Fluorescence (F), DCMU-enhanced fluorescence (F^DCMU) and a ratio of these two measurements (F ratio) were found to be useful indicators of light history but not physiological condition of natural phytoplankton assemblages. Changes in the fluorescence properties of unialgal continuous and batch cultures at different growth rates and following nutrient addition were observed only during nutrient starvation. Nutrient deficiency in Lake Ontario was not revealed either by seasonal patterns of fluorescence or by short term changes in the F ratio following nutrient additions. This result however is not conclusive evidence of nutrient sufficiency because of the insensitivity of the fluorescence ratio as an indicator of growth rate. The depression of F and F^DCMU observed in surface waters of Lake Ontario occurred during conditions of high light and low mixing rates. The result suggests that ''photoinhibition" of photosynthesis as measured by conventional primary productivity techniques, may occur in nature only under these particular conditions. A general relationship between temperature gradients or water column stability and the difference in fluorescence between 1 and 10 meters was observed. This relationship was due to both vertical structure in the assemblage and the physiological effect of light on fluorescence. Diurnal patterns of fluorescence were found to be due to the physiological effect of light on fluorescence rather than a circadian rhythm. The physiological effect was dependent on both the duration and intensity of exposure of the cells to light as well as the sensitivity of the assemblage to light. Differences were associated with seasonal changes in species composition with spring and winter populations exhibiting the greatest sensitivity. / Thesis / Master of Science (MSc)