Trace metal cycling in the surface waters of Lake Erie: Linking ecologicaland geochemical fates

The highly productive plankton that comprise the microbial food web are ide ally suited for trace metal scavenging, the removal of particle-reactive tr ace metals from the aqueous phase. Scavenging is considered to be the key f actor controlling the concentrations of trace metals in the surface waters of large lakes during thermal stratification. Observed characteristics of t he various plankton size fractions in the microbial food web (ability to sc avenge trace metals from the dissolved phase; potential to regenerate these metals back into the dissolved phase; population dynamics) were incorporat ed into a dynamic microbial food web model of trace metal cycling in the su rface waters of Lake Erie. The model yields estimates of epilimnetic trace metal residence times under the assumption of steady-state conditions: Cs = 352 d, Cd = 19 d, Zn = 23 d, Gd = 45 d. These trace metal residence times were considerably greater than the residence times predicted when microzoop lankton grazing activity was eliminated from the model simulations (Cs, +46 %; Cd, +58%; Zn, +53%; Gd, +80%). The increase of residence time by microzo oplankton grazing is attributed to the trace metal regeneration that result s from incomplete assimilation by the grazer of metal previously scavenged by the prey item. The results illustrate the important influence of the mic robial food web activities on the geochemical fates of trace metals in the pelagic surface waters of large lakes during thermal stratification.