A Six‐Year Water, Phosphorus, and Nitrogen Budget for Shagawa Lake, Minnesota

Abstract

AbstractWater, phosphorus, and nitrogen budgets of eutrophic Shagawa Lake, Minnesota were determined from 1967–1972 to assist in defining the significance of the loading from the city of Ely secondary waste water treatment plant to the trophic state of the lake. Ely's municipal waste water accounted for about 80% of the P, 24% of the N, and only 2% of the water to Shagawa Lake while the major tributary, Burntside River, accounted for 66% of the water, 42% of the N, and only 11% of the P to the lake. An average of 30% of the P was retained within the lake.The water and nutrient budgets, along with field and laboratory studies, indicated that high levels of P from the municipal waste water treatment plant were mainly responsible for the eutrophic condition of Shagawa Lake. These studies led to the construction of a tertiary waste water treatment plant designed to remove 99+% (⩽ 0.05 mg/liter) of the P from the secondary waste water effluent. Tertiary waste water treatment should increase the N:P ratio supplied to the lake from 10.5 to 50, and would tend to produce a body of water in which P is the limiting factor in primary production. The P specific loading value of 0.73 g P m−2 year−1 will be reduced to 0.15 g P m−2 year−1.