Complex patterns of phosphorus delivery in the Lake of the Woods watershed

Lake of the Woods (LoW) is a large, transboundary lake that continues to experience harmful algal blooms despite large declines in phosphorus (P) inputs from point sources. Tributary runoff is considered the largest source of P to the lake; however, there are few monitoring data within the Canadian portion of the basin (～60% of total area), to guide management. To address this gap, we monitored five rivers of contrasting land use within the lower Rainy River region, an acknowledged “hot spot” of P delivery in the basin. Total P (TP) concentrations were consistently high at all five rivers (volume-weighted range: 19–215 μg/l) despite differences in agriculture across sites (7–27%), suggesting ‘natural’ background P levels are high in this landscape. Furthermore, TP concentrations were strongly correlated with total suspended sediment and geogenic metals, iron (Fe) and aluminum, and TP was especially high during events (>400 μg/l) indicating erosion is an important source of P delivery. However, equally high TP and Fe during periods of slow, stagnant flow in the summer and under winter ice suggest erosion is not the only source of P. Instead, we suggest redox release of P from streambed sediment is also important. This observation is significant, because internal P release within the tributaries, especially during the summer could ‘seed’ downstream algal blooms. The strong sensitivity of TP to both high and low flow conditions indicates that frequent, all-season, multi-year measurements are needed to understand the mechanisms of P delivery in this basin.