Field testing a metal bioaccumulation model for zebra mussels

A kinetic model of trace element bioaccumulation in zebra mussels, employing experimentally determined trace element influx and efflux rates following food and water exposures, was field-tested in the Hudson and Niagara Rivers and Lakes Erie and Ontario. Ultraclean measurements of water column trace element concentrations in these waters and on suspended particles were used to predict metal concentrations in zebra mussels that were measured independently by the NOAA Mussel Watch program. Field concentrations of Ag, Cd, Cr, and Hg ranged from sub-picomolar (Ag) to low nanomolar (Cr) and displayed partition coefficients between particulate and aqueous phases of 120 x 10⁵ L kg^-1. Despite variation in bioaccumulation factors (BAF) between locations by up to 6x (for Ag), our model predicted mean body burdens of Ag, Cr, Hg, and Se that differed from measured tissue concentrations at the same sites by only 30% on average. Cd predictions matched measured values at the two lake sites but exceeded measurements at the three river sites by 2.6-fold. Furthermore, the model predicted that, under all environmental conditions likely to prevail in natural waters, Ag, Cd, and Hg are predominantly accumulated from ingested particles, that Cr is accumulated mostly from the dissolved phase, and that the relative uptake pathway for Se varies with environmental conditions. The highest BAF was for Cd (1564 x 10⁴) and the lowest BAFs were for Cr and Se (1.22.5 x 10⁴ and 0.5 2.8 x 10⁴, respectively), with Ag and Hg being intermediate (2.012 x 10⁴ and 1.425 x 10⁴, respectively). The good agreement of the model with field measurements suggests, for these elements, that (a) accumulation of these elements in zebra mussels is in fact proportional to influx from food and water (that is, the organism is not actively regulating internal concentrations), and (b) we can account for the processes governing metal bioaccumulation in these animals. We conclude that for these elements the zebra mussel will be effective as a bioindicator of ambient metals in freshwater systems.