Predicting thermal tolerance limits of marine forests' seaweed species under a changing climate

Marine seaweed forest habitats, such as kelp forests, have experienced a global decline over the past few decades due to the adverse effects of climate change (CC), particularly due to extreme marine heatwaves (MHWs). Characterized by a period of excessive warmth above a specific threshold, MHWs are predicted to increase in intensity, duration and frequency in the near future. However, significant knowledge gaps in our understanding of MHWs' impacts on marine forests hinder the development of accurate predictions for their resilience and the implementation of effective conservation policies.Thermal tolerance (TT) assessments with ectothermic organisms show there is an exponential relationship between tolerance time and temperature, where heat tolerance decreases with increased exposure time to a thermal challenge. To better understand TT of marine forests macroalgae species, we adapted Rezende's and Jorgensen's unifying TT limit frameworks and applied it for the first time in macroalgae. Our assessments involved dynamic ramping temperature experiments, where thermic stress and critical thermal maxima were evaluated via chlorophyll fluorescence quantification, to determine specific thermal performance curves and estimate tolerance probability functions for each species. Our fitted estimates closely resemble empirical values obtained from the literature across different thermal regimes, allowing us to predict species' thermal limits and the intensity of selection under changing temperature regimes and projected MHWs. Our findings have significant implications for the conservation and management of marine forest ecosystems, as this framework could help us predict species' thermal tolerance and anticipate their responses to CC.