A dramatic recovery of Lake Spokane water quality following wastewater phosphorus reduction

Abstract

Lake Spokane became hypereutrophic due to nutrient input from a municipal wastewater facility. Following a 1977 reduction in wastewater total phosphorus (TP) from about 5 to 0.5 mg/L, lake water quality and trophic state recovered rather quickly, going from hypereutrophy to meso-eutrophy in the first 7 years. After TP reduction, mean summer (Jun–Oct) inflow TP declined from 86 to 25 μg/L during that 7-year period. Mean summer epilimnetic chlorophyll (Chl) declined from 21 to 11 μg/L, and the mean volume-weighted (v-w) hypolimnetic seasonal minimum dissolved oxygen (DO) increased from 1.4 to 4.5 mg/L over that same period. Recent data (2010–2014) demonstrate continued recovery to meso-oligotrophy with the 5-year average minimum hypolimnetic v-w DO at 6.5 mg/L and mean inflow TP and epilimnetic Chl at 15 and 4 μg/L, respectively. The areal hypolimnetic oxygen deficit (AHOD) rate now averages 0.67 ± 0.12 g/m2 per day, which is 84% less than the pre TP-reduction AHOD (median 4.2 g/m2 per day). This recovery in DO indicators may be the clearest case of recovery from severe eutrophication for a reservoir, which usually have higher AHODs than lakes. The recovery confirms the close link among TP inflow concentration, Chl, and DO in reservoirs, despite their relatively large watersheds and inflows that produce high nutrient loadings compared to natural lakes. The results show that reduction of phosphorus recovered the lake to meso-oligotrophy, even though nitrogen was initially limiting as much or more than phosphorus during hypereutrophy, and despite markedly increased inflow nitrogen since 2000.