Seagrasses are most vulnerable to marine heatwaves in tropical zones: local‐scale and broad climatic zone variation in thermal tolerances

Summary Under a changing climate, it is imperative that we understand how species may respond to temperature impacts, which can differ among populations of the same species due to local drivers. Thermal tolerance data, which can be used to assess an organism's upper thermal limits, is valuable to identify species and/or populations' susceptibility to thermal stress. This study assessed the variation in thermal tolerance of six seagrass species at both broad latitudinal ( c . 500–1000 km) and local scales ( c . 25 km). Photosynthesis–temperature curves (15–45°C) were conducted, by measuring oxygen in closed incubation chambers, and thermal optima ( T opt ) was extracted. We found that T opt varied by almost 10°C among six species, and T opt for the same species differed by up to 4°C across both broad and local scales, but with no consistent patterns across latitude. This highlights that thermal performance does not necessarily reflect thermal geography of a seagrass species range, and that other environmental variables may play a role in how species respond to temperature. Overall, while some seagrass species may benefit from small increases in temperature, marine heatwaves are likely to have negative implications for five of the six species assessed, with greater impacts occurring in tropical regions.