The role of cladocerans reflecting the trophic status of two large and shallow Estonian lakes

The role of pelagic cladoceran communities is discussed on the basis of a comparative study conducted in two Estonian lakes, the moderately eutrophic Lake Peipsi (N_tot 700, P_tot 40 μg l^?1 as average of ice-free period of 1997–2003) and in a strongly eutrophic Lake Võrtsjärv (N_tot 1600, P_tot 54 μg l^?1). The cladoceran community was found to reflect the differences in the trophic state of these lakes. In L. Peipsi, characteristic species of oligo-mesotrophic and eutrophic waters co-dominated (making up 20% or more of total zooplankton abundance or biomass), whereas in L. Võrtsjärv only species of eutrophic waters occurred. In L. Peipsi, the dominant cladocerans were Bosmina berolinensis and Daphnia galeata, while Chydorus sphaericus was the most abundant cladoceran in L. Võrtsjärv. The cladocerans of L. Peipsi (mean individual wet weight 25 μg) were significantly (threefold) larger than those of L. Võrtsjärv (8 μg). The mean wet biomass of cladocerans was higher and total cladoceran abundance was lower in L. Peipsi compared to L. Võrtsjärv (biomass varied from 0.133 to 1.570 g m^?3; mean value 0.800 g m^?3 in L. Peipsi and from 0.201 to 0.706 g m^?3, mean 0.400 g m^?3 in L. Võrtsjärv; the corresponding data for abundances were: 8,000–43,000 ind. m^?3, mean 30,000 ind. m^?3 for L. Peipsi, 50,000–100,000, mean 52,000 ind. m^?3 for L. Võrtsjärv). Based upon differences in body size, cladocerans were more effective transporters of energy in L. Peipsi than in L. Võrtsjärv. Cladocerans proved to be informative indicators of the trophic status and of the efficiency of the food web in studied lakes.