Sea surface temperature and chlorophyll concentration effects on suspended blue mussel Mytilus sp. health biometrics in Irish waters, 1992-2022

Over a 30 yr period (1992-2022), suspended blue mussels Mytilus sp. in Irish waters exhibited significant declines in biometrics, marked by reductions in wet tissue weight (17%) and condition index, alongside increases in shell:dry tissue and moisture:dry weight ratios (17%). These changes coincided with complex sea surface temperature (SST) dynamics as episodic warming and cooling phases occurred. SST increased overall, with warmer temperatures—particularly minimum and maximum SST—being positively associated with tissue growth and condition at broader spatial scales. Conversely, cooler periods with increased cold days (<10°C), notably 2002-2011, corresponded with elevated moisture:dry weight ratio, indicating stress or nutritional limitation. Chlorophyll concentrations, used as a proxy for food availability, remained relatively stable throughout, although a slight but significant rise in minimum values suggested a potential baseline shift. Importantly, the influence of environmental drivers varied by scale: SST exerted the strongest predictive power when all data were analysed, while at finer spatial resolutions—especially within transitional waters—chlorophyll metrics showed stronger, positive correlations with biometric health. By contrast, northern and coastal sites exhibited negative or mixed chlorophyll relationships, potentially reflecting poor food quality or eutrophic stress. These findings highlight the scale-dependent nature of mussel responses to environmental change, and underscore the need to consider both thermal and trophic drivers when evaluating bivalve health under future climate scenarios.