calcifiers remains an important topic of study, especially under ocean global change associated with multiple stressors. We explore the impact of multiple stressor on pteropod calcification in the southern Salish Sea (Washington, U.S.), a coastal estuarine system that exhibits a high degree of spatial and temporal variability in multiple environmental parameters across sampling locations. We hypothesized that such variability is associated with differences in pteropod calcification. Shell thickness and shell density across pteropod life history stages was compared with high-resolution outputs from a realistic model of regional circulation and biogeochemistry to explore how the mean and variability of multiple stressors (aragonite saturation state (Ω_ar), temperature, food availability) influence calcification. We found that both the mean and variability in multiple stressors play a major role in calcification in pteropods, with a generalized linear model explaining more than 60% of the variance. We suggest two different modes of shell building: stable conditions of lower mean Ω_ar trigger the loss of shell thickness and density. In the more variable habitats, i.e., where the variability occurs over diel and seasonal scales, shell thickness increases at higher Ω_ar variability and greater food availability, which might partially compensate for the loss of shell density. This plastic response appears to be consistent across life stages and could represent a response mechanism that allows some compensatory calcification under less favourable conditions. However, compensation is very limited, as evident by lower shell growth resulting in shell sizes comparable to early life stages. These results substantially improve the understanding of the variability in multiple stressors on the calcification process under multiple stressors and provide a foundation for the development of two new proxies for calcification monitoring, and with implications for marine carbon dioxide removal strategies.