Differential photophysiological responses of intertidal seaweeds to increasing thermal and desiccation stressExtreme Events

Maike Kaffenberger^1,2, Maike Kaffenberger^1,2, Ian Hawes², Chris Battershill², Leigh Tait³, David Schiel¹

¹University of Canterbury, New Zealand - ²University of Waikato, New Zealand - ³National Institute of Water and Atmospheric Research, New Zealand

The intertidal zone is one of the most physiologically stressful environments on earth. Organisms must contend with substantial daily fluctuations in temperature and desiccation stress, which are becoming more variable due to the increasing frequency and duration of marine and atmospheric heat waves. These extreme events are especially stressful for non-mobile intertidal organisms, like seaweeds, which facilitate other intertidal organisms, but are incapable of actively avoiding temperature or desiccation stress. When these extreme climatic events coincide with prolonged emersion durations during spring tide, seaweed can suffer severe desiccation and oxidative damage that exceeds their physiological limits, leading to die offs of seaweeds and associated organisms. To assess the vulnerability of ecologically important habitat-forming intertidal seaweeds in New Zealand to emersion stress during marine and atmospheric heatwaves, we simulated heat wave scenarios in controlled laboratory experiments. We tested how varying water temperature, air temperature, and relative humidity over consecutive emersion cycles affected photoinhibition (damage to photosystem II) and the potential for recovery upon rehydration. There is a clear interaction between coinciding elevated air and seawater temperatures, which together can take intertidal seaweeds well past their compensation points. Different species have varying responses in photoinhibition and recovery and therefore in resilience levels and desiccation thresholds. While both temperature and humidity influence desiccation, relative humidity has a more pronounced effect on the extent of dehydration and the associated physiological stress. This study provides insights into species-specific resilience in the face of multiple heat-related stressors associated with a changing climate.