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Characteristics of zebra mussel (Dreissena polymorpha) populations in infested reservoirs, northwest Bulgaria

The zebra mussel populations in the Reservoirs Ogosta and Rabisha (northwest Bulgaria, Danube River basin) were studied with respect to distribution, shell morphometry, abundance and biomass. In the reservoirs, the zebra mussels occurred at depths between 3.5 and 10.0 m and only on hard and mixed substratum. Based on principal component analysis, the majority of total variance of morphometric data was explained by the shell size and only a small proportion by the shell shape of the zebra mussels. In the Rabisha Reservoir, shell plasticity in respect to site location was observed. The absolute abundance and total biomass of zebra mussels differed among the sites within both reservoirs. The mean values in the Rabisha population were much higher than in the Ogosta population. The relations of shell morphometry, abundance and biomass of zebra mussels to the physicochemical conditions in the reservoirs were discussed.