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1 Polish Journal of Ecology

1 2004

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Time of Chironomus plumosus (L.) generations in natural conditions of lowland reservoir

Kajak Z., Prus P.

Polish Journal of Ecology

2004

Vol. 52, nr 2

211-222

The time of Chironomus plumosus generation in the field conditions (shallow, eutrophic dam reservoir) was estimated tobe about 3 weeks in the spring. This estimate has been possible due to simultaneous mass appearance of young larvae (the new generation) and the lack of older larvae at this time. Later in the season usually there was some amount of the youngest larvae, indicating the permanent emergence of imagos and the egg-laying, but without clear peaks of numbers and boundaries between successive generations. This regularity and the relatively low total numbers of larvae during the summer indicate the heavy fish pressure on the benthos, not allowing for the mass appearance of young larvae and the estimate of the generation time. Fish pressure is probably weak in spring, during a spawning period, but then increase in the summer. The generation number could be theoretically as high as 5 during the vegetation season (May-October), assuming about 3 weeks for full larval development, as it was estimated at optimal feeding and oxygen conditions and low fish pressure in the spring. However some limiting factors like: oxygen defficits, the annoyance by fish and bestrewing of larval tubes with the mud transported by the water flow (range 150-500 m^3 s^-1 of the total inflow) increase in the summer. These factors can slow down larval development, resulting in observed lower generation number: 3 to 4 during a year.