Contaminant Accumulation by Unionid Mussels: An Assemblage Level Assessment of Sequestration Functions Across Watersheds and Spatial Scales

Freshwater mussels (Unionida) perform important functions that are integral to keeping streams, rivers, and lakes operating as holistic ecosystems. Some of these functions improve water quality for humans through their filtration activities such as nutrient cycling and feces and pseudofeces production. In this study, we estimated the magnitude of contaminant sequestration by mussel assemblages using data at polluted and relatively unpolluted sites from watersheds in the upper Mississippi River (Minnesota, Wisconsin, Iowa, and Illinois, USA), the Clinch River (Virginia and Tennessee, USA), and the upper Neuse River (North Carolina, USA). Data from these rivers represented a range of (1) spatial scales from wadable streams to large rivers, (2) population sizes from tens of thousands to hundreds of millions of mussels, (3) survey techniques from qualitative to quantitative, and (4) chemical classes from inorganic to organic contaminants. We estimated that mussels in two relatively unpolluted reaches of the upper Mississippi River sequestered 1.42 × 1013 µg of total metals, metalloids, and ions (i.e., 14.2 metric tons). Mussels in the relatively unpolluted upper Neuse River sequestered between 22.2 and 53.3 million ng of polycyclic aromatic hydrocarbons (PAHs; i.e., 22.2–53.3 mg). Mussels at a polluted site in the Clinch River (Pendleton Island) sequestered 168 million ng of PAHs, compared to 1.45 billion ng of PAHs sequestered at relatively unpolluted sites. Mussels at unpolluted sites in the Clinch River had a 10 times greater sequestration capacity despite having lower tissue concentrations. The accuracy (precision and bias) associated with estimating assemblage-level contaminant sequestration by mussels varied as a function of survey design, spatial scale, population size, and contaminant type. This preliminary assessment of sequestration of contaminants by mussels outlines a framework for understanding the contributions these organisms make in supporting water quality and highlights the need to protect and conserve mussels and the ecosystem functions and services they provide.