Drought and management effects on biophysicochemistry in a rapidly-flushed reservoir

Bellinger BJ, Richter A, Porras A, Davis SL. 2017. Drought and management effects on biophysicochemistry in a rapidly flushed reservoir. Lake Reserv Manage. 34:182198.During a recent drought period, a municipal reservoir was affected by increased duration and magnitude of cyanobacterial blooms, deep water hypoxia, and abundant Hydrilla verticillata growth. We analyzed over 20 yr of water quality data from sources monitoring Lake Austin, Texas, USA, to relate changing hydraulic pulsing rates during periods of water surplus and deficit to system biophysicochemistry. During the drought period, average daily discharges through the reservoir system were significantly reduced to conserve water supply. In response, growing season hydraulic residence times in Lake Austin, typically a run-of-river system with residence of about 10 d, increased 2- to 8-fold. Nutrient loadings to Lake Austin decreased during low discharge, but only total phosphorus (TP) concentrations declined appreciably in the reservoir. Late summer lower flows across years created epilimnetic conditions conducive to cyanobacteria blooms because of stable stratification, water temperatures exceeding 25 C, and nitrate concentration minima with TP and ammonia maxima. Drought-induced discharge reductions promoted earlier onset of warm, stratified waters, and lower hypolimnetic oxygen concentrations. The physicochemical changes coupled with longer hydraulic residence altered the timing of algal bloom initiation, duration, and peak, and quadrupled the probability of cyanobacterial bloom occurrence. The largest cyanobacteria blooms were observed when growing season discharges were consistently below 20 m(3)/s (i.e., hydraulic residence time >2 weeks). Concomitantly, the nonnative macrophyte hydrilla rapidly spread in the warm, reduced flow conditions. By linking flushing rates to reservoir stratification and potential phytoplankton bloom magnitude, our results will allow managers to anticipate summer water quality conditions.