Marine heatwaves: Winners, losers, and the metrics that countExtreme Events

Shinae Montie¹, Mads Thomsen^2,3, Karen Filbee-Dexter^1,4, Thomas Wernberg^1,4, MHWs International Working Group MHWs International Working Group

¹University of Western Australia, Australia - ²University of Canterbury, New Zealand - ³Aarhus University, Denmark - ⁴Norwegian Institute of Marine Research, Norway

Marine heatwaves (MHWs) are becoming more intense and frequent globally, causing widespread impacts on biological fitness, species distributions, and ecosystems. However, the effects of MHWs are not uniform, as organisms have varying temperature tolerances and abilities to relocate during extreme thermal stress. Here, we present a comprehensive global synthesis of published organismal responses to MHWs. We extracted and analysed over 6,000 log response effect sizes from more than 300 publications to test whether MHW impacts vary across different temperature-derived metrics (i.e., maximum intensity, cumulative intensity, maximum severity, number of MHW days, maximum sea surface temperature, and maximum degree heating weeks), taxonomic identity, ecological response (i.e., performance, abundance, survival), and biogeographical region. Moreover, more than 40% of effect sizes were species-specific, allowing us to assess MHW effect sizes relative to species’ thermal niches and range locations derived from global species distributions. Broadly, MHW impacts were overwhelmingly negative across major taxa groups, from microscopic phytoplankton to large mammals, with the only significantly positive effects observed for zooplankton. Corals and other sessile invertebrates were most severely affected, primarily due to severe bleaching and high mortality rates. The most negative effects occurred near species’ warm range edges, whereas positive effects were typically observed near their cool range edges. No single temperature-derived MHW metric fully explained all impacts, emphasising the need to assess biological effects against multiple metrics. Finally, regression models improved significantly when species range location was combined with taxonomic identity, regardless of the MHW metrics used. Our comprehensive global synthesis provides an important evidence base for decision-makers to assess the impacts of MHWs on marine biodiversity, fisheries, and ecosystems. With the intensification of MHWs globally, this knowledge is critical for guiding adaptive decision-making to safeguard marine resources, support ecosystem-based management, and ensure the resilience of marine habitats in the face of climate change.