Newnans Lake, Florida: Removal of Particulate Organic Matter and Nutrients Using a Short-Term Partial Drawdown

Spillways at lake outlets are commonly employed to reduce water level fluctuations and promote year-round lake access. However, stabilized water levels may cause accelerated accumulation of sediment on the lake bottom. This can change aquatic plant communities, cloud the water, and eliminate hard-bottom nesting areas for many sportfish. These effects may be particularly pronounced when spillway design does not allow for bottom water drainage and outflow is restricted to less turbid surface waters. Such a structure controls the single surface-outflow of shallow, algal-dominated Newnans Lake, Florida. A 90-day removal of this spillway flushed 60 metric tons (dry weight) of sediment (containing 15% total Kjeldahl nitrogen and 0.5% total phosphorus) from the lake. This quantity was small compared to the likely stores in the lake, but the removal was accomplished at no cost. Data suggest low lake stage at the start of dewatering, resulting in small hydraulic head and low flow through the lake system depressed removal rates. Elevated concentrations of particulate organic matter (POM), total Kjeldahl nitrogen (TKN), and total phosphorus (TP) were noted during the first month of dewatering when adequate head differential was still present. Storms stirred the water column and promoted flushing of resuspended matter. In situ and laboratory tests did not demonstrate net oxidative removal of organic matter from exposed areas of the lake bottom. Production of organic matter under high solar radiation and nutrient availability likely replaced material lost through oxidation. Consolidated sediments remained firm after reflooding, providing improved habitat for rooted macrophytes and fish spawning. Short-term partial drawdowns are inexpensive and effective in flushing organic matter and nutrients when they are initiated at high lake stage and coincide with frequent storm events. Routine application of this management technique may produce a periodic rejuvenation of the lake ecosystem