Spatial, seasonal, and annual bacterioplankton dynamics in recently infested by the species Dreissena polymorpha Ogosta Reservoir were studied for the first time during three year period. Bacterioplankton total number was higher in spring in ecotone zones, than in summer at thermocline. NH4-N, PO4-P, turbidity, dissolved oxygen, COD and chlorophyll-a correlate positively, while transparency and Ca2+ negatively with bacteria. Nematode species composition, included 22 species studied (13 rarely found and Rhabditis brevispina new for Bulgaria) belonging to nine families. The D. polymorpha impact is positive on nematodes and phytoplankton, negative on zooplankton and bacterioplankton, but weak positive on larger bacteria, rods and attached bacteria.

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During a long-term study (2003-2007) of the chironomids in Vaya Lake 27 taxons were established. There were determined 18 species belonged to 11 genera. These results were compared to the only ones so far provided by Zashev and Angelov 1958. There is a complete change in the species composition of the Chironomidae compared with the results obtained in the period 1953-1957, which is mainly related to the increased anthropogenic impact on the lake and the changed connection with the Black Sea.