Global impacts of marine heatwaves on coastal foundation species

Anthropogenic climate change has driven the oceans to absorb roughly 90% of excess heat, leading to multi-decadal warming and associated biological changes, including range shifts that alter marine communities and ecosystem services. Marine heatwaves (MHWs)—discrete, prolonged periods of anomalously warm ocean temperatures—have increased in intensity and duration and are projected to intensify further. MHWs, defined as periods of at least five days above the seasonally varying 90th percentile of temperature, can be categorized by intensity and described by metrics such as mean, maximum, and cumulative intensity, duration, timing, and rate of onset. Prior studies show MHWs affect ecological performance and can cause mass mortality across taxa, with outsized ecosystem effects when foundation species (macroalgae, seagrass, hard and soft corals) are impacted. However, a coherent global understanding of how MHW characteristics mediate responses of foundation species, and how responses vary across species’ ranges and biogeographic regions, remains lacking. This study addresses that gap by combining a widely used MHW framework with globally distributed ecological observations to assess how coastal foundation species respond to strong, summer MHWs across 85 marine ecoregions, testing how MHW metrics, absolute temperatures, ecoregion, and species’ position within their geographic range predict biological responses.

The paper synthesizes extensive prior work documenting that MHWs have intensified and lengthened over the past century and that such extremes can exceed thermal thresholds, reducing organisms’ capacity for plastic or behavioral responses and causing declines from sublethal effects to mass mortalities across producers, invertebrates, fishes, birds, and mammals. Foundation species losses can precipitate ecosystem collapse and regime shifts, undermining biodiversity and ecosystem services (e.g., tourism, fisheries, carbon sequestration). A standardized hierarchical definition of MHWs and associated metrics enables comparability across events and studies. Previous research indicates MHW duration and cumulative intensity can predict biological impacts, that trailing (warm) range-edge populations are especially vulnerable due to proximity to thermal limits, and that responses vary by region due to oceanographic context and acclimation. Despite these insights, global-scale, cross-taxonomic analyses linking specific MHW characteristics to foundation species responses, while accounting for biogeography and species’ range position, have been limited.

