A Comparison of Methods for Normalizing Residues of Organic Contaminants in Zebra Mussels (Dreissena polymorpha), with Implications for Biomonitoring Programs

There has been little consistency to date in the method used to express concentrations of organic contaminants in zebra mussels collected from the Great Lakes. Concentrations have been reported on a wet whole (with shells), dry whole, wet soft tissue, dry soft tissue, and lipid weight basis. This study examined residues of organic contaminants in samples of zebra mussels collected from 24 sites in the lower Great Lakes and St. Lawrence River, to determine if the way in which the residues were normalized could affect the precision of estimated concentrations or the spatial trends observed. Variation in the moisture content of soft tissues was minimal, so normalizing to wet or dry soft tissue weight would yield consistent results. Moisture content of whole mussels was more variable, and relative proportions of soft tissues and shell vary with size, season and location-potentially confounding spatial trends. Lipid content also varies greatly among locations, seasons and years, and lipid-normalized residues were much more variable than residues normalized to whole or soft tissue weight. Furthermore, site-to-site trends based on lipid-normalized data did not always agree with those based on other components of the organism, and thus should not be interpreted in isolation.