Abstract: The plankton community and primary productivity in waters under three stocking types and two management models were evaluated in an enclosure experiment. The stocking types included monoculture of freshwater pearl mussel (Hyriopsis cumingii), polyculture of H. cumingii, silver carp Hypophthalmichthys molitrix) and bighead carp(Aristichthys nobilis), and polyculture of H. cumingii and gibel carp (Carassius auratus gibelio), while the management models included fertilizing and fertilizing plus feeding. During the period of the experiment, the phytoplankton was mainly consisted of the species with grain size <20 um, and Cyanophyta, Chlorophyta and uglenophyta were dominant, while the number and mass of Bacillariophyta, Cryptophyta, Xanthophyta and Pyrrophyta were lower. The number and the mass of hytoplankton varied within the ranges of 0.35×10⁹-1.26×10⁹ ind·L^-1 and 32-86 mg·L^-1, respectively. In the enclosure under polyculture of H. cumingii, H. molitrixand A. nobilis, biomass of phytoplankton were relatively higher, and the proportions of Cyanophyta and Chlorophyta in total phytoplankton was higher than those in the other enclosures. Protozoa was highest in number, while rotifer highest in weight of zooplankton. Number of cladocera and opepod was low in the enclosures. In the enclosure under monoculture, ooplankton biomass was the highest, and the proportion of rotifer in total ooplankton’s biomass was lower while that of copepod was higher, compared with those in polycultured enclosures. In the enclosure under polyculture of H. cumingii, H. molitrix and A. nobilis, the proportion of protozoa in number of zooplankton was higher while that of copepod was lower than those in the enclosures under monoculture or polyculture of H. cumingii and C. auratus gibelio. In the experiment, the primary productivity varied within 4.11-5.45 g O₂·m^-2·d^-1, water respiration within 2.39-4.12 g O₂·m^-2·d^-1, and the ratio of primary productivity to water respiration (P/R) within 1.25-1.99. The primary productivity was highest in the monocultured enclosure, but lowest in the enclosure under polyculture of H. cumingiiand C. auratus gibelio. This study indicated that the phytoplankton and zooplankton in the waters for culturing freshwater pearl mussel tended to become smaller-sized, and high number and mass of phytoplankton (especially Bacillariophyta) might be the factors resulting in an enhanced growth of the pearl old mussel in the enclosure under polyculture of H. cumingii, H. molitrixand A. nobilis.

Key words: Alpine meadow, Zoker-mound, Successional stage, Diversity, Functional group