Study scope and datasets: The authors compiled nine long-term ecological datasets capturing abundance or cover of macroalgae and seagrass, bleaching in scleractinian hard corals, and mass mortality events (MMEs) in gorgonian soft corals. Datasets emphasized representative, repeat surveys in shallow waters (≤10 m) for consistency with SST-based MHW detection. After deduplication and removal of records confounded by non-temperature stressors, the pool comprised 9027 locations and 25,951 individual site-year data points globally, generally at species resolution (hard corals at order level). Marine heatwave detection: MHWs were identified from NOAA OISSTv2 (1/4°) SST using the Hobday et al. framework via the heatwaveR package. A MHW is ≥5 consecutive days above a seasonally varying 90th percentile, with climatology and threshold computed over an 11-day window and smoothed with a 30-day running window for a fixed 1983–2012 baseline. Intensity categories: Moderate (1–2×), Strong (2–3×), Severe (3–4×), Extreme (>4×) relative to the local difference between climatological mean and 90th percentile. Event metrics extracted included mean and maximum intensity (SST anomaly), cumulative intensity (degree-days), maximum absolute temperature (°C), and duration (days). Event selection: Analyses focused on Strong-or-greater events. Outside the tropics (23.44°N–23.44°S), only summer events were retained (June–September for Northern Hemisphere; December–March for Southern Hemisphere); all months considered within the tropics. Biological response matching: For macrophytes (macroalgae and seagrass), pre-MHW density or percent cover (typically averaged over 2–3 years) was compared with measurements 6–12 months post-MHW (same season) to capture lagged responses; the response was the change (post minus pre). For corals and gorgonians, bleaching/MMEs recorded during or within 16 weeks after MHWs were used, expressed as the percentage of individuals impacted. Where consecutive MHWs occurred within the analysis window, event metrics were combined; events crossing window boundaries were trimmed to the defined period. Spatial aggregation and ecoregions: Responses from sites within 8 km sharing environmental features were averaged to reduce spatial autocorrelation. Localities were assigned to marine ecoregions (Spalding et al.). This yielded 1322 observations across 85 ecoregions: macroalgae (139), seagrass (60), hard corals (1047), gorgonian soft corals (76). Analytical approach: 1) Temporal trend: Using the initial 25,951 site-year points and 2314 strong summer MHW responses (pre-aggregation), the percentage of annual observations showing negative responses was computed and regressed against year. 2) Global/ecoregion descriptive analyses: For 1322 aggregated observations, responses were binned as mild (<10%), moderate (10–50%), or severe (≥50%) inhibition (loss) or facilitation (gain), globally and by ecoregion; ecoregion averages were calculated where n ≥ 3. For hard coral bleaching ranges (mild, moderate, severe), conservative numeric values (1%, 11%, 51%) were assigned. 3) Global GLMs: Two sets of generalized linear models were run—one for macrophytes (Gaussian errors modeling change in cover/density) and one for corals/gorgonians (quasibinomial errors modeling proportion impacted). Predictors: mean, maximum, and cumulative MHW intensity, duration, maximum absolute temperature, marine ecoregion, and species’ point-in-range (position within latitudinal range from OBIS; for corals, common reef-forming genera ranges with equator as trailing edge). Models were weighted by the number of sites within each aggregated locality. Collinearity was addressed by iteratively removing variables with VIF ≥ 10. For Gaussian models, AIC guided model choice. Residuals were checked; outliers removed (one coral, three macrophyte cases). 4) Ecoregion-level GLMs: For ecoregions with ≥10 observations (28 ecoregions; two excluded due to excessive zeros), separate GLMs related responses to MHW metrics with similar structures and weighting. For macrophytes, analyses focused on a single dominant foundation species per ecoregion where possible (e.g., Ecklonia radiata in Bassian and Cape Howe; Macrocystis pyrifera in Northern/Southern California; Laminaria hyperborea in North Sea; Zostera muelleri in Tweed-Moreton). Analyses excluded cases with insufficient species-level data or only categorical coral bleaching data. All analyses used R/RStudio and heatwaveR for MHW detection.

• Global trend: The proportion of negative responses among annual observations increased significantly over time (p = 0.021, R^2 = 0.15), indicating growing impacts of strong, summer MHWs on foundation species. - Geographic scope of impacts: Negative responses were detected in 79 of 85 ecoregions; six ecoregions exhibited no observed responses (mostly due to limited data, and all were regions without site-level hard coral bleaching). - Magnitude by ecoregion and group: • Gorgonian soft corals experienced the highest MMEs in the Western Mediterranean, with a single MHW causing an average 44.0% mortality. • Hard coral bleaching averages were highest in South India and Sri Lanka (69%), Maldives (51%), and East African Coral Coast (40.3%). Least impacted ecoregions included New Caledonia, North and Central Red Sea, Andaman Sea Coral Coast, Cocos-Keeling/Christmas Island, Southern China, and the Banda Sea, where average bleaching remained below 1% across 80 MHW-associated observations. • Seagrass exhibited the greatest average loss in the Tweed-Moreton ecoregion (28.6% loss), while Torres Strait and Northern GBR showed small average losses (5.7%). • Macroalgae declined most in Northern California (39.3% loss in density), while moderate gains were recorded in the Bassian ecoregion (11.8% gain in cover). - Range-edge effects: Point in range was a significant predictor in global GLMs for both macrophytes (p = 0.004) and corals (p = 0.027). Macrophytes showed strong losses toward warm trailing edges (20.1% loss) and gains toward cool leading edges (11.0% gain). Corals/gorgonians had higher proportions impacted at warm edges (23.5%) compared to mid-range (22.9%) and cool edges (21.0%). - MHW characteristics as predictors: Global GLMs found macrophyte responses significantly related to mean intensity (p = 0.012), duration (p = 0.034), and ecoregion (p = 0.009). Coral/gorgonian responses were significantly related to mean and cumulative intensity, maximum absolute temperature, duration (all p < 0.001), and ecoregion (p = 0.001). - Ecoregion-level modeling: Significant relationships between responses and one or more MHW metrics were identified in 21 of 28 ecoregions with ≥10 observations. For hard corals, maximum absolute temperature most commonly best predicted bleaching levels; for macrophytes, mean intensity was most frequently the strongest predictor. In the Western Mediterranean, gorgonian MMEs related significantly to mean intensity; in the Adriatic Sea, to duration. Some ecoregions showed no significant relationships, potentially due to low sample sizes or generally low bleaching (e.g., Mascarene Islands, Eastern Philippines). - Resilience hotspots: Despite MHWs exceeding 31°C, New Caledonia and Southern China exhibited little to no bleaching (≤2.5% and ≤12.5% maximum, respectively), suggesting localized resilience or mitigating environmental conditions. - Overall pattern: Increasing MHW intensity and duration generally exacerbated negative impacts. While occasional positive responses of macrophytes occurred during lower-intensity events, most macrophyte responses were negative and became more severe with higher intensity.

