Anthropogenic climate change has caused significant warming of the upper oceans, leading to a multidecadal increase in global temperatures and the intensification of marine heatwaves (MHWs). MHWs, defined as prolonged periods of extreme ocean temperatures exceeding the 90th percentile of a 30-year baseline, are increasing in both frequency and intensity. These extreme temperatures negatively impact marine species at all levels, from molecular to population, often exceeding critical thermal thresholds. The effects range from reduced ecological performance (growth, photosynthesis, reproduction) to mass mortality events (MMEs). Declines in foundation species – macroalgae, seagrass, and corals – are particularly concerning due to their critical role in maintaining ecological processes and supporting biodiversity. The loss of these foundation species can lead to ecosystem collapse and significant disruptions to ecosystem services. While individual studies have documented MHW impacts, a comprehensive global understanding of how MHW characteristics affect these key species is lacking. This gap hinders prediction and the development of effective management strategies. This study addresses this need by combining an established MHW framework with globally distributed ecological observations to assess the responses of coastal foundation species to MHWs.

Existing literature extensively documents the detrimental impacts of MHWs on a wide array of marine species. Studies have shown reduced ecological performance, including decreased growth, photosynthesis, and reproductive success, across various taxonomic groups. Mass mortality events have also been reported globally, affecting primary producers, invertebrates, fish, birds, and mammals, with significant ecological and socioeconomic consequences. The disproportionate impact on foundation species such as macroalgae, seagrass, and corals has been highlighted, emphasizing their critical role in maintaining ecosystem structure and function. Several studies have investigated the relationship between specific MHW characteristics (duration, cumulative intensity) and biological responses, but a coherent global-scale analysis of how these characteristics impact functionally similar foundation species remains absent. The importance of considering species' location within their range has also been noted, with warm-range edge populations predicted to be particularly vulnerable to future MHWs.

This study utilized nine long-term datasets documenting changes in macroalgae and seagrass abundance, coral bleaching, and gorgonian soft coral MMEs. Data spanned 85 marine ecoregions, encompassing 2314 initial observations. The study focused on strong or greater intensity MHWs occurring during summer months in each hemisphere. To account for spatial proximity and common environmental features, observations from geographically close sites (<8 km) were averaged, resulting in 1322 observations. Linear regression was used to examine temporal trends in negative responses. Global and ecoregion-level trends were visually explored, and generalized linear models (GLMs) were employed to investigate relationships between foundation species responses and MHW characteristics (mean, maximum, and cumulative intensity, duration, maximum absolute temperature), marine ecoregion, and point in species range. Separate GLMs were run for macrophytes and corals due to differing response scales. Further GLMs were run for individual ecoregions with sufficient datapoints (≥10).

The study found a significant increase (p=0.021, R²=0.15) in the proportion of negative responses to MHWs over time. Impacts were observed in 79 of the 85 ecoregions analyzed, with variations in response magnitude across species and ecoregions. The Western Mediterranean showed the highest average mortality in gorgonian soft corals (44.0%), while South India and Sri Lanka experienced the highest average coral bleaching (69%). The Tweed-Moreton ecoregion exhibited the largest seagrass cover loss (28.6%), whereas Northern California showed the most dramatic macroalgae decline (39.3%). Critically, responses were strongly modulated by location within a species' range, with negative impacts most pronounced at warm-range edges. GLMs revealed significant relationships between species responses and various MHW characteristics. For macrophytes, mean MHW intensity, duration, and ecoregion were significant predictors; for corals, mean and cumulative intensity, maximum absolute temperature, duration, and ecoregion were significant. Ecoregion-level analyses (21 of 28 ecoregions) generally showed declines in foundation species performance with increasing MHW intensity and duration, though the most influential MHW characteristics varied among species and ecoregions. Some ecoregions, notably New Caledonia and the Southern China Sea, exhibited minimal coral bleaching despite high-magnitude MHWs, potentially highlighting resilience or localized factors. In contrast, some ecoregions showed positive responses, often during lower-intensity events, with the increase potentially attributable to some species being at the cooler edge of their temperature range.

This study provides strong evidence of the widespread and detrimental effects of intense summer MHWs on global coastal foundation species. The increasing frequency and intensity of these events, along with the vulnerability of warm-range edge populations, pose a significant threat to coastal ecosystems. Losses in foundation species lead to decreased biodiversity, altered community composition, reduced ecosystem services (including fisheries, tourism, carbon sequestration, and storm protection), and can even trigger ecosystem regime shifts. The identification of ecoregions with minimal impacts, despite high-magnitude MHWs, offers potential insights into resilience mechanisms and highlights the need for further investigation into protective factors (local oceanography, cloud cover, thermal acclimation). This study emphasizes the importance of considering multiple MHW characteristics, species' range position, and local environmental factors when predicting future impacts. The findings inform management strategies for mitigating the effects of MHWs, such as targeted conservation efforts in areas and species of high concern.

This global-scale analysis underscores the significant impact of marine heatwaves on coastal foundation species. The observed increase in negative responses over time, particularly at warm-range edges, highlights the urgent need for conservation efforts and climate change mitigation. Further research should focus on identifying and understanding the mechanisms underlying resilience in certain ecoregions and developing predictive models that incorporate multiple stressors and species interactions. This knowledge is critical for informing effective management and adaptation strategies to safeguard coastal ecosystems.

While this study represents a significant advance in understanding global MHW impacts, several limitations should be acknowledged. Data availability and potential publication bias may influence the observed patterns. The focus on strong summer MHWs might not fully capture the impacts of other seasons. The analysis primarily assessed direct relationships between MHW characteristics and foundation species responses, potentially overlooking indirect effects or compound stressors. Furthermore, the dataset's spatial resolution may not fully capture the variability of MHW events within ecoregions.