Marine heatwaves (MHWs) are a major concern worldwide due to their increasing impacts in recent years, and these extreme events may trigger deoxygenation of coastal waters affected by sewage and eutrophication. Here we investigate the combined effects of MHWs and nutrient enrichment on the water quality and biodiversity of the Bay of Santa Catarina Island (Brazil). We used historical (1994–2020) sea surface temperature data from satellites and in situ physical, chemical and biological parameters to assess temporal trends. Oxygen levels have been decreasing whilst phosphorus levels have been increasing in the bay. During the austral summer of 2020 a regional sea surface heatwave was detected by satellite, lasting for 9 days and coinciding with our research cruise. During this period, seawater temperatures reached 29.8°C and anoxia was detected for the first time in the bay. A decrease in macrobenthic and phytoplankton community richness correlated with decreases in oxygen both through time and towards more urbanized areas. Overall, poor wastewater treatment is a key stressor that combined with MHWs to degrade coastal waters. Mitigation strategies are needed to minimize the impact of MHWs, including improved sewage treatment, restoration and conservation of wetlands and the use of nature-based technologies to promote coastal ecosystem recovery.

Marine heatwaves (MHWs) are defined as extended periods in which sea surface temperatures (SST) exceed a climatological threshold (Hobday et al., 2016). They have received much attention lately due to their harmful impacts on marine ecosystems, ranging from habitat shifts and changes in populations' genetics to high mortality of several marine species (Coleman et al., 2020;Smale et al., 2019). Recent studies have identified an increase in their frequency, duration, intensity, and spatial extension over most of global oceans and linked these trends to long-term warming (

Understanding how climatic and density‐dependent processes affect demography is crucial for predicting population responses to climate change. For marine invertebrates with complex life cycle such as decapod crustaceans, increasing temperatures might affect survival and development of early pelagic stages, whereas high density can increase competition and thus reduce growth and fecundity of older life stages. In this study, we investigate the effects of warm ocean events, body size and density on the population dynamics of the intertidal Sally lightfoot crab (Grapsus grapsus) at the Brazilian oceanic islands. Firstly, we assessed the trends of marine heatwaves (MHW) and positive temperature anomalies (ΔSST+) at the equatorial St Peter and St Paul (SPSP) Archipelago and Rocas Atoll and the subtropical Trindade Island. We then jointly analyzed short‐term count, capture‐recapture and fecundity data, and long‐term population monitoring data (2003–2019) using an integrated population model. Warm ocean events have become more frequent and intense only at the equatorial islands. Increasing MHW frequency positively influenced recruitment in the high‐density SPSP population, while MHW intensity and ΔSST+ frequency had negative impacts. Conversely, no climatic effects were observed for the low‐density Rocas population, which has the largest crabs. Despite a lack of warming in Trindade, this subtropical population with intermediate density and body size was negatively affected by ΔSST+. Our findings revealed population‐specific responses to climate change when accounting for local life history and ecology. Thus, environmental and density‐dependent effects should be broadly considered in future conservation studies regarding ocean warming impacts on marine invertebrate populations.