9508087 Lehman Differences in availabilities and rates of supply of phosphorus to the ecosystems of the Great Lakes of North America are known to be major factors controlling rates of productivity, biomass development, and species composition. Nonetheless, epilimnetic concentrations of phosphate are uniformly low in all the Great Lakes during summer, and radiotracer studies demonstrate that turnover times of the ambient pools are minutes to tens of minutes, as the result of biological processes. In situ concentrations during the productive summer months have generally proven intractable to accurate measurement, but it appears that, in common with oceanic observations of ammonium dynamics, bacteria dominate fluxes at ambient levels, whereas phytoplankton dominate when concentrations are modestly increased. There are differences between turnover dynamics and equilibrium partitioning of tracer P in Great Lakes of differing trophic condition, and along inshore-to-offshore gradients within individual Great Lakes. The main hypothesis to be tested in this research is that the relative magnitudes of microbial recycling processes, and of P recycling by zooplankton, change predictably with overall primary production, and with an index of P deficiency, such that different proportions of recycled P are incorporated by algae versus bacteria in different lakes. This research effort will combine explicit kinetic models for P fluxes with experiments designed to quantify the relative contributions of bacterial, protozoan, and metazoan recycling rates in Lakes Erie, Huron and Michigan. A new analytical method adapted from sea water for fresh water investigations (magnesium inducted coprecipitation of P) now makes tractable the investigation of P dynamics and regeneration rates by is tope dilution methods.
