Global impacts of marine heatwaves on coastal foundation species.

With increasingly intense marine heatwaves affecting nearshore regions, foundation species are coming under increasing stress. To better understand their impacts, we examine responses of critical, habitat-forming foundation species (macroalgae, seagrass, corals) to marine heatwaves in 1322 shallow coastal areas located across 85 marine ecoregions. We find compelling evidence that intense, summer marine heatwaves play a significant role in the decline of foundation species globally. Critically, detrimental effects increase towards species warm-range edges and over time. We also identify several ecoregions where foundation species don't respond to marine heatwaves, suggestive of some resilience to warming events. Cumulative marine heatwave intensity, absolute temperature, and location within a species' range are key factors mediating impacts. Our results suggest many coastal ecosystems are losing foundation species, potentially impacting associated biodiversity, ecological function, and ecosystem services provision. Understanding relationships between marine heatwaves and foundation species offers the potential to predict impacts that are critical for developing management and adaptation approaches.

A marine heatwave drives significant shifts in pelagic microbiology.

Marine heatwaves (MHWs) cause disruption to marine ecosystems, deleteriously impacting macroflora and fauna. However, effects on microorganisms are relatively unknown despite ocean temperature being a major determinant of assemblage structure. Using data from thousands of Southern Hemisphere samples, we reveal that during an "unprecedented" 2015/16 Tasman Sea MHW, temperatures approached or surpassed the upper thermal boundary of many endemic taxa. Temperate microbial assemblages underwent a profound transition to niche states aligned with sites over 1000 km equatorward, adapting to higher temperatures and lower nutrient conditions bought on by the MHW. MHW conditions also modulate seasonal patterns of microbial diversity and support novel assemblage compositions. The most significant affects of MHWs on microbial assemblages occurred during warmer months, when temperatures exceeded the upper climatological bounds. Trends in microbial response across several MHWs in different locations suggest these are emergent properties of temperate ocean warming, which may facilitate monitoring, prediction and adaptation efforts.