Nutrient sequestration in the Lake Winnipeg watershed

Lake Winnipeg, Manitoba, has been subjected to significant increases in nutrient loading over the last few decades, and consequently has experienced significant and widespread algal blooms. The objective of our study was to identify sources of nutrients in the Lake Winnipeg basin, and quantify their removal (sequestration) into 28 of the largest lakes and reservoirs located in the Saskatchewan, Dauphin, Red, and Winnipeg river sub-basins, thus preventing their transport downstream to Lake Winnipeg. Discharges were determined daily, and nutrient parameter concentrations determined once or twice each month upstream and downstream from each of the lakes and reservoirs for three years. Concentrations of P and N in source waters of the Lake Winnipeg basin varied substantially, with the lowest concentrations occurring in pristine headwaters of the Saskatchewan River (mean TP = 14 μg/L; mean TN = 217 μg/L) and some of the highest concentrations occurring in small streams that originated within agricultural landscapes in the headwaters of the Dauphin River sub-basin (mean TP = 133 μg/L; mean TN = 1313 μg/L). Twelve reservoirs in the Saskatchewan River sub-basin collectively sequestered 92% of the TP inputs and 68% of the TN inputs to the sub-basin. In P-rich lakes, relatively more N was sequestered than P compared with nutrient impoverished lakes. A total 13,215 t/yr TP was discharged into Lake Winnipeg while 8234 t TP/yr, was sequestered into the lakes and reservoirs. The Red River sub-basin was the principal source of nutrients to Lake Winnipeg and should be the focus of nutrient management in the Lake Winnipeg basin.