Nutrient loading has been linked to many issues including eutrophication, harmful algal blooms, and decreases in aquatic species diversity. In the Great Lakes, algal blooms continue to plague Lake Erie and Saginaw Bay despite reduction in point source loading to the lakes. Though many watershed nutrient models exist in the literature, there is generally a tradeoff between the scale of the model and the level of detail regarding the individual sources of nutrients and basin characteristics. To examine the link between watershed nutrient sources, landscape processes, and in-stream loads in the Lower Peninsula of Michigan a spatially explicit nutrient loading model was developed. The model is composed of two parts: GIS models that predict nutrient sources and a statistical model that predicts nutrient loads from the source models and basin characteristics. Observations collected during baseflow, melt, and rain conditions from 2010-2012 were used to calibrate and validate the model. The model predicts nutrient loads and provides information on the sources of nutrients within each watershed and the relative contribution of different sources to the overall nutrient load. The model results indicated that there is a high degree of variability in nutrient export rates, even within the same land use class, and export rates can be as much as 15 times greater for nitrogen and as much as 9 times greater for phosphorus depending on the season. The model performed as well as other regional scale nutrient models and showed less bias due to land use. In addition, nitrate isotope data agreed with the model's predictions of the relative source of nutrients. This work has the potential to provide valuable information to environmental managers regarding how and where to target efforts to reduce nutrient loads in surface water.
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