Habitat-Forming Macroalgae Show Greater Resilience to Marine Heatwaves Than Models Predict, Yet Chronic Ocean Warming...

Temperate macroalgal forests globally have undergone climate-induced range contractions and localised extinctions in response to warming and marine heatwaves, with further significant losses projected to occur. Managing these foundational habitats into the future requires improved understanding of the expected changes in distributions and the timeframes over which these changes are likely to occur. Here, we combine observations at species range edges with species distribution models to examine the impacts of marine heatwaves and scrutinise the potential impacts of future climate scenarios on macroalgal distributions for inshore waters along the temperate southern Australian coastline. We compare modelled to realised changes in distributions at the equatorward range edge of three major habitat-forming macroalgal species in Australia, Ecklonia radiata, Phyllospora comosa and Durvillaea spp. Monitoring revealed that no substantial range contractions occurred at the equatorward range edge for these species between 2019 and 2024, despite several moderate to strong marine heatwaves during this period. However, species distribution model predictions for this period indicated that marine heatwaves (MHWs) induced considerable temporary reductions (e.g., 30%-70%) in environmental suitability for these species, implying greater resilience to MHWs than models suggest. Future projections indicate that long-term ocean warming may cause moderate (8-152 km for E. radiata under RCP2.6) to severe (832 km for Durvillaea under RCP8.5) range contractions in eastern Australia by 2100. Projections may overestimate macroalgal losses, as they do not consider factors that confer resilience to species under short-term extremes (e.g., local adaptation potentially delaying projected contractions at the warm range edge). Considering spatial variation in adaptation is therefore essential to refine models and generate more realistic projections of climate-mediated macroalgal loss.