Low levels of oxygen (O) 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 NH-N deep in the lake (20-33 m) by supplying O for 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 d) due to very high rates during spring turnover of the water column, demonstrating that natural turnover provides O for nitrification. During stratification, denitrification was highest at the continuously aerated station (4.06 mg N m d) and lowest at the nonaerated station (3.02 mg N m d). At the periodically aerated station, aeration pauses did not restrict the increase in temperature but resulted in accumulation of NH-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.

Diurnal variation in the density, biomass, and individual size of the predatory cladoceran Leptodora kindtii in Lake Vesijärvi (southern Finland) was studied. Day and night sampling was conducted in July and August at 16 different study stations with net hauls from the bottom to the surface. Independently on the month, estimated biomasses were more than two times higher at night than at day. This was due to both higher density and larger individual length at night. The day‐night differences could not be explained by vertical or horizontal migrations or by bottom‐dwelling behavior by Leptodora, but were attributed to net avoidance. When light was present, more Leptodora were able to avoid the net haul than in the darkness. The avoidance was more effective by large adult individuals (>5 mm) than by juveniles, which amplified the day‐night difference in the population biomass estimate. In order to evaluate the role of large pelagic invertebrate predators in aquatic food webs, nighttime sampling should be used.