In looking for factors affecting Chironomus plumosus (L.) abundance and its fluctuations − the comparison of its numbers has been done in the bottom (at 6 m depth) and in mesocosms trays with the same bottom sediment elevated 0.4-1.2 meter above the bottom. The experiment was carried out, in a shallow, strongly eutrophic, polymictic, lowland dam reservoir, where theabundance of Chironomus is among the highest in nature. The total abundance was on the average several times higher and more stable and larvae developed more quickly in trays over the bottom than in the same mud at the bottom. All this was probably due to better and more stable oxygen conditions above the bottom than at its very surface. Usually no differences in the abundance were found between levels 0.4, 0.8 and 1.2 m above the bottom, what indicates that conditions at all these levels were similar and different from those at the very bottom. The average density, and especially spring peak numbers in and above the bottom, were much higher in the year 1993 than in 1996 (the ratio of average numbers 1993/96 being similar 2.3 and 2.6 for the bottom and trays respectively). It was probably due to a higher food supply (mostly small diatoms) by rivers and also due to weaker spring water flow in 1993. The very strong decline from high spring peak numbers to summer minimum seems to result from interconnections between larvae and perhaps also increased fish pressure, after the spawning period.
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