Season-Specific Effects of Marine Heatwaves on Macrofaunal Community Composition and Ecosystem Processes

Marine HeatWaves (MHWs) increasingly modify marine ecosystems, and this is most prominent when high extreme summer temperatures exceed species’ tolerances. Less is known about the effects on ecosystem structure and processes of relative extreme temperatures in other seasons, despite heatwaves being increasingly common throughout the year. We conducted four repetitive laboratory experiments, in winter, spring, summer and autumn, to investigate the effects of seasonal MHWs on the processes and structure of natural benthic communities. Intact sediment cores were exposed to climatologically normal and strong MHW temperatures for 15 days. Community respiration and nutrient fluxes were measured every 5 days, and the community composition and their vertical distribution in the sediment were assessed at the end of each experiment. Our results showed season-specific changes in community metabolism due to MHWs, with the strongest impact in summer, followed by autumn, spring and least in winter. Notably, no mortality-induced differences in the composition over the whole communities between control and MHW treatments were detected. In summer, a migration into deeper sediments was observed, with the fauna possibly seeking a thermal refuge, highlighting a response to high absolute temperatures. Surprisingly, community metabolism was constant year-round under climatologically normal temperatures despite a difference of 7.4 °C between winter and summer, strongly suggesting seasonally adapted communities. In spring and autumn, strong heatwaves with temperatures of 8.5 °C caused distinctly increased respirations rates compared to summer, where this temperature is normal. Together, these findings add a seasonal perspective to the marine heatwave literature by demonstrating that the responses of ecosystem processes depend on the seasonal context of the system and vary with both relative and absolute high temperatures in a natural community adapted to strong seasonality. More generally, incorporating seasonality into marine heatwave assessments may improve predictions of ecosystem vulnerability under future climate extremes.