Drawdown II: Water quality and ecological responses to a managed hydrologic drawdown during autumn

Author(s): Lizotte, R.; Jenkins, M.

A water drawdown of Roundaway Lake, a tributary of the Big Sunflower River, was initiated in mid-autumn to alleviate critical low river flow. While water releases have been demonstrated to alleviate critical low flows, effects of these releases on water quality in contributing tributaries is necessary to improve water resource management decisions. The purpose of the present study was to assess the responses of lake surface water chemical and ecological components including nutrients, phytoplankton and ecoenzyme activities. Lake drawdown began on October 17, 2016 and finished after 14 days when outflow was <0.1% of peak flows and shallowest depths occurred 35 days after drawdown with decreased depths of 56%, 23% and 90% at upstream, lake, and downstream sites, respectively, relative to pre-drawdown depths. Control pond depths during the study period ranged from 82-113% relative to pre-drawdown depths with changes due to evaporation and rainfall. Water samples were collected on days -3, 0, 1, 2, 3, 7, 14, 21, 28, 35, and 42 at the three drawdown sites and an adjacent control pond site (no drawdown) to account for natural seasonal variations. Chemical analyses included soluble nutrients (PO4-P, NH4-N, NO2-N, NO3-N), total nutrients (TP, TN) and organic carbon. Ecological analyses included algal chlorophyll, phycocyanin and photosynthetic efficiency as well as a suite of five ecoenzyme activities. Nutrient changes were greatest at sites with the largest changes in water depth. Upstream dissolved organic nitrogen (NH4-N, NO2-N, NO3-N) increased by >100%, while organic carbon exhibited bimodal changes. Downstream PO4-P, C:N ratios, and C:P ratios all increased by >100%, organic carbon increased by 50% and TP decreased by 45%. Lake nutrients exhibited modest bimodal changes in NH4-N, NO2-N and organic carbon while control pond NO2-N decreased by 50%. Similar to nutrients, algal responses were strongest upstream and downstream. Upstream phycocyanin concentrations increased by >70% while photosynthetic efficiency decreased by 75-80%. Downstream chlorophyll and phycocyanin concentrations decreased by 75-85% and photosynthetic efficiency decreased by 66-90%. Lake and control algal responses were modest with chlorophyll concentrations decreasing by 22-30% and photosynthetic efficiency decreasing by 25-45%. Ecoenzyme activity responses were modest at most sites with bimodal changes to Β–glucosidase:alkaline phosphatase ratios upstream and leucine aminopeptidase in the lake. Greatest changes occurred downstream where Β–N-acetylglucosaminidase, fluorescein diacetate and alkaline phosphatase decreased by 76%, 77% and 98%, respectively. The study contributes valuable information supporting water resource management goals to sustain river and lake ecosystem integrity.

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