An increase of xenodiversity in plankton and benthos in the eastern Gulf of Finland was observed from 1998 to 2004. Nonindigenous species account for 4.8% of all species found and up to 96% of total biomass. Invasive benthic omnivores, the alien amphipods Gmelinoides fasciatus and Pontogammarus robustoides and the predaceous fish Perccottus glenii with their versatile diets strongly affect the community structure. Invasive sessile seston-feeders that directly (through grazing and water clearance) and indirectly (through recycling of nutrients) interact with other ecosystem components, are mainly represented by the zebra mussel Dreissena polymorpha, which affect the structure of benthic and planktonic communities as well as benthicpelagic coupling. The invasive predatory cladocerans Cercopagis pengoi and Evadne anonyx and larvae of D. polymorpha are only temporary components in the zooplankton, which is limiting their overall effect. Alien benthic bioturbators, the polychaetes Marenzelleria neglecta and the oligochaete Tubificoides pseudogaster account for a high proportion of total abundance and biomass but their effects on native species need further research.

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The non‐indigenous predaceous cladoceran Cercopagis pengoi (Ostroumov) was found for the first time in the Gulf of Finland in 1995. After this invasion, the diet of the Baltic herring Clupea harengus mebras and its seasonal dynamics were investigated in the coastal waters of SE Finland. The specimens of C. pengoi were not present in plankton samplings until the end of July when the surface water temperature reached 13.5 °C. From the beginning of August to the end of October Cercopagis was found in all the plankton samples. Before the occurrence of Cercopagis, the diet of herring consisted mostly of Eurytemora affinis and Bosmina coregoni maritima. During August to mid‐October, C. pengoi was the main food source. In November, after the disappearance of C. pengoi from the plankton, B. coregoni maritima became the main prey object. Both the smallest and largest size classes of the herrings investigated consumed C. pengoi. Females with overwintering eggs are the most attractive prey for herrings. The overwintering eggs of C. pengoi, due to their very hard capsules, seem to pass undamaged through the stomach and intestines of herring. C. pengoi started to play a highly important role in the zooplankton community in the waters of SE Finland, and its abundance is not too dependent upon the annual temperature fluctuations. The Baltic herring population has substantially changed its diet in this area, and now it prefers the new‐comer C. pengoi.

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