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

We studied the distribution and population structure of the immigrant preda ceous cladoceran species Cercopagis pengoi and its role in the zooplankton community during a 1-week cruise at the beginning of September 1997. Sample s were collected at 20 stations within a coastal area of 500 km(2) in the G ulf of Finland. Temperature above the thermocline was about 16-18 degrees C . Phytoplankton was dominated by filamentous blue-green algae, metazooplank ton by rotifers, cladocerans, cyclopoids, and calanoids. Ciliates (maximum density 2 x 10(5) 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 popu lation consisted of maturing parthenogenic females with a low percentage of males and gametogenic females. On the basis of population structure, we su ggest that the population was still in the growth phase, parthenogenesis be ing the dominant reproduction strategy. Variation in zooplankton community structure and density was associated with differences in salinity, the stat ions nearest to the coast favouring cladocerans and cyclopoids. Rotifers an d ciliates were most abundant at the eastern stations. Since density of Cer copagis was also highest at the eastern stations, this predator may have be en an important factor in the structure of the zooplankton community. The p otential effects of Cercopagis on the pelagic community structure and dynam ics in the Gulf of Finland are discussed. (C) 1999 International Council fo r the Exploration of the Sea.