The study demonstrates that strong, summer MHWs are pervasive stressors for coastal foundation species and that their negative ecological impacts are increasing over time. Because foundation species underpin habitat structure, biodiversity, productivity, and ecosystem services, their decline can trigger regime shifts and long-term loss of ecological function. Documented examples include local extinction of bull kelp in New Zealand after the 2017–2018 MHW and a persistent shift from kelp forests to warm-affinity algal turfs in Western Australia following the 2011 extreme MHW, leading to knock-on effects on seagrass, fisheries, megafauna, and carbon release from sediments. A key mechanistic insight is the strong modulation of impacts by species’ position within their ranges: warm trailing-edge populations suffer the greatest detriments, consistent with proximity to upper thermal limits, implying accelerated equatorward range contractions. Although some poleward (cool edge) expansions and performance gains occur, new populations may take decades to form comparable, functionally rich ecosystems. The study’s ecoregion-level analyses link specific MHW metrics to biological responses, enabling identification of high-risk combinations (e.g., frequent strong events overlapping with many warm-edge populations) and potential climatic refugia where bleaching remains low despite high temperatures (e.g., New Caledonia, Southern China). These findings support the development of predictive tools and management interventions, such as early warnings, adaptive fisheries closures, and strategic focus on resilient species or locations. Recognizing compound and indirect pathways (e.g., disease outbreaks, herbivore booms following predator loss) is essential for comprehensive risk assessments.

This work provides a globally coherent analysis linking marine heatwave characteristics to biological responses of coastal foundation species across 85 ecoregions. It shows that strong, summer MHWs are driving increasingly negative impacts, particularly at species’ warm trailing edges, and that MHW intensity, duration, maximum absolute temperature, ecoregion, and range position are key predictors of response. The results highlight escalating risks to biodiversity, ecological functioning, and ecosystem services as foundation species decline, while also revealing ecoregions that may exhibit resilience. The framework enables improved prediction of impacts and supports targeted management and adaptation strategies. Future research should: expand analyses to non-summer MHWs; disentangle direct versus compound/indirect stressor effects; assess recovery trajectories and long-term resilience; integrate mechanistic laboratory and mesocosm experiments to identify physiological tipping points; and further refine forecasting to support proactive management in high-risk regions.

• Potential publication or sampling bias and the use of multiple datasets with varying methodologies. - Focus on strong, summer MHWs; responses to non-summer events were not assessed and may differ. - Some responses may reflect compound or indirect effects (e.g., disease, sedimentation, light limitation, trophic cascades) rather than direct thermal impacts alone. - Hard coral data often at order level and, in some cases, bleaching reported as categorical ranges replaced by conservative numeric values, reducing precision. - Spatial autocorrelation addressed by averaging within 8 km, but residual non-independence may remain. - Limited sample sizes in some ecoregions and many zero-response records constrained ecoregion-level modeling; two ecoregions were excluded from GLMs due to excessive zeros. - Use of SST-based MHW detection and shallow-water focus may not capture fine-scale thermal variability or subsurface conditions. - Outlier removal and VIF-based variable exclusion may influence model structure; quasibinomial and Gaussian assumptions may not capture all data features.