Abstract
Increasing ocean warming and marine heatwaves will have important consequences for the world’s seagrasses. Predicting impacts of thermal stress on seagrasses depends on understanding their temperature-productivity responses. Here, we examine these responses for eelgrass ( Zostera marina ) and how they are influenced by plant acclimation to seasonal and localized physical conditions. Eelgrass was collected seasonally from two sites in Nova Scotia (NS) Canada that were characterized by contrasting physical conditions, with temperature regime being a dominant feature. Laboratory incubations measured oxygen production and consumption rates by eelgrass leaves and roots-rhizomes from 5 to 40 °C. Unimodal temperature dependence of net photosynthetic rate and whole plant net productivity was modelled and maximum productivity ( P max ) and optimal temperatures ( T opt ) were estimated. Both net photosynthetic rate and net plant productivity were more peaked and bell-shaped across increasing temperature for plants from a warm highly variable temperature regime compared to plants from a cool less variable regime. P max and T opt were highest in warm relative to cool regime plants across all seasons. Furthermore, P max was lower and T opt was higher when seasonal ambient temperatures increased. T opt for whole plant productivity (22.5 to 28.5 °C) was used to create a thermal risk map for eelgrass, which indicated that 20%, 8.5%, 4.6%, and 1.3% of predicted eelgrass habitat in NS exceeds 22 °C, 24 °C, 26 °C and 28 °C, respectively, for at least 1 h annually. Our results show that even northern temperate eelgrass can be heat stressed, and that plant acclimation to seasonal and localized conditions should be considered when evaluating seagrass thermal risk.