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Dreissena polymorpha inhabits about 46.5% of the bottom area in Lake Dąbie. It is most abundant in the northern part of the lake and sporadically occurs in its southern part. The average density of the zebra mussel is 891 individuals/m2 , while its wet weight is 1374 g/ m2 . In the settled areas the density of the zebra mussel reaches 1734 individuals/ m2 . There were presently established 7 age-groups from 0 to 6+. The highest number of specimens occurred in the age-groups between 1+ and 3+, while the lowest number - in the groups 0, 5 and 6+. Strong eutrophication and pollution of Lake Dąbie on its southern side, combined with sedimentation of the extensive amounts of seston carried in by the Płonia and Regalica rivers, do not create favourable conditions for development of Dreissena. D. polymorpha plays a very positive role in the lake ecosystem through its biofiltrating action, contributing thus to an increase of water clarity and to limitation of phytoplankton development through cumulating nutrients in its biomass. Electrophoretic analysis of 9 enzymatic loci revealed strong polymorphism of the studied population of the zebra mussel: percent of polymorphic loci - 100, mean number of alleles in locus – 26 3.4, coefficient of expected heterozygosity per locus in the population, HS - 0.335, percentage of separate genotypes - 69%, in this number unique genotypes - 58%. A very strong scattering of the alleles was stated within the entire population. In each aggregation, on average, 90% of the specimens have a separate genotype. Each of the five defined groups of D. polymorpha, representing profiles I-V, respectively, had similar genetic composition. The values of the genetic similarity among the studied groups of the zebra mussel ranged from 0.96 to 0.99.
Key words: Dreissena polymorpha, distribution, age composition, ecological role, enzymatic polymorphism