Changes in the plankton community following introduction of filter-feedingplanktivorous fish

1. We conducted enclosure experiments in a shallow eutrophic lake, in which a biomass gradient of the filter-feeding planktivore, silver carp, Hypopht halmichthys molitrix Valenciennes, was created, and subsequent community ch anges in both zooplankton and phytoplankton were examined. 2. During a summer experiment, a bloom of Anabaena flos-aquae developed (ap proximate to 8000 cells mL(-1)) solely in an enclosure without silver carp. Concurrent with, or slightly preceding the Anabaena bloom, the number of r otifer species and their abundance increased from seven to twelve species ( 1700-14 400 organisms L-1) after the bloom in this fish-free enclosure. Pro tozoans and bacteria were generally insensitive to the gradient of silver c arp biomass. 3. During an autumn experiment, on the other hand, large herbivorous crusta ceans were more efficient than silver carp in suppressing the algae, partly because the lower water temperature (approximate to 24 degrees C) inhibite d active feeding of this warm-water fish and also formation of algal coloni es. Heterotrophic nanoflagellate and bacterial densities were also influenc ed negatively by the crustaceans. 4. Correspondence analysis (CA) was applied to the weekly community data of zooplankton and phytoplankton. A major effect detected in the zooplankton community was the presence/absence of silver carp rather than the biomass o f silver carp, whereas that in the phytoplankton community was the fish bio mass before the Anabaena bloom, but shifted to the presence/absence of the fish after the bloom.