Lake Winnipeg, Canada, has been experiencing extensive nuisance and toxic algal blooms for over two decades. Blooms are linked to increased phosphorus (P) loadings from tributaries, including the Red River, which contributes 70% of inflowing P. Red River P loads were identified to be increasing nearly 20 years ago; however, studies have not assessed trends in the major P fractions (i.e., particulate and dissolved phosphorus) or in individual seasons. Drivers of trends have also not been identified. Our study assessed annual and seasonal trends in concentrations and loads of total phosphorus (TP), particulate phosphorus (PP), and total dissolved phosphorus (TDP) and determined if trends were associated with changes in hydrologic, climatic, and/or anthropogenic conditions. TP loads have more than tripled since 1960, primarily due to increases in TDP during the snowmelt and summer seasons. As a result, the dominant fraction of P in the Red River has shifted from PP to TDP. Trends in P are positively associated with increasing discharge and precipitation. Trends also correspond to increased release of treated municipal wastewater and greater P fertilizer application in the basin. In addition, results indicated that management actions to reduce PP losses from agricultural lands have been effective, but new management actions targeting TDP losses are needed, especially for the summer season. Development of strategies to address these emerging needs would benefit from assessments of sources associated with increasing P loadings as well as hotspot analyses to target locations where management can efficiently prevent P losses to the Red River.
Yates et al. (Fri,) studied this question.