Effect of filter-feeding fish silver carp on phytoplankton species and size distribution in surface water: a field study in water works.

Silver carp were introduced into the pre-sedimentation pond to control excessive phytoplankton in raw water. The effectiveness of the filter-feeding silver carp on phytoplankton control and the effect of silver carp on phytoplankton community were investigated. The results showed that Microcystis could be effectively removed by silver carp stocked in the pre-sedimentation pond, and simultaneously, the concentration of single-cell phytoplankton increased obviously. The difference in phytoplankton species and single-cell phytoplankton size between in the water and in the gut of silver carp indicated that phytoplankton smaller than 5 microm, such as Chamydomonas and Platymonas, were almost not be filtered by silver carp, phytoplankton with the size between 5 and 20 Csm could be partly filtered, and large size phytoplankton, mainly colony-forming Microcystis could be filtered almost completely. These filter-feeding characteristics directly caused the phytoplankton size distribution biased toward miniaturization. Therefore, this biological treatment using silver carp could be applied only to deal with groups of Microcystis-dominated eutrophic water, and was not appropriate in water bodies where single-cell micro phytoplankton were dominant. Especially when silver carp are used in water treatment, a cautious attitude should be taken based on the evaluation of phytoplankton biomass and species structure features in raw water.