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A review of the 1976 Great Lakes phosphorus load estimates developed by the Pollution From Land Use Activities Reference Group (PLUARG), the Water Quality Board (WQB) and Task Group III (TG) (the bilateral technical group which developed phosphorus Target Loads for the Great Lakes as part of the Fifth Year Review of the United States - Canadian Water Quality Agreement) was conducted. The load estimates were examined to determine the sources of differences between estimates and, where possible, to explain the reasons for the differences.

A trophic evaluation of the nearshore zone of the Great Lakes has been undertaken. Trophic status is described as a composite of three parameters descriptive of both water quality and trophic conditions. These parameters are total phosphorus, chlorophyll a, and Secchi depth. Phosphorus is assumed to be a primary variable in determining trophic conditions. Chlorophyll a is important as a measure of algal biomass. Secchi depth is a measure of water clarity, inversely related to algal biomass. The relationship between these three parameters in the Great Lakes nearshore zone is determined with linear regression techniques. These relationships are used as the basis for a Composite Trophic Index (CTI). The CTI is related to trophic conditions in the nearshore zone. The resulting delineation of trophic status in this region is presented. The nearshore trophic conditions are related to phosphorus contributing areas in the Great Lakes Basin. Nearshore and open water trophic conditions are also compared. Ontario Ministry of the Environment