Relative Distributions of Dreissena bugensis and Dreissena polymorpha in the Lower Don River System, Russia key words: zebra mussels, quagga mussels, dreissenid abundances, invaders, mineral content AbstractA survey was conducted in the lower Don River system in Russia to confirm the presence of Dreissena bugensis, and to compare its distribution relative to that of Dreissena polymorpha. In 1999 and 2001-2002, dreissenid mussels were collected at 15 sites in the main river, in connecting reservoirs, and in a major tributary, the Manych River. Collections were made near stations where long-term monitoring data on total mineral (sum of principal ions) and calcium content were available. Both dreissenid species were found at all sites, with D. bugensis comprising 4-75% of all dreissenids at individual sites. D. bugensis was relatively more abundant than D. polymorpha in the Manych River where total mineral and calcium content was significantly higher than in the Don River, suggesting the two species may have different calcium requirements. Examination of archived samples indicated that D. bugensis was present in the Don River system as early as the 1980s, presenting the unresolved enigma of why D. bugensis has not displaced D. polymorpha as the dominant species as typically found over shorter time periods in other water bodies. IntroductionThe expansion of species outside of their native ranges has become a world-wide problem (CARLTON and GELLER, 1993;LODGE et al., 1998). Once introduced into a new environment, these invasive species often rapidly increase in number and may cause complex changes in food web structure and ecosystem processes. Given the importance of the problem, information on the biology and distribution patterns of these species within their native ranges may provide a better understanding of how populations respond in newly invaded systems. Two dreissenid species, Dreissena bugensis (ANDRUSOV, 1897) (quagga mussel) and Dreissena polymorpha (PALLAS, 1771) (zebra mussel), provide examples of invaders that now occupy vast areas far removed from their original places of origin. Dreissena bugensis originated from a quite restricted region in the Bug River where it was first described (ANDRUSOV, 1890). It was later found in the nearby Bug-Liman Delta, and in reservoirs in the Ukraine (ZHURAVEL, 1951;KHARCHENKO, 1995). Within Eastern Europe, the first finding of the quagga mussel far removed from its natural range occurred in 1992 in the mid-Volga Internat. Rev. Hydrobiol.

We examined trends in expansion patterns and relative abundances of Dreissena bugensis in reservoirs and major river systems in eastern Europe. Based on our own data and data from the literature, it is apparent that trends were variable across river basins and not easily related to environmental conditions. In some cases these did not conform to the patterns typically found for dreissenids. In the early period of expansion beyond its native range in the Dnieper-Bug delta and estuary, D. bugensis rapidly replaced Dreissena polymorpha in the upper Dnieper River system, but increased only gradually and over time became less abundant relative to D. polymorpha in the DonManych River system. Contrary to the Dnieper and Don River systems, in the Volga River system considerable spatial variability in relative abundances was apparent, particularly in northern reservoirs. Moreover, even though D. bugensis usually displaces D. polymorpha as the dominant dreissenid, the latter can remain dominant in certain types of habitats where conditions are less favourable for the former. Suggested factors that may be responsible for differences in invasion patterns in the river systems may include differential responses to temperature, or to some other factor(s) associated with geographical latitude, the level of water mineralization, and selective predation by molluscivorous fish. In particular, the northward expansion of D. bugensis seems to be limited by temperature. The lack of long-term data on appropriate scales precludes linking these differences to specific features within the environment, but our comparisons indicate that the expansion of D. bugensis relative to D. polymorpha is more complex than previously believed.

No abstract