The low concentrations of dissolved trace metals observed in the surface waters of the lower Laurentian Great Lakes of North America during summer months are generally attributed to the sedimentary loss of biogenic particles from the epilimnion, in accordance with established scavenging models based solely upon the sorptive loss of solutes to particle surfaces. The majority of particles in the pelagic systems of these lakes are biotic. Among the most productive are the autotrophic picoplankton and bacteria (0.2-2 microns) that have a high potential to scavenge trace metals. The ecological fate of picoplankton in the microbial food web is predominately consumption by microzooplankton (mixotrophic and heterotrophic protozoans, 2-200 microns).

The hypothesis that microzooplankton can regenerate significant amounts of trace metal into the dissolved phase through the incomplete assimilation of trace metals from their prey was tested in the laboratory and in the field. Radionuclides were used to follow the fate of trace metals ingested in a particulate form by microzooplankton. Rapid regeneration of trace metals from the particulate to the dissolved phase (<0.2 microns) was observed in a laboratory model of a simple Great Lakes microbial food chain, composed of a mixotrophic nanoflagellate (OCHROMONAS) grazing Cs-137, Cd-109, Zn-65, and Gd-153-radiolabeled picocyanobacteria (SYNECHOCOCCUS). Most of the trace metal consumed as prey was regenerated by OCHROMONAS; regenerated Gd-153, Zn-65, and Cd-109 had reduced bioavailability, in comparison with inorganic forms of the same elements. Trace metal regeneration was also observed in the natural plankton community sampled from the pelagic region of Lake Erie during thermal stratification. Cd-109 and Zn-65-radiolabeled SYNECHOCOCCUS was used to label the picoplankton community in lake water and to trace the effect of natural grazing activity on the size fractionation of these metals. Trophic transfer and recycling of regenerated trace metal by microbial food-web organisms was observed. Most regenerated trace metal radionuclides remained in the dissolved phase.

A model of trace metal fate in surface waters under steady-state conditions was based on observed trace metal scavenging and biological characteristics of the microbial food web. Trace metal residence times of Cs (514 d), Cd (29 d), Zn (32 d), and Gd (66 d) predicted by the model were 46%, 62%, 58%, and 84% greater, respectively, than residence times predicted if microzooplankton grazing activity was eliminated from the model simulations.

These results are the first unequivocal demonstration of trace metal regeneration by microzooplankton grazing activity, and illustrate the importance of the microbial food web in determining the geochemical fates of particle-reactive trace metals in the pelagic surface waters of large lakes during thermal stratification.