Mud (with or without Chironomus) taken from the bottom of a shallow (6 m) eutrophic reservoir, was maintained (about 3 weeks) in trays supported 30 cm and 70 cm above the bottom. At the end of the experiment it contained 7-9 times more Chironomirs plrimosus individuals than did the mud at the bottom. Numbers of young ( S 12 mm) larvae were up to 18 times higher, and youngest (26 mm) larvae (absent in the bottom at the end of the experiment) were abundant. Such patterns were more evident at the higher (70 cm) than the lower (30 cm) level. Numbers of the oldest (1 18 mm) larvae at the higher level were only 6 times higher (whilst more than 8 times at the lower level) than at the bottom, probably due to quicker development and emergence in higher trays. So the production of the population increased with the increasing distance from the bottom. In the tray at the bottom (which could protrude 1-2 cm above it) numbers were twice those outside it, and the youngest larvae occurred.
There was a slight current at the research site. Nevertheless, short time oxygen deficits occurred periodically, and were probably the main cause of considerable reductions in Chironomus numbers at the bottom during the experiment.
1985; see also reviews by KAJAK, 1988; RESH and ROSENBERG, 1984). This paper deals with the second type of field experiments. A completely new methodologyrelocating the bottom mud to various heights above the bottom was applied. This method permits many experimental variants, with only a few being described here as examples. The aim of this work was to check wheather Chironornirs numbers and the size (age) structure of the population in the mud elevated over the bottom would be different from that in the bottom, and wheather it would depend on the distance above the bottom.