Rates and routes of trace element uptake in zebra mussels

The zebra mussel, Dreissena polymorpha, a nonindigenous invasive species, i s now widespread throughout the eastern half of North America. Because zebr a mussels are ubiquitous and because they effectively filter particulate ma tter out of suspension, the cycling and residence times of particle-reactiv e metals will likely be affected in waters with zebra mussels. This study d escribes experiments designed to assess the possibility of using this speci es as a bioindicator of metals in ambient freshwater environments. Laborato ry exposures of zebra mussels to Ag-110m, Cd-109, Cr-51, C-14, Hg-203, and Se-75 were employed to measure their assimilation efficiencies (percentage of ingested element that crosses gut lining) from eight food types (four al gal species and bacteria, seston, and mineral assemblages), absorption effi ciencies from water (percentage of element pumped by the mussel that is abs orbed by the animal), and rates of depuration of these elements from mussel s following long-term exposures to food and water. Assimilation efficiencie s of elements from foods ranged from 4 to 29% for silver (Ag), 19 to 72% fo r cadmium (Cd), 42 to 85% for carbon (C), 2 to 19% for chromium (Cr), 4 to 40% for mercury (Kg), and 8 to 46% for selenium (Se). Absorption efficienci es from the dissolved phase were 1.87% for Ag, 1.02% for Cd, 0.47% for Cr(I II), 0.27% for Cr(VI), 1.17% for Hg, and 0.03% for Se. Efflux rate constant s (d(-1)) following long-term exposure to food and water were 0.067 and 0.0 84 for Ag, 0.013 and 0.011 for Cd, 0.019 and 0.011 for Cr, 0.050 far Hg (fo od only), and 0.026 and 0.035 for Se. These loss rates corresponded to biol ogical half-lives ranging from 8 d for Ag to 76 d for Se. Loss rates of tra ce elements from zebra mussel feces followed the following sequence: Cr < A g < Se < Hg less than or equal to Cd, with average retention half-times bei ng 59, 43, 11, 6.7, and 5.1 d, respectively, which indicates that geochemic al cycling rates from zebra mussel biodeposits are element specific. Egesti on patterns of the radioisotopes indicated two digestive phases, extracellu lar and intracellular digestion. The extent of intracellular digestion rang ed from 7 to 40%, depending on the food source, and correlated with assimil ation efficiency for Ag, Cd, and Hg. The bioaccumulation parameters measure d for D. polymorpha can be used in kinetic models to quantify the relative importance of food and water as sources of metals and to predict on a site- specific basis the tissue concentrations of metals in these mussels, as sho wn for Cd. Because D. polymorpha accumulates metals from dissolved and part iculate sources in proportion to ambient concentrations, this species can b e an effective bioindicator of freshwater metal contamination.