MEPS 362:193-200 (2008)  -  DOI: https://doi.org/10.3354/meps07441

ABSTRACT: We studied the potential of an introduced species, the predatory cladoceran Cercopagis pengoi, to influence native zooplankton populations in the Baltic Sea. Feeding rates (FRs) of different instars of C. pengoi on Eurytemora affinis, a dominant copepod species, were determined experimentally. The FRs of C. pengoi varied from 0.7 to 4.8 prey predator^–1 d^–1 and were significantly higher in older instars. Based on the experimental results and long-term zooplankton abundance data from the Gulf of Finland, we estimated the in situ predation rates. Our results imply that at maximum abundance, the C. pengoi population feeding in dense prey patches could consume as many as 10⁵ E. affinis m^–3 d^–1. This could explain the observed drastic decrease in copepod abundances in the eastern Gulf of Finland, the region with the highest C. pengoi abundance after the expansion of this species. Such a decline may strengthen food competition between other zooplanktivores, i.e. planktivorous fish and mysids, feeding on the same copepod prey in regions where C. pengoi may reach high abundances. In the Gulf of Finland, especially in the eastern parts, a collapse of E. affinis, a key species in the pelagic system, would cause major changes in virtually all trophic levels, because the food web structure is simple with only a few species of zooplankton, planktivores, and piscivores.

KEY WORDS: Introduced species · Predation · Food web dynamics · Cercopagis pengoi · Eurytemora affinis