Potential nutrient limitation to chlorophyll production in surface waters of southern Lake Huron was assayed monthly from April to December 1975. Natural phytoplankton assemblage responses to nutrient enrichment were determined from chlorophyll production in laboratory incubations under seasonally varied light and temperature conditions. Experimental treatments included a complete treatment containing P, N, Si, EDTA, vitamins, and trace metals; treatments with deletions from the complete treatment; complete treatments and different P concentrations; and treatments with single nutrient additions. Effects were most pronounced during summer and fall when P, EDTA, FeEDTA, vitamins, and Si were added simultaneously. The intensity of the effect resulting from additions of phosphorus and other nutrients varied seasonally. Individual additions of EDTA, N, and Si had little effect as did deletion of N from the complete treatment. Addition of P alone resulted in limited growth. Deletion of phosphorus from the complete enrichment, however, reduced growth drastically during most experimental period.