Seasonal and year-to-year variation of numbers of Chironomus plumosus L. and Tubificidae in a lowland reservoir : regularities, causes, mechanisms

During 9 years of studies very regular Chironomus population dynamics was stated in a eutrophic, lowland dam reservoir. There were usually two peaks of the abundance: the higher one at spring (up to 80 thousands ind. m^-2) and the much lower in autumn. The duration of the spring Chironomus generation was about 3 weeks. The constant presence of young larvae during the summer did not result in the high total abundance of larvae, mainly due to the strong predation of fish and swallows on various stages of Chironomus. The smaller predators pressure in the spring (due to fish breeding) and in the autumn (due to lower temperatures) resulted in the mentioned two peaks. The spring peak abundance was positively correlated with the chlorophyll concentration in water (feeding resource for larvae) and negatively with the water flow. There was also negative correlation of the water flow and the chlorophyll concentration, as well as abundance of Chironomus and Tubificidae during the vegetation season (April-October). Tubificidae correlated strongly positively with the spring Chironomus numbers (with a month lag). The slight positive correlation of these benthic components abundance occurred for the whole vegetation season. Tubificidae occurred in generally high numbers up to 400 thousands m^-2, but various in different years, and with no regular changes during the season.