Marine heatwaves and eutrophication jeopardize the seagrass Halodule wrightii and associated infauna.

Coastal ecosystems such as seagrass meadows endure challenges imposed by marine heatwaves (MHWs) and eutrophication. Although the isolated impacts of these stressors are well documented, their combined effects are yet to be better understood. To fill this knowledge gap, we conducted a mesocosm experiment evaluating the combined effects of stress caused by a MHW and exposure to effluent from a ruptured sewage pond -used as a proxy for eutrophication- on the seagrass Halodule wrightii and its associated infauna. Our results showed that H. wrightii and its infauna tolerated the conditions of the treatment simulating a MHW, with no negative responses observed, even with temperatures reaching 32 °C. However, when combined with eutrophication, the negative impacts were significantly amplified, demonstrating a synergy between these stressors. This included increased ammonium release, as well as decreased photosynthetic efficiency, and toxicity to H. wrightii, along with notable physiological stress and a reduction in associated infauna. These results align with the ecological changes observed in a coastal lagoon that experienced an acute eutrophication event and a MHW, resulting in a dystrophic crisis, browning of the waters, hypoxia, and mass mortality events. This study highlights how simultaneous stressors, at multiple scales, interact to impact key coastal ecosystems and biodiversity, increasingly threatened by climate change and rising coastal pollution.