Effects of marine heatwaves on primary and secondary production in macroalgae-amphipod systems

Marine heatwaves are becoming more frequent and intense as climate change progresses, with potential consequences for the functioning of marine ecosystems, particularly macroalgal beds and their associated mesoherbivores. While the direct effects of heatwaves on macroalgae have been well studied, the interactions between species at different trophic levels that affect ecosystem functioning remain underexplored. The aim of this study was to investigate how marine heatwaves affect primary and secondary productivity in marine ecosystems. We conducted a mesocosm experiment combining the macroalga Sargassum filipendula and the mesoherbivore amphipod Cymadusa filosa under two temperature scenarios: a current summer temperature (27 °C) and a heatwave scenario (32 °C), with and without herbivores. The experiment lasted 30 days, with 5 days of marine heatwave. All replicates were kept at 27 °C for ten days. Then, the 'heatwave' treatment replicates were exposed to 32 °C for five days. Subsequently, all replicates were returned to 27 °C and maintained for 15 days until the end of the experiment. We evaluated the variation in macroalgal biomass and the variation in amphipod biomass and abundance. The results showed that heatwaves reduced primary and secondary productivity, with the greatest effects observed on primary producers. The reduction in primary productivity suggests that these extreme events may compromise the ability of macroalgae to support the base of the coastal food web and facilitate the occurrence of an abundant and diverse associated fauna. Thus, changes in mesoherbivore biomass may have significant implications for higher trophic levels, affecting the dynamics and stability of marine ecosystems. These results suggest that marine heatwaves affect the functioning of marine ecosystems by reducing productivity, potentially altering the flow of energy and matter along the food web, and affecting ecosystem services such as carbon storage by algae.