Monitoring stormwater contaminants in the Puget Sound nearshore: an active biomonitoring tool using transplanted mussels (Mytilus trossulus)

Stormwater delivers a diverse range of contaminants to receiving waters including Puget Sound. Monitoring stormwater pollutants and their effects on biota is critical to informing best management practices aimed at recovering Puget Sound health. In the winter of 2012/13, the Washington Department of Fish and Wildlife’s Toxics-focused Biological Observation System (TBiOS) team conducted a pilot study using transplanted mussels to characterize the extent and magnitude of contamination in nearshore biota of Puget Sound. Mussels are now a key TBiOS indicator organism for tracking contaminants in the nearshore, and the Stormwater Action Monitoring (SAM) program has adopted mussels for nearshore stormwater monitoring as well. SAM now serves as the primary funder of nearshore mussel monitoring in Puget Sound and the first two SAM mussel monitoring surveys were conducted during the winters of 2015/16 and 2017/18, with future surveys planned on a biennial basis. These mussel surveys utilized native bay mussels (Mytilus trossulus) from a local aquaculture source that were transplanted into anti-predator cages to locations along the Puget Sound shoreline. Monitoring sites covered a broad range of upland land-use types, from rural to highly urban, and concentrations of organic contaminants and metals were measured in the mussels after a two to three-month winter deployment period. Data from the first two years of mussel surveys (2012/13, 2015/16) indicates polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) were the most abundant organic contaminants of those tested in the nearshore. Concentrations of both contaminants were significantly higher in the most urbanized areas and were positively correlated with impervious surface in upland watersheds adjacent to the nearshore. Patterns of PAHs (i.e. PAH fingerprints) in mussels from different locations demonstrate how mussels might be useful as indicators of sources for this particular class of stormwater contaminants in Puget Sound