Evidence of adaptive local thermal resilience in marine forests, over a latitudinal gradientBiology & Ecophysiology

Sofía Hernández-Chan^1,2, Patrícia G. Cardoso1¹, Laurids N. Enevoldsen³, Paul E. Renaud³, Fernando Lima⁴, Francisco Arenas¹,

¹Interdisciplinary Centre for Marine and Environmental Research (CIIMAR), Portugal - ²Instituto de Ciências Biomédicas Abel Salazar da Universidade do Porto (ICBAS), Portugal - ³Akvaplan-niva, Fram Centre for Climate and Environment, Norway - ⁴Centro de Investigação em Biodiversidade e Recursos Genéticos (BIOPOLIS–CIBIO), Portugal

Kelps play a pivotal role in coastal ecosystems, supporting biodiversity, sequestering carbon, and mitigating coastal erosion. However, over the past few decades, they have experienced a global decline as a result of climate change. In particular, the projected near-future increase in intensity, duration and frequency of extreme weather events, such as marine heatwaves (MHWs), poses a considerable threat to these marine forests: exposure to prolonged periods of excessive warmth beyond specific physiological thresholds can easily lead to death. Today, significant knowledge gaps in our understanding of MHWs’ impacts on marine forests still hinder the development of accurate predictions for their resilience and the implementation of effective conservation policies. The recent (re)introduction of thermal death time (TDT) assessments with ectothermic organisms shows there is an exponential relationship between exposure time and temperature. Thus, determining TDT functions improves the accuracy of predicted specific thermal tolerance (TT) limits to extreme events like MHWs, which are characterised by their duration (time) and intensity (temperature). Here, we adapted Rezende’s and Jorgensen’s unifying TT frameworks and applied them 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 species and populations’ specific TT functions. Our methodology successfully allowed us to compare and identify the specific TT boundaries of three habitat founder species, over a latitudinal gradient from the Northern Iberian Peninsula to the Arctic Circle. Our findings have significant implications for the conservation and management of marine forest ecosystems, helping us identify local TT adaptation processes and anticipate species’ responses under changing temperature regimes and projected MHWs.