Stormwater Action Monitoring 2019/2020 Puget Sound Nearshore Mussel Monitoring Survey

Monitoring pollutants from contaminated stormwater and their effects on the marine biota of Puget Sound is critical to inform best management practices and remediation efforts in this large and diverse estuary. The Stormwater Action Monitoring (SAM) Status and Trends in Receiving Waters program conducts monitoring in Puget Sound small streams and nearshore marine waters to provide a regional assessment of whether collective stormwater management actions are leading to improved receiving water conditions. The SAM Puget Sound Nearshore Mussel Monitoring studies focuses on the bioaccumulation of pollutants in caged native bay mussels (Mytilus trossulus) to evaluate the current status and trends of nearshore conditions.

In the winter of 2019/2020 the Washington Department of Fish and Wildlife (WDFW), with the help of citizen science volunteers, other agencies, tribes, and non-governmental organizations, conducted the third of a series of biennial, nearshore mussel monitoring efforts under the Stormwater Action Monitoring (SAM) program. The first SAM Puget Sound Nearshore Mussel Monitoring survey was conducted in the winter of 2015/2016 (Lanksbury et al., 2017) and the second in the winter of 2017/2018 (Langness and West, 2020).

The 2019/2020 monitoring survey provided the first opportunity to evaluate changes in contamination of nearshore biota residing inside the urban growth areas (UGAs) of Puget Sound occurring between the first three surveys conducted in 2015/2016, 2017/2018 and 2019/2020 (hereafter referred to as 2016, 2018 and 2020 respectively). In each survey year, relatively uncontaminated mussels from a local aquaculture source were transplanted to over forty SAM monitoring locations along the Puget Sound urban growth area (UGA) shoreline, covering a broad range of upland land-use types from rural to highly urban. Two reference sites were established, one in Penn Cove and one in Hood Canal. The Hood Canal reference site was sampled each survey year and subsequently used to establish regional scale thresholds as it had no obvious sources of contamination and had consistent low contaminant concentrations (lower than Penn Cove). At the end of each study, after approximately three months of exposure, the concentration of several major contaminants in the mussels’ soft tissues were measured including several classes of organic chemicals, such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs, or flame retardants), and chlorinated pesticides (including dichlorodiphenyltrichloroethane compounds, or DDTs) and six metals (arsenic, cadmium, copper, lead, mercury, zinc).

To characterize changes in the spatial patterns of nearshore biota contamination, the changes in the distribution of mussel tissue contaminant concentrations along the Puget Sound UGA nearshore between each survey year (2016, 2018, 2020) were examined. The spatial extent of organic contaminant concentrations in each survey year showed that the concentration of organic contaminants in the UGA nearshore were consistently greater than the clean reference site. Most of the sampled UGA nearshore in each survey year (2016, 2018, 2020) had Σ₁₆PAHs, TPCBs, Σ₁₁PBDEs , and Σ₆DDTs concentrations above the Hood Canal reference site concentration indicating mussels accumulated these contaminants at nearshore sites within the UGA, with the spatial extent of these contaminants showing little to no decline during the three survey years. Similarly, the spatial extent of metal concentrations in most survey years showed that concentrations in the UGA nearshore for these contaminants were largely greater than the clean reference site. Most of the sampled UGA nearshore in two or more surveys had arsenic, cadmium, copper, lead, mercury, and zinc concentrations above the Hood Canal reference site concentration. The spatial extent of arsenic, cadmium, lead, and zinc contamination showed little to no decline during the three survey years. However, copper and mercury contamination declined between 2016 and 2020, with 87% of the UGA nearshore in the 2020 survey with copper concentrations equal to or less than the Hood Canal Reference, and 95% of UGA nearshore in the 2020 survey with mercury concentrations equal to or less than the reference.

To track temporal changes in contaminant concentrations of nearshore biota, the changes in the central tendency concentrations of key contaminants in mussels between each survey year (2016, 2018, 2020) were evaluated. Three of the four most frequently detected organic contaminants (Σ₁₆PAHs, Σ₁₁PBDEs, and Σ₆DDTs) had significantly lower central tendency concentrations in mussels from the 2020 survey than in the 2016 and/or 2018 survey. TPCBs data were inconclusive as there was no significant difference in mean concentration values attributable to survey year. Although additional sampling years are needed to infer conclusions regarding any significant trends in these organic contaminant concentrations, the declining Σ₁₁PBDEs concentrations but stable TPCBs concentrations were congruent with the temporal pattern in two other WDFW-TBiOS indicator species (English sole and Pacific herring) reported in the Toxics in Aquatic Life Vital Sign (Puget Sound Partnership 2022c). Two of the six metals measured (cadmium and zinc) had significantly higher concentrations in mussels from the 2018 and 2020 surveys than in the 2016 survey, and two other metals (copper and mercury) had significantly lower concentrations in mussels from the 2020 survey than in both the 2016 and 2018 surveys. Although a statistically significant increase or decrease was observed in these metal concentrations, additional sampling years are needed to infer conclusions regarding any significant trends. Arsenic concentrations in mussels significantly increased in the 2018 survey and then decreased in the 2020 survey, indicating variable concentration conditions of these metals in the UGA nearshore. Lead data were inconclusive as there was no significant difference in mean concentration values attributable to survey year.

Lastly, to evaluate where contaminant concentration changes were associated with levels of nearshore land development (using % impervious surface in adjacent nearshore watersheds as a proxy), additional spatial analyses were performed on the organic contaminant data where significant differences among concentration means between survey years was observed. The observed decline in Σ₁₆PAHs concentrations in the 2020 survey occurred within areas with a medium level of land development, where the adjacent nearshore watershed unit to a site had 20 to <40% impervious surface (IS). The decline in Σ₁₁PBDEs concentrations occurred within areas with the least (< 10 % IS), medium (20 to < 40 % IS), and high levels of development (40 to 100 % IS).