Chironomus plumosus — what regulates its abundance in a shallow reservoir?

Abstract

A high increase of the abundance of the main dominant benthic species, Chironomus plumosus,was reached by elevating the local mud with benthos in trays 40 and 70 cm above the bottom in 6 m deep site of a shallow polymictic, highly eutrophic reservoir. The differences of numbers between the trays and the bottom varied during the season (May–August). The maximum ratio of Chironomus numbers trays: bottom (almost 8), was found in early summer (June/July), when abundance in the bottom was the lowest. Young larvae were especially more numerous in trays. The main causes of much higher Chironomusnumbers in trays than in the bottom in early summer could be differences in oxygen deficit. In August-September numbers of larvae in the bottom were moderate (several times lower than maximal in spring, but several times higher than minimal in early summer). In trays in summer numbers were only 1.5 times higher than in the bottom. The reasons for these relatively low numbers in summer, despite prolonged occurrence of imagos and young larvae, could be insufficient numbers of eggs due to heavy predation by swallows on egg laying females, or the negative influence of big larvae on the survival of youngest ones.