Seasonal and spatial patterns in phosphorus and nitrogen delivery: Implications for monitoring and management

There is growing recognition that management efforts to limit harmful algal bloom (HAB) production in lakes need to consider tributary loadings of both phosphorus (P) and nitrogen (N). This may be the case for Lake of the Woods (LoW), which experiences annual HABs, and has been historically monitored for P, but not N. Ongoing agricultural intensification within the basin, including expansion of tile-drained row crop production, is creating new sources of N that may be entering rivers to a greater extent in the winter/spring, when sampling is typically less frequent. To address this gap, we investigated seasonal P and N inputs to the Rainy River, the largest tributary that feeds the LoW, from seven tributaries that drain the Lower Rainy River basin. Total P (TP) concentrations were consistently high at all seven tributaries and exceeded water quality guidelines, and total Kjeldahl-N (TKN) levels, which include ammonium (NH 4 -N) but largely reflect organic N, were also high relative to reference conditions for the region. In contrast, nitrate (NO 3 -N) levels were generally low, especially in the growing season. Notably, NO 3 -N, TKN, and TP concentrations were highest in tributaries with more agricultural development. There were no clear seasonal patterns in TP or TKN, whereas NO 3 -N was up to 10 times higher in the winter compared with the growing season. Higher N losses from agricultural areas that were especially clear in the winter suggest that N export is sensitive to regional trends of agricultural intensification and winter warming and warrant increased scrutiny of N inputs to the basin.