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Relative abundance of two dreissenid species, Dreissena rostriformis bugensis and Dreissena polymorpha, in the total dreissenid community was calculated for 15 sites in the lower Don River system, Russia, between 1977 and 2010 to determine relative trends in their sympatric occurrence. The proportion of D. r. bugensis first increased at most stations reaching a maximum by 1999. However, after 1999, this species consistently decreased at 14 of the 15 sites. Degree of decline seems to correlate strongly to calcium content and total mineral content of the water. At sites in the Manych River with a higher Ca 2+ and total mineral content, the proportion of D. r. bugensis in the total dreissenid population declined from 65–75% in 1999 to 14–22% by 2009–2010, whereas at sites in the Don River with lower Ca 2+ and total mineral content its proportion declined from 25–50% to only 1 %. However, Ca 2+ and total mineral content cannot explain the synchronous and consistent long-term decrease in relative numbers as observed. D. r. bugensis normally displaces D. polymorpha over time due to superior physiological characteristics. Reasons for the decline of D. r. bugensis in the total dreissenid community are unclear, but given its synchronicity despite different times of invasion and hence species interactions we assume a macroregional factor affecting all the populations examined.

Relative abundance of two dreissenid species, Dreissena polymorpha and Dreissena rostriformis bugensis in the Lower Don River system, Russia