Tolerance of organisms composing an Arctic kelp community to ocean warming and marine heatwaves

Abstract The Arctic region is experiencing rapid warming and an increase in the frequency and intensity of marine heatwaves (MHWs), posing an increasing risk to coastal ecosystems. Kelp communities in the Arctic are vital habitats that support biodiversity and resilience but are particularly vulnerable to increasing temperature anomalies. In this study, we conducted a mesocosm experiment over 23 days in summer to assess the effects of warming and MHWs on three kelp species ( Saccharina latissima , Laminaria digitata and Alaria esculenta ) and associated community members: coralline algae, snails, sea urchins and mussels. Three warming scenarios were tested, all of which were offset from a naturally variable control. The four conditions were a control (i.e. ambient seawater) which ranged from ~7.5 to 10°C (naturally variable), a constant high‐temperature treatment (+1.8°C from the control), a treatment simulating a single 13‐day heatwave with a maximum offset temperature of +2.8°C from the control, and a treatment simulating two consecutive 5‐day MHWs with a maximum offset of +3.9°C. For the three kelp species, net photosynthetic rate, maximum quantum yield, chlorophyll a content, carbon to nitrogen ratio and growth rate were investigated. For the associated species, only growth rates were evaluated. Laminaria digitata showed no significant changes in any of the measured parameters, suggesting a robust tolerance to heat stress. Conversely, the maximum quantum yield for S. latissima decreased in the two heatwaves treatment. In addition, A. esculenta displayed a decrease in chlorophyll a content and net photosynthesis over time in the two heatwaves treatment. The growth rates of coralline algae, mussels, snails and sea urchins were not affected by the treatments. Synthesis : These results suggest that Arctic kelp communities are fairly tolerant to warming and short‐term MHWs, despite the few minor effects on photosynthesis by some of the kelp species. We note that the future response of kelp communities must account for the potential indirect effects of climate change, such as predation and competition, along with any consequential effects of warming anomalies, as those presented here.