Hydrobiologia 200/201 : 241-250, 1990. R. D. Gulati, E. H. R. R. Lammens, M.-L. Meijer & E. van Donk (eds), Biomanipulation - Tool for Water Management. © 1990 Kluwer Academic Publishers. Printed in Belgium. 241 Ecological consequences of a manual reduction of roach and bre eutrophic, temperate lake Bo Riemann, t Kirsten Christoffersen, ~ Helle Jerl Jensen, Jens Peter Mailer, Claus Lindegaard & Suzanne Bosselmann Freshwater Biological Laboratory, University of Copenhagen, Helsingorsgade 51, DK-3400 Hillerod, Denmark; tPresentaddress: The International Agency for14C determination, The Water Quality Institute, Agern AII~ 15, DK-2970 Horshohn, Denmark Key words: biomanipulation, fish reduction, zooplankton, phytoplankton, zoobenthos Abstract A biomanipulation experiment was carried out in the eutrophic lake, Frederiksborg Slotsso (Denm During 1987 and 1988, densities of roach (Rutilus rutilus) and bream (Abramis brama) were reduced, using seine and pounding nets, and large-sized perch (Percafluviatilis) were added instead. Nutrients, oxygen, phytoplankton, zooplankton, fish and zoobenthos were measured two years after the manipulation compared with results obtained two years before the manipulation. A total amount of 6524 kg wet weight of roach and bream was removed. Roach and bream consti 45 ~o of the total fish biomass after the reduction, compared with 78 ~o before the manipulation. R ment of roach decreased, and mortality rates of young-of-the-year perch were lower after the fish reduction. After the manipulation, decreases in phytoplankton biomass coincided with increa zooplankton biomass during spring and autumn periods, although, the mid-summer level of the bio of cyanobacteria did not change. Inorganic nutrients generally increased, but no significant change found, either in the oxygen budget or in the community structure or quantitative distribution ofzoo after the fish manipulation. Although the effects of the fish manipulations were not as pronounced a found in lakes with lower nutrient regimes, the results indicate positive changes in the wate Nevertheless, it is probably necessary to continue a fish reduction programme to maintain o improve the water quality. Introduction A theoretical model of the structure and func- tioning of eutrophic lakes predicts that plankti- vorous fish control the growth of macrozooplank- ton (Riemann etal., 1986; Persson et al., 1988). The model, and direct observations on the impact offish communities in eutrophic lakes (Johansson & Persson, 1986) suggest a possible scenario for changing the structure of the food chain by mani- pulations of the fish communities. In eutrophictemperate lakes, pelagic food chains are characterized by high levels of phyto- plankton, relatively low densities of macrozoo- plankton, and large numbers of cyprinid fish populations (Riemann & Sondergaard, 1986; Persson etaL, 1988).These often undesirably high levels of phytoplanktonoccur primarily because of an increase in the biomass and changes in the community structure of phyto- plankton owing to an increase of nutrients (Kristiansen, 1986; Sondergaard et al., 1988).