Scavenging of 137Cs, 109Cd, 65Zn, and 153Gd by Plankton of the Microbial Food Web in Pelagic Lake Erie Surface Waters

Abstract

Inorganic dissolved 137Cs, 109Cd, 65Zn and 153Gd were added, at concentrations well below their respective solubility limits, to screened water (< 210 μm) sampled from the pelagic epilimnion of Lake Erie during the summers of 1994 and 1995. The hypotheses tested were that scavenging occurs in all of the ecologically significant size fractions that comprise the microbial food web (picoplankton, 0.2 to 2 μm; nanoplankton, 2 to 20 μm; microplankton, 20 to 210 μm), and that scavenging by plankton is directly related to the respective particle-reactivity of the elements (Gd > Zn > Cd » Cs). Size-selective filtration at intervals over periods of 22 to 30 h established that picoplankton and nanoplankton were the dominant scavenging phases in this environment. Scavenging of 153Gd, 65Zn, and 109Cd by plankton was more similar than predicted on the basis of their anticipated particle-reactivity and 137Cs was weakly scavenged. Except for the picoplankton, 65Zn was the element most readily scavenged by the plankton size fractions. High accumulation of 109Cd in the picoplankton may reflect the sorption of this element by calcite associated with autotrophic picoplankton. These experiments quantify the partitioning of trace metals within the plankton of the microbial food web (0.2 to 210 μm), a dynamic community of particles that dominates the seston in the pelagic surface waters of Lake Erie during thermal stratification, and suggest that plankton dynamics should be considered in predictions of the geochemical fate of trace metals in this environment.