Carbon mass of the non-indigenous predatory fishhook water flea Cercopagis pengoi (Ostroumov, 1891) from the eastern Gulf of Finland, the Baltic Sea, was for the first time measured using the high temperature combustion method. Prior to the analysis, individual dry weight of Cercopagis was determined; altogether ca. 500 organisms were examined. Mean individual dry weight of C. pengoi for July-September was estimated as 34.0 µg; carbon mass averaged 15.8 µg; carbon content, calculated as percent of dry weight, averaged 43.4%. Those values varied over months, mainly because of different population structure of C. pengoi and variation in their diet due to seasonal dynamics of the food objects. However, relations between carbon mass and dry weight for different months did not differ statistically (p<0.001). Therefore, the general polynomial regressions (k=2), describing carbon mass-to-dry weight and carbon content-to-dry weight relationships, were calculated for the entire dataset of individual measurements of C. pengoi body metrics. These data will contribute to adequate evaluation of food web structure and ecosystem alterations in various water bodies invaded by C. pengoi which has got a strong potential to pelagic food web transformations that may impact the overall energy balance and decrease the size of fish stocks.
