Nitrogen cycling in Lake Wingra

Monitoring of the forms of N in water and sediment, and investigations of N cycling in Lake Wingra, a small, eutrophic, hardwater lake in southern Wisconsin, were conducted. Nitrification in the waters appeared to be by heterotrophs, while autotrophic nitrification dominated in the sediments. Rapid turnover of /sup 15/N-NH/sup +//sub 4/ occurred in the waters, but not in the sediments, while /sup 15/N-NO/sub 3//sup -/ turnover was slow in the waters. In the sediments, denitrification and immobilization of /sup 15/N-NO/sub 3//sup -/ was rapid. About 80 percent of the added /sup 15/N-NO/sub 3//sup -/ was denitrified, the remainder being immobilized. Detrital-/sup 15/N was slowly mineralized in the sediments. Sediment-water interchange occurred at a significant rate. About 40 mu g NH/sub 4//sup +/-N/liter sediment/day was formed from organic-N, and subsequently released to the overlying water. Estimates of the overall average N content in various compartments indicated that about 50 percent of the ''available'' N was in the water, 20 percent in the macrophytes, and 30 percent in the sediments, while over 97 percent of the total N in the lake (to 30 cm sediment depth) was as sediment organic N. Consideration of seasonal changes in the NH/sub 4//sup +/-N content ofmore » the sediments gave an average release rate of NH/sub 4//sup +/-N of 7 to 29 mg m/sup -2/ day/sup -1/. This would supply from 3 to 14 metric tons of N to the lake water yearly.« less