Changes in sediment characteristics following drawdown of Big Muskego Lake, Wisconsin

Changes in surficial sediment physical and chemical composition and interna l P loadings due to lake drawdown and refill to promote macrophyte growth i n shallow Big Muskego Lake, Wisconsin were examined. Thirteen percent of th e sediment area was exposed for similar to1 year, while over 80 % of the se diment area was exposed for a period of 6 months. Mean sediment (upper 10 c m) moisture content and organic matter content decreased (92 % to 86 % and 45 % to 39 %, respectively), while sediment density increased from a mean o f 0.078 g/mL to 0.149 g/mL due to lake draw down, suggesting consolidation of sediment and some oxidation of organic matter in the sediment. Porewater ammonium-nitrogen and soluble reactive phosphorus, and laboratory-based ra tes of phosphorus release from sediment, increased substantially shortly af ter lake refill, but then declined 1 year later. Total phosphorus and extra ctable phosphorus forms of the surficial sediment declined in both concentr ation and mass after drawdown, which may be attributed to uptake by biota. These positive (i.e., sediment consolidation) and negative (i.e., sediment nutrient release) impacts of drawdown on sediment dynamics need to be consi dered in shallow lake rehabilitation programs.