Toxic Contaminants in Puget Sound's Nearshore Biota: A Large-Scale Synoptic Survey Using Transplanted Mussels (Mytilus trossulus)

In the winter of 2012-13 the Washington Department of Fish and Wildlife, with the help of citizen science volunteers, other agencies, tribes, and non-governmental organizations, conducted the first synoptic, Puget Sound-wide assessment of toxic contaminants in nearshore biota. This project was funded by EPA’s National Estuary Program (NEP) in support of Washington State’s Action Agenda and their goal of restoring the health of Puget Sound. The Washington Department of Fish and Wildlife and Department of Natural Resources awarded this grant in their role as Lead Organization for NEP’s Marine and Nearshore Protection and Restoration. This project was funded as a cross-cutting study, which drew together concepts related to three NEP-supported focal efforts in the Puget Sound: (1) Toxics and Nutrients, (2) Marine and Nearshore Protection and Restoration, and (3) Watershed Protection and Restoration. This study focused on toxic contaminants generated primarily from terrestrial sources, and conveyed to Puget Sound nearshore habitats via stormwater and other hydraulic watershed processes.

In this study we used native mussels (Mytilus trossulus) as indicators of the degree of contamination of nearshore habitats. We transplanted relatively uncontaminated mussels from an aquaculture source to 108 locations along the Salish Sea shoreline, covering a broad range of upland land-use types from rural to highly urban. At the end of the study we determined three biological endpoints (mortality, growth and condition index) and measured the concentration of several major contaminant classes in mussels: polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs, or flame retardants), chlorinated pesticides (including dichlorodiphenyltrichloroethane compounds, or DDTs) and six metals (lead, copper, zinc, mercury, arsenic, cadmium).

Overall, PAHs, PCBs, PBDEs, and DDTs were the most abundant organic contaminants measured in this study. PAHs and PCBs were detected in mussels from every site, and highest concentrations were observed in four of Puget Sound’s most urbanized embayments (466 - 5030 ng/g dry weight (dw) for Σ₄₂ PAHs, and 38 - 216 ng/g dw for total PCBs in mussels from Elliott Bay, Salmon Bay, Commencement Bay, and Sinclair Inlet). Although lower in overall concentration, PBDEs and DDTs followed a similar pattern. In addition, although PCBs were elevated mainly along urbanized shorelines, PAHs were elevated in mussels from some non-urban shorelines (some near marinas or ferry terminals). The other organic contaminants were detected in mussels at fewer than 22% of study sites, and at low levels.

We observed significant positive correlations between both our proxies of nearshore watershed land development (impervious surface and road area), and levels of PAHs, PCBs, PBDEs, and DDTs. Variability in contaminant concentration increased exponentially with increasing impervious surface (or road area), suggesting other, unmeasured landscape factors may more fully explain the variation in mussel contaminant concentrations. These factors may include proximity to point sources (e.g., outfalls) or focal non-point sources (e.g., marinas or ferry terminals).

PAH analyte pattern analysis suggested the majority of mussel sites were dominated by pyrogenic (i.e. combustion) sources; however, atypical patterns at a few locations (Salmon Bay, Bremerton Shipyard-Charleston Beach, Hylebos Waterway, and the Thea Foss Waterway) suggested petroleum sources may be contributing a larger proportion of PAHs to the mussels in those areas. PCB congener-ratio analysis suggested urban embayments in the Central Puget Sound (Elliott Bay, Commencement Bay, and Sinclair Inlets) are sources of PCBs for non-urban areas. The PCB pattern in mussels became �"lighter” with distance from urban areas; that is, more highly chlorinated PCB congeners with greater molecular weight tended to be less abundant in mussel tissue with increasing distance from urban shorelines. Lighter congeners tend to migrate faster through the environment than heavier congeners.

Although the condition index of mussels declined at 72% of the study sites, condition index was not linked to impervious surface or road area; this decline was likely the result of natural processes related to normal declines in food supply and slowing of growth during winter months. However, there was a weak positive correlation between mortality and both impervious surface and road area, suggesting greater survival of mussels with decreasing contamination. Growth was not linked to either factor, however the short deployment time (60 days) and season (winter) probably hampered our ability to measure growth adequately.

All six metals were found in mussels from all the study sites, though their concentrations were relatively low and did not vary greatly from baseline (starting) values. There was a weak, positive relationship for lead, with impervious surface and road area, weaker relationships with copper, and a weak relationship between zinc and impervious surface. There was no link between mercury, arsenic, or cadmium with either factor, suggesting the concentration of these metals in mussels is not predictable from levels of impervious surface or road area.

Wild and transplanted mussels sampled simultaneously from six sites had similar concentrations of organic contaminants and metals, suggesting that caged mussels behaved similarly to wild-growing mussels. However this study was not designed to make such a comparison; sample size for these pairings was low and important factors such as tidal elevation were uncontrolled, so caution should be exercised when comparing contaminant levels between the two types.

These findings suggest toxic contaminants are entering the nearshore food web of the Salish Sea, especially along shorelines adjacent to highly urbanized areas. Some contaminants such as PAHs exhibited a wider, less predictable distribution, than the other organic chemicals, perhaps related to sources that may occur on rural or less developed landscapes (e.g., roadways, creosote pilings, marinas, and ferry terminals). We recommend that Washington State develop a long-term, regional, nearshore sampling program using caged mussels as a sentinel species to monitor status and trends of contaminants in nearshore biota. Success of such a large-scale field-intensive study is predicated on participation by citizen science volunteers to conduct the field work, and by partner groups interested in monitoring pollution in their nearshore areas to maximize spatial coverage in the Sound.