NITROGEN DYNAMICS IN SEDIMENT DURING WATER LEVEL MANIPULATION ON THE UPPER MISSISSIPPI RIVER

Nitrogen (N) has been linked to increasing eutrophication in the Gulf of Mexico and as a result there is increased interest in managing and improving water quality in the Mississippi River system. Water level reductions, or ‘drawdowns’, are being used more frequently in large river impoundments to improve vegetation growth and sediment compaction. We selected two areas of the Upper Mississippi River system (Navigation Pool 8 and Swan Lake) to examine the effects of water level drawdown on N dynamics. Navigation Pool 8 experienced summer drawdowns in 2001 and 2002. Certain areas of Swan Lake have been drawn down annually since the early 1970s where as other areas have remained inundated. In the 2002 Pool 8 study we determined the effects of sediment drying and rewetting resulting from water level drawdown on (1) patterns of sediment nitrification and denitrification and (2) concentrations of sediment and surface water total N (TN), nitrate, and ammonium (NH4 þ). In 2001, we only examined sediment NH4 þ and TN. In the Swan Lake study, we determined the long-term effects of water level drawdowns on concentrations of sediment NH4 þ and TN in sediments that dried annually and those that remained inundated. Sediment NH