Thompson Spit (WA 50.3) is in the southern Salish Sea, south of Protection Island, Washington, USA. This site is located adjacent to the Washington State Department of Natural Resources Protection Island Aquatic Reserve. The primary submerged vegetation at this site is Phyllospadix spp., which thrives in the rocky intertidal. Between 2012-2024 seagrass density along the Thompson Spit SeagrassNet transect increased. Only transect B (0 m, MLLW) is monitored at this site.