Distribution of the non-indigenous Cercopagis pengoi in the coastal waters of the eastern Gulf of Finland

Abstract

Abstract We studied the distribution and population structure of the immigrant predaceous cladoceran species Cercopagis pengoi and its role in the zooplankton community during a 1-week cruise at the beginning of September 1997. Samples were collected at 20 stations within a coastal area of 500 km2 in the Gulf of Finland. Temperature above the thermocline was about 16–18°C. Phytoplankton was dominated by filamentous blue-green algae, metazooplankton by rotifers, cladocerans, cyclopoids, and calanoids. Ciliates (maximum density 2 × 105 L −1) were dominated by oligotrich ciliates and Mesodinium rubrum. The maximum density of different metazoan taxa varied from a few (Cercopagis) to a thousand (rotifers) per litre. Most of the Cercopagis population consisted of maturing parthenogenic females with a low percentage of males and gametogenic females. On the basis of population structure, we suggest that the population was still in the growth phase, parthenogenesis being the dominant reproduction strategy. Variation in zooplankton community structure and density was associated with differences in salinity, the stations nearest to the coast favouring cladocerans and cyclopoids. Rotifers and ciliates were most abundant at the eastern stations. Since density of Cercopagis was also highest at the eastern stations, this predator may have been an important factor in the structure of the zooplankton community. The potential effects of Cercopagis on the pelagic community structure and dynamics in the Gulf of Finland are discussed.