The modeled and observed response of Lake Spokane hypolimnetic dissolved oxygen concentrations to phosphorus inputs

Lake Spokane, a reservoir in eastern Washington State, was previously hypereutrophic due to phosphorus discharges from the City of Spokane wastewater treatment plant (WWTP). This reservoir subsequently recovered to a meso-oligotrophic state after implementation of advanced phosphorus removal. The present study tested whether the mechanistic Lake Spokane water quality (WQ) model realistically represents the sensitivity of this reservoir's hypolimnetic oxygen concentrations to phosphorus inputs. We compared the observed relationship between the mean summer input total phosphorus concentration (TP_IN) and the minimum volume weighted hypolimnetic dissolved oxygen concentration (DO_MIN) to model values for conditions ranging from hypereutrophic to oligotrophic. Prior to advanced phosphorus removal, TP_IN and DO_MIN averaged 86 ± 37 (SD) µg/L and 1.4 ± 1.3 mg/L, respectively. Currently (2010–2014), these values average 14 ± 3 µg/L and 6.5 ± 0.8 mg/L, respectively. By contrast, the model's DO_MIN response for similar TP_IN concentrations was much less pronounced, with hypereutrophic and contemporary DO_MIN averaging 3.8 ± 0.4 and 4.7 ± 0.04 mg/L, respectively. The model also has a structural DO deficit (saturated DO − DO_MIN) of 5.3 mg/L that was evident when all TP inputs to the reservoir were set to zero. Similarly, when all WWTP effluent sources were set to TP_EFF = 0 µg/L, the reservoir epilimnetic TP concentrations were ≈8 µg/L higher than the Spokane River inputs. The water quality model indicates that even if effluent phosphorus concentrations are reduced to zero, the dissolved oxygen goals for Lake Spokane cannot be met.