Lake Simcoe in Ontario, Canada, is a large lake surrounded by a mix of urban, agricultural, and less developed areas and is showing adverse effects from excess nutrient inputs, including low hypolimnetic oxygen concentrations. Knowledge of both the quantity and quality of nutrients and seston entering the lake is important because large reductions in phosphorus (P) loads have been proposed to help restore the lake and its coldwater fishery. We examined land use effects on P quality (i.e., bioavailability) and its relationship to seston in the tributaries of Lake Simcoe. Indicators of agricultural impacts were examined in 13 tributaries of Lake Simcoe, which were selected to represent a range of land use types. Bioavailability of P was assessed through analysis of different forms of P and stoichiometric indicators of nutrient status in seston. Nutrient sources were examined using the δ15N of seston. The percentage of cropland in the subwatershed had a strong relationship with P as reflected in higher soluble reactive P concentrations and lower indicators of P deficiency. Cropland land use effects were complicated; they contributed highly bioavailable P to a P deficient lake, and at the same time, contributed high seston loads causing turbidity, resulting in light deficiency. In the Lake Simcoe watershed, animal manure application on cropland could be a source of nutrients related to the δ15N variability and, correspondingly, bioavailable P. Management efforts should therefore include best management practices to reduce manure application to croplands and to prevent runoff from areas where manure is stored
