The relative importance of top-down and bottom-up factors in the regulation of trophic level biomass has been discussed in the first chapter. Based on the Hairston-Smith-Slobodkin hypothesis of different modes of biomass control, depending on the number of trophic levels and primary productivity of the system, several theories has been argued. Models and results obtained from the whole-lake studies and enclosure experiments as well illustrated contradictory data and very often unpredictability in determination of dominant type of biomass control at the base of food web and at the intermediate trophic levels.
The purpose of the dissertation was to conduct the whole-lake studies
on the regulation of zooplankton and phytoplankton biomass and point out the time-scale issue. Plankton community structure and biomass were investigated for three years (1996-1998) in two small neighboring oligotrophic lakes located near the Arctic circle (European Russia, Northern Karelia). One of the lake was fishless while in the other the planktivorous fish has been registered for the several years. The comparative analysis of zooplankton biomass and species abundance showed the strong negative effect of fish on the zooplankton species and size structure, but no effect on the total biomass of the trophic level in those lakes in 1996. The invasion of the stickleback to the lake which served as the reference "No Fish" lake in 1996 in June 1997 lead to devastating changes in zooplankton biomass and complete elimination of five previously dominant grazer species. This natural experiment demonstrated the effect of trophic cascade and its consequences in the small lakes, showing the prevailing "top-down" control of phytoplankton in oligotrophic ecosystems.
The last chapter was devoted to the observed shift in the linear growth rate of adult Holopedium gibberum which was considered the evolutionary response to strong size-selective predation.
The long-term consequences of strong top-down regulation existing in the lakes of this type were supported by the fact that even on the second year after the fish appearance there were still no convergence observed between zooplankton communities of those neighboring lakes.