Shallow Lakes ’95

Abstract

A high increase of the abundance of the main dominant benthicspecies, Chironomus plumosus, was reached by elevatingthelocal mud with benthos in trays 40 and 70 cm above the bottomin6 m deep site of a shallow polymictic, highly eutrophicreservoir.The differences of numbers between the trays and the bottomvariedduring the season (May–August). The maximum ratio ofChironomusnumbers trays: bottom (almost 8), was found in early summer(June/July), when abundance in the bottom was the lowest.Younglarvae were especially more numerous in trays. The main causesofmuch higher Chironomus numbers in trays than in thebottom inearly summer could be differences in oxygen deficit. InAugust–September numbers of larvae in the bottom weremoderate(several times lower than maximal in spring, but several timeshigher than minimal in early summer). In trays in summernumberswere only 1.5 times higher than in the bottom. The reasons forthese relatively low numbers in summer, despite prolongedoccurrence of imagos and young larvae, could be insufficientnumbers of eggs due to heavy predation by swallows on egglayingfemales, or the negative influence of big larvae on thesurvival ofyoungest ones