Qualitative and quantitative analyses of plankton were carried out during biomanipulation studies conducted in 1993-1996 in the Malta[n]ski Reservoir (64 ha, mean depth 3.1 m), Poland. The taxonomic composition and size fractions of phytoplankton, as well as number and biomass of phyto-, zoo- and bacterioplankton, were investigated. Despite the removal of all fish before the experiment and extensive stocking with predatory fish, attempts to biomanipulate a top-down control of phytoplankton in the Malta[n]ski Reservoir were not successful except for the first season. Macrozooplankton filtration (including cladocerans and calanoids) was found to control the development of small organisms only, such as pico-, nano- and bacterioplankton. Microplankton were also under the influence of filter feeders, but these were not effectively eliminated. Zooplankton grazing stimulated the development of large phytoplanktonic organisms both by providing intermediate disturbances for their growth and by supplying them with nutrients released from grazed small-size organisms. A similar influence on micro- and nanophytoplankton was exerted by microzooplankton (including rotifers and nauplii). The presence of picophytoplankton and bacterioplankton, however, was positively correlated with microzooplankton number.
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