Landscape characteristics driving spatial variation in total phosphorus and sediment loading from sub-watersheds of the Nottawasaga River, Ontario

Eutrophication from agricultural runoff is a global problem, often resulting in formation of anoxic zones in receiving water bodies. The Nottawasaga River Watershed (NRW) is dominated by agricultural land-use, and drains into Nottawasaga Bay, Georgian Bay (Lake Huron). A fundamental feature of the NRW is the Minesing Wetlands, a Ramsar site and the largest inland wetland in southern Ontario. We used total phosphorus (TP) concentration-discharge relationships to estimate annual loading from six sub-watersheds and compared these against published models, which did not offer a way to account for the unique properties of the Minesing Wetlands. We developed predictive loading models specifically for the NRW to account for these characteristics, which accurately predict daily summer base-flow TP (r² = 0.76, p = <0.0001) and total suspended solids (TSS; r² = 0.65, p = <0.0001) loads for 11 subwatersheds using geomorphic and land-cover variables. Drainage area and % pasture land were the most significant predictive variables driving spatial variability in TP and TSS loading rates among subwatersheds. The positive relationship between TP and % wetland (r² = 0.22, p = 0.0063) also suggested that the Minesing Wetlands are a source of nutrients to the Nottawasaga River. Watershed geomorphology (e.g. slope) was a good predictor of land cover, and produced accurate loading estimates. This study is the first to offer a new approach to predict TP and TSS loading rates during the growing season using readily available geospatial data.