The deep, cold and well-flushed waters of Puget Sound, WA (USA) are experiencing areas of eelgrass (Zostera marina L.) decline. Eelgrass faces anthropogenic stresses ranging from eutrophication and sedimentation to shoreline hardening, ship traffic, and aquaculture, which are currently being evaluated with a weight-of-evidence analysis. Since 2000, the Washington State Department of Natural Resources’ Submerged Vegetation Monitoring Program has assessed status and trends in eelgrass area and depth distribution throughout Puget Sound. Over this same time period, WA Department of Ecology has been monitoring nitrogen in the Sound’s waters; increasing concentrations of nitrate have been measured, linked to anthropogenic sources. The human-derived nitrogen comes on top of the already high background nitrogen level from the Pacific Ocean upwelling. The result is very high phytoplankton productivity, most evident in the more poorly flushed parts of the Sound. In many of these areas, nitrate concentrations have increased 4-10 times over the last 10 years while eelgrass beds have declined, with eelgrass losses seen at the deep edge where light is most limited. Nitrogen loading is by no means the only stressor impacting eelgrass in the Sound. Sediment loading also shades eelgrass and is derived from river input to the Sound and surface runoff that results from watershed deforestation, agriculture, and impervious surfaces, in addition to the fine sediment from glacial melting. There are a variety of additional direct physical impacts to eelgrass, including aquaculture, shoreline hardening, dredging and filling, boating and fishing practices, and overwater structures all contribute to direct physical impacts on eelgrass and each was evaluated in terms of its spatial extent and type of threat. The weight-of-evidence analysis shows that the nitrogen stressor has the broadest spatial extent and most lethal impacts to eelgrass and is the primary stressor of eelgrass in Puget Sound. The Puget Sound Partnership’s goal of a 20% increase in eelgrass area by 2020 cannot be achieved with existing management practices; the stresses on eelgrass must be reduced to create gains in eelgrass area and insure the health of Puget Sound
