Short-term resilience, long-term costs: Reduced growth and increased erosion in the kelp Ecklonia radiata (phylum Ochrophyta) following repeated marine heatwaves.

Marine heatwaves (MHWs) have increased in frequency by 34% since 1990 and are projected to rise further with global ocean change, posing significant risks to marine ecosystems. Kelps (order Laminariales) provide essential habitats and play key ecological roles, but they are increasingly threatened by MHWs. Tasmania, SE Australia, is a global warming hotspot, but the impacts of recurrent MHWs on the physiological performance of the ecologically dominant kelp Ecklonia radiata remain poorly understood. To address this, we investigated how the frequency of MHWs influenced the physiological and biochemical performance of E. radiata, both during and after MHWs, to evaluate immediate responses and recovery potential. In laboratory experiments, juvenile sporophytes were exposed to three experimental treatments: no-MHW, a single 6-day MHW, and double 6-day MHWs followed by 7-day recovery periods. Ecklonia radiata sporophytes were resilient to the single 6-day MHW, but double MHWs negatively impacted recovery, with reduced growth rates and increased tissue erosion. Although photosynthetic rates remained unaffected, changes in pigment ratios and increased antioxidant activity indicated a mitigation of physiological stress. We propose that energy may be diverted from growth toward repair processes and the maintenance of essential functions. These findings suggest there was cumulative stress caused by repeated MHWs, leading to progressive physiological decline. More frequent MHW events may hinder E. radiata's recovery capacity, with potential ecosystem implications, considering its key ecological role.