Marine heatwaves shift intertidal marine communities in the SW Atlantic.

Background: Accelerating rates of ocean warming are a global threat to coastal marine ecosystems. More frequent heat extremes, also known as marine heatwaves, are likely to cause severe impacts on marine ecosystems as species may be unable to tolerate, adapt to, or recover from these events. Methods: In this study, we examined the association between the occurrence of marine heatwaves and benthic cover at intertidal reefs in the SW Atlantic. To investigate these effects, we monitored Results: Our results revealed that temporal changes in macroalgal cover during the study period were highly correlated (>80% association) with marine heatwaves, which were the dominant extreme weather conditions in this region. Stronger (intensity effect) and prolonged (weeks) events result in a significant decrease (38% loss) in macroalgal cover with impacts on benthic richness and diversity both locally and regionally. The impacts were more pronounced and drove succession on dominant macroalgal taxa (brown and red algae), which were the least resilient to high temperatures. Although there were indications of macroalgal recovery after 2020, the community did not return to the pre-heatwave composition, revealing that assemblage succession over these macroalgal beds may occur at multi-year time scales. Our study supports previous research indicating that marine heatwaves are becoming more frequent along the Brazilian coast. Given the prolonged period for assemblages' recovery, we can expect marked decreases in coastal biodiversity in the SW Atlantic.