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Low levels of oxygen (O2) in the hypolimnion layer of lakes are harmful to benthic animals and fish; they may also adversely affect nutrient cycles. Artificial aeration is often used in lake management to counteract these problems, but the effects of aeration on nitrogen (N) cycling are not known. We studied the effects of hypolimnetic aeration on N dynamics and temperature in a eutrophic lake by comparing continuous and pulsed aeration with a nonaerated station. Aeration decreased the accumulation of NH4+–N deep in the lake (20–33 m) by supplying O2for nitrification, which in turn provided substrate for denitrification and promoted N removal. Aeration also increased the temperature in the hypolimnion. Denitrification rate was highest in the nonaerated deep areas (average, 7.62 mg N m−2d−1) due to very high rates during spring turnover of the water column, demonstrating that natural turnover provides O2for nitrification. During stratification, denitrification was highest at the continuously aerated station (4.06 mg N m−2d−1) and lowest at the nonaerated station (3.02 mg N m−2d−1). At the periodically aerated station, aeration pauses did not restrict the increase in temperature but resulted in accumulation of NH4+–N and decreased the contribution of denitrification as a nitrate reduction process. Our findings demonstrate that hypolimnetic aeration can substantially affect N cycling in lakes and that the effect depends on the aeration strategy. Because N is one of the main nutrients controlling eutrophication, the effects of aeration methods on N removal should be considered as part of strategies to manage water quality in lakes. Continuous hypolimnetic aeration prevented NH4+accumulation in a lake at 20–33 m depth.Continuous aeration promoted denitrification by enabling nitrification.Periodical aeration did not restrict temperature changes, but NH4+was accumulated.Denitrification rate decreased during aeration pauses.Benthic N removal in a lake can be enhanced via hypolimnetic aeration.

The effect of sediment resuspension on the nitrogen to phosphorus (N:P) ratio of lake water as well as on concentrations of total and dissolved nitrogen and phosphorus were studied experimentally in shallow eutrophic Lake Kirkkojärvi. In addition the denitrification rate and potential were measured, using the isotope-pairing technique concomitantly with the resuspension experiments. Resuspension increased the concentration of total phosphorus (TP) relatively more than the concentration of total nitrogen (TN), thus decreasing the TN to TP ratio, regardless of the phase of the growing season. Resuspension also had various effects on the dissolved nitrogen and phosphorus concentrations. The denitrification rate was highest in May, when the supply of nitrate was high. In August, when the denitrification potential peaked due to high temperature, the low availability of nitrate limited nitrogen removal. However, sedimentary denitrification during the open-water season was so intense that it also lowered the N:P ratio in the water directly and not only by lowering the ratio in the sediment prone to resuspension.