The Effect of Dreissena polymorpha on Bacterioplankton, Nematode Fauna and their Relations to Environmental Factors in Ogosta Reservoir (Danube Basin)

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.