Intertidal eelgrass decline under combined effects of wasting disease and warming

Ocean warming threatens vital coastal ecosystems, including eelgrass (Zostera marina) meadows. At meadows along the Pacific coast of North America, outbreaks of seagrass wasting disease are correlated with warm water anomalies. The San Juan Islands in the Salish Sea are a hotspot for seagrass wasting disease; outbreaks have increased since 2013 and can infect 90% of plants in individual meadows. From 2019-2021, we assessed the interactive stressors of warming temperatures and disease outbreaks in intertidal meadows in the San Juan Islands, and we compared disease outbreaks along a latitudinal gradient, including meadows on the Central Coast of British Columbia and the Oregon coast, where wasting disease is less prevalent. Our surveys captured complex site-specific dynamics, with extreme regression of the upper intertidal area and/or decreased density of the meadow coinciding with high incidence of wasting disease at some sites. Outbreaks of wasting disease may serve as an early warning for subsequent loss of portions of intertidal meadows. In June 2021, we documented extreme temperatures during the heat dome, exceeding 35ºC during low tides at some intertidal meadows in Washington. However, shoot densities remained similar to prior summers at some locations over the short term (days to weeks following the heating event). These results highlight the complexity of seagrass response to warming events, with both duration and intensity of exposure contributing factors to ecosystem impacts. Sustained warming that facilitates wasting disease infection may be particularly damaging for seagrass meadows under climate change.