Metabolic traits and thresholds to inform marine ecological conservation and restoration

The combined effects of anthropogenic pressures and climate change pose significant threats to key habitat-forming species, such as seagrasses. Understanding species' responses to environmental stressors and identifying their tolerance thresholds are essential for effective conservation and restoration efforts in coastal environments. Through a mesocosm experiment, we assessed Posidonia oceanica's metabolic responses under ecologically realistic conditions across three seasonal periods (February-March, June-July, and October-November) when plants were naturally acclimated to different temperature regimes. Within each period, we tested plant responses to small temperature variations (ambient and two increasing steps of 2°C) crossed with four turbidity levels (0-34 mg/L), enabling construction of ecologically realistic thermal performance curves. Our findings reveal that turbidity may impair P. oceanica functioning, including decreased thermal performance and narrowed thermal tolerance window, impairing photosynthesis and potentially limiting growth. Metabolism increased with temperature up to a thermal optimum (Topt) identified at 23 °C for all turbidity and exposure time treatment levels. We demonstrate the relevance of stressor properties on P. oceanica responses, with individuals exposed to the more extreme treatment (high turbidity (34 mg/L) and increased exposure time (7 days)) presenting a reduced optimal thermal tolerance with respect to control. We advocate integrating metabolic traits into monitoring protocols as early warning indicators of ecosystem stress. This approach can strengthen both conservation and restoration initiatives by informing policy decisions, particularly in the context of increasing coastal development and climate change.