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Nitrogen Cycling in Lake Wingra¹

ABSTRACT

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.

Key Words: nitrification • denitrification • mineralization • immobilization • sediment-water interchange • eutrophication

¹ Research supported by the Coll. of Agric. and Life Sci., Univ. of Wisconsin, Madison, WI 53706, and by the Eastern Deciduous Forest Biome, US-IBP funded by the National Science Foundation under Interagency Agreement AG-199, 40-193-69 with the Atomic Energy Commission, Oak Ridge National Lab. Contribution No. 96, from the Eastern Deciduous Forest Biome, US-IBB.

² Research Assistant and Professor, Dep. of Soil Sci., and Research Associate, Inst. for Environ. Studies, Univ. of Wisconsin. Present address of the senior author is Lecturer, College of Science and Technology, Port Harcourt, Nigeria.

Received for publication June 9, 1975.