Nitrogen Cycling in Lake Wingra
1976
Isirimah, N. O. | Keeney, D. R. | Dettmann, E. H.
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 ¹⁵N-NH₄⁺ occurred in the waters, but not in the sediments, while ¹⁵N-NO₃⁻ turnover was slow in the waters. In the sediments, denitrification and immobilization of ¹⁵N-NO₃⁻ was rapid. About 80% of the added ¹⁵N-NO₃⁻ was denitrified, the remainder being immobilized. Detrital-¹⁵N was slowly mineralized in the sediments. Sediment-water interchange occurred at a significant rate. About 40 µg NH₄⁺-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% of the “available” N was in the water, 20% in the macrophytes, and 30% in the sediments, while over 97% of the total N in the lake (to 30 cm sediment depth) was as sediment organic N. Consideration of seasonal changes in the NH₄⁺-N content of the sediments gave an average release rate of NH₄⁺-N of 7 to 29 mg m⁻² day⁻¹. This would supply from 3 to 14 metric tons of N to the lake water yearly. This is about the same order of magnitude as all external N inputs. Thus, even if the controllable point sources of N were eliminated, the biomass N in the lake water would not be significantly decreased.
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