“Wasting” Time: Phenology and Ecological Impact of Seagrass Wasting Disease in Eelgrass Meadows

Seagrass meadows provide numerous ecosystem benefits, from supporting biodiversity to sequestering carbon, but these essential marine habitats are threatened by many factors, including coastal development and rising water temperatures. Eelgrass (Zostera marina), a widespread temperate species, is also threatened by eelgrass wasting disease, an infectious disease caused by the protist Labyrinthula zosterae. The disease is currently outbreaking at sites in Puget Sound and the San Juan Islands, WA, with mean prevalence of 60% in 2017 and 75% in 2018. However, the ecological consequences of an outbreak remain poorly understood, with most studies of wasting disease limited to laboratory inoculation experiments. In this study, we examined the effect of wasting disease infection on eelgrass productivity in the field. We coupled in situ shoot marking with high-resolution imagery of individual seagrass blades to capture shoot-specific relationships between growth rates and wasting disease lesion development. We applied a novel artificial intelligence application to determine disease prevalence and severity from the digital imagery, and compared disease and growth metrics measured over repeated 5-day periods in June and July 2019 to understand temporal dynamics of the outbreak. Our results suggest that water temperature may be a critical driver of lesion development, leading to lower eelgrass productivity. Additional analyses of rhizomes from infected eelgrass suggest that individuals with higher levels of infection had compromised sucrose concentrations in belowground tissue. This research furthers our understanding of the phenology and interactions between wasting disease and eelgrass growth in natural conditions. In a warming ocean with increased incidence of marine heat waves, eelgrass are likely vulnerable to the interactive effects of rising temperature and infectious disease.