Mechanisms of Resilience: Contextualizing the health of subtidal Zostera marina meadows around the San Juan Islands, Washington

Abstract

Eelgrass (Zostera marina) is a vital ecosystem engineer in the Salish Sea and is established as an indicator for ecological health in Puget Sound. Declines within eelgrass meadows around the San Juan Islands from monitoring through 2000-2020 have gained attention, as losing eelgrass threatens the many ecosystem services they provide, potentially disrupting entire food webs. Seagrass wasting disease (SWD), caused by the protist Labyrinthula zosterae, and heat stress are primary concerns for these meadows; both of these stressors are moderated by cooler and deeper waters. In this study we provide new information on 11 subtidal eelgrass meadows around the San Juan Islands during the summer of 2023. Through specimens collected via snorkel survey, we report information on body size, leaf count, and disease, using one-way ANOVA tests to compare variables between locations. Sites significantly differed in canopy height, normalized sheath length, leaf count, and second youngest leaf area (p < 0.05). The prevalence of disease differed by survey site, from 0% at False Bay and Beach Haven to 74% at Fisherman’s Bay (p < 0.05). The severity of disease between sites did not differ (p > 0.05). Sites were distinct in their environmental conditions. Variance in turbidity, depth, and water flow may have an influence on SWD susceptibility. In the face of rising global temperatures, this study reinforces the importance of holistic analyses of environmental conditions and future projections to identify resilience in subtidal eelgrass meadows.