Rates and routes of trace element uptake in zebra mussels

Abstract

The zebra mussel, Dreissena polymorpha, a nonindigenous invasive species, is now widespread throughout the eastern half of North America. Because zebra mussels are ubiquitous and because they effectively filter particulate matter out of suspension, the cycling and residence times of particle‐reactive metals will likely be affected in waters with zebra mussels. This study describes experiments designed to assess the possibility of using this species as a bioindicator of metals in ambient freshwater environments. Laboratory exposures of zebra mussels to 110mAg, 109Cd, 51Cr, 14C, 203Hg, and 75Se were employed to measure their assimilation efficiencies (percentage of ingested element that crosses gut lining) from eight food types (four algal species and bacteria, seston, and mineral assemblages), absorption efficiencies from water (percentage of element pumped by the mussel that is absorbed by the animal), and rates of depuration of these elements from mussels following long‐term exposures to food and water. Assimilation efficiencies of elements from foods ranged from 4 to 29% for silver (Ag), 19 to 72% for cadmium (Cd), 42 to 85% for carbon (C), 2 to 19% for chromium (Cr), 4 to 40% for mercury (Hg), and 8 to 46% for selenium (Se). Absorption efficiencies from the dissolved phase were 1.87% for Ag, 1.02% for Cd, 0.47% for Cr(III), 0.27% for Cr(VI), 1.17% for Hg, and 0.03% for Se. Efflux rate constants (d 21 ) following long‐term exposure to food and water were 0.067 and 0.084 for Ag, 0.013 and 0.011 for Cd, 0.019 and 0.011 for Cr, 0.050 for Hg (food only), and 0.026 and 0.035 for Se. These loss rates corresponded to biological half‐lives ranging from 8 d for Ag to 76 d for Se. Loss rates of trace elements from zebra mussel feces followed the following sequence: Cr < Ag < Se < Hg ≤ Cd, with average retention half‐times being 59, 43, 11, 6.7, and 5.1 d, respectively, which indicates that geochemical cycling rates from zebra mussel biodeposits are element specific. Egestion patterns of the radioisotopes indicated two digestive phases, extracellular and intracellular digestion. The extent of intracellular digestion ranged from 7 to 40%, depending on the food source, and correlated with assimilation efficiency for Ag, Cd, and Hg. The bioaccumulation parameters measured 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 shown for Cd. Because D. polymorpha accumulates metals from dissolved and particulate sources in proportion to ambient concentrations, this species can be an effective bioindicator of freshwater metal contamination.