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Effects of Hypolimnetic Aeration on Nitrogen Transformations in Simulated Lake Sediment-Water Systems¹

ABSTRACT

Mechanical aeration of the anoxic hypolimnion of small, eutrophic, thermally stratified lakes and impoundments has been proposed as a temporary water quality improvement technique. We hypothesized that proper manipulation of this technique could lead to significant N loss through nitrification-denitrification. This hypothesis was examined by using simulated sediment-water and batch incubation systems, and the use of ¹⁵N.

Aeration of an NH₄-enriched hypolimnion should lead to disappearance of ammonium and formation of nitrate. If the sediment had a high oxygen demand, nitrate should rapidly disappear once aeration was discontinued. Nitrate should also disappear, although more slowly, during aeration due to reactions at the sediment-water interface and in the surficial sediment. The amount of N removed from the sediments by nitrification-denitrification would vary widely, depending on the lake, but would not be as large as predicted from nonlabeled N mass balances due to assimilatory nitrate reduction (immobilization).

The results of this work indicate that nitrification is rapid, even at 10°C, when the hypolimnion is aerated, and that denitrification and nitrate immobilization is equally rapid when aeration is stopped and the hypolimnion again becomes anaerobic. However, at least in Cox Hollow Lake sediments, a considerable amount of nitrate (up to about 60%) is immobilized rather than denitrified. Thus, the net loss of N from the lake system is much less than would be predicted from strictly inorganic N mass balance calculations, and hypolimnion aeration does not offer much promise as a method for removing N from lakes.

Key Words: eutrophication • lake treatment • redox potential • Wisconsin

¹ Research supported by the College of Agricultural and Life Sciences, Univ. of Wisconsin, Madison, and by USEPA Project no. R-802692-01-0. Presented in part before Div. A-5, American Society of Agronomy, Los Angeles, Calif., 16 Nov. 1977.

² Research Associate, Professor, Research Associate, Dep. of Soil Science, Univ. of Wisconsin, Madison, WI 53706. The senior and third authors are currently Assistant Professor, Center for Wetland Resour., Louisiana State Univ., Baton Rouge, LA 70803; and Research Chemist, USDA, SEA-FR, Beltsville, MD 20705, respectively.

Received for publication November 9, 1978.