Impacts of marine heatwaves on benthic estuarine populations

Marine heatwaves (MHWs) are among the most severe consequences of climate change for marine ecosystems, particularly affecting tropical organisms that live near their upper thermal tolerance limits. The impacts of these Heating effects can vary over time, including immediate, delayed, and cumulative effects. Extreme heat events can trigger unprecedented ecosystem changes and biodiversity loss, underscoring the urgency of understanding how marine organisms respond. We aimed to quantify and classify MHWs in an estuarine complex in southern Brazil and to evaluate the effects of strong MHWs on the density of 24 benthic species over time. Seasonal density data from three rocky shores, collected between 2015 and 2022, were used to calculate the effect size of MHWs on each species during and after the events. Cumulative effects were assessed using linear regression between effect size during MHWs and the number of years since the onset of MHWs. We found an increase in the average intensity, duration, and frequency of MHWs from 1982 to 2023, with four strong events during the study period. No cumulative effects were detected. However, 67% of the species evaluated were affected by strong MHWs, with predominantly negative effects. Although physiological plasticity may have enabled population recovery following these events, delayed effects were observed, resulting in reduced species density in subsequent years. Despite signs of resilience on part of the community, both negative and positive responses among other organisms can compromise ecological functions and drive shifts in benthic community composition. Our findings emphasize the need for continuous monitoring and targeted management strategies to conserve estuarine benthic fauna in the face of the predicted increasing MHW frequency and intensity.