Latitudinal Variation in Seagrass Wasting Disease from Puget Sound to Alaska

Widely regarded as critical marine habitat, seagrass provides ecologically and economically valuable services in coastal areas worldwide. Eelgrass (Zostera marina) is one of five seagrass species native to the Pacific Northwest and is commonly infected by the marine protist Labyrinthula zosterae, the causative agent of seagrass wasting disease. Infection from L. zosterae causes necrotic lesions, limiting growth and compromising the health of eelgrass beds. To assess levels of wasting disease in subtidal and intertidal eelgrass meadows across a broad latitudinal gradient, we measured disease prevalence and severity in eelgrass beds ranging from the Puget Sound and San Juan Islands, WA; Calvert Island, British Columbia; and in Prince of Wales Sound, Alaska in the summers of 2017 and 2018. Collectively, we surveyed 5,536 eelgrass blades across both years. Disease prevalence ranged from 46-100% in 2018 intertidal meadows compared to 33-95% in subtidal meadows this same year and was higher in 2018 compared to 2017 across both tidal regimes and all regions. At sites with both intertidal and subtidal meadows, subtidal meadows almost always had lower disease prevalence, though disease varied considerably between geographic regions, sites, depths, and years. To determine potential environmental drivers of disease, we also measured site characteristics: density, canopy height, water temperature, and salinity. Understanding broad-scale patterns of disease and environmental drivers of wasting disease outbreaks has the potential to inform key decisions regarding eelgrass conservation along the West Coast, USA.