Scavenging of Cs-137, Cd-109, Zn-65, and Gd-153 by plankton of the microbial food web in pelagic Lake Erie surface waters

Inorganic dissolved Cs-137, Cd-109, Zn-65 and Gd-153 were added, at concent rations well below their respective solubility limits, to screened water (< 210 mu m) sampled from the pelagic epilimnion of Lake Erie during the summ ers of 1994 and 1995. The hypotheses tested were that scavenging occurs in all of the ecologically significant size fractions that comprise the microb ial food web (picoplankton, 0.2 to 2 mu m; nanoplankton, 2 to 20 mu m; micr oplankton, 20 to 210 mu m), and that scavenging by plankton is directly rel ated to the respective particle-reactivity of the elements (Gd > Zn > Cd mu ch greater than Cs). Size-selective filtration at intervals over periods of 22 to 30 h established that picoplankton and nanoplankton were th dominant scavenging phases in this environment. Scavenging of Gd-153, Zn-65, and Cd -109 by plankton was more similar than predicted on the basis of their anti cipated particle-reactivity and Cs-137 was weakly scavenged. Except for the picoplankton, Zn-65 was the element most readily scavenged by the plankton size fractions. High accumulation of Cd-109 in the picoplankton may reflec t the sorption of this element by calcite associated with autotrophic picop lankton. These experiments quantify the partitioning of trace metals within the plankton of the microbial food web (0.2 to 210 mu m), a dynamic commun ity of particles that dominates the seston in the pelagic surface waters of Lake Erie during thermal stratification, and suggest that plankton dynamic s should be considered in predictions of the geochemical fate of trace meta ls in this environment.