The freshwater pearl mussel,Hyriopsis cumingii,is commercially important for freshwater pearl production in China and contributes more than 95% of the annual pearl production in the world.In traditional farming ponds,H.cumingii is co-cultured with some planktivorous fishes(silver carp Hypophthalmichthys molitrix and bighead carp Aristichthys nobilis),and huge amounts of organic manure are used to provide natural food(phytoplankton and detritus)for the mussel and fishes.Excessive fertilization with organic manure results in serious environmental problems.Previous studies found that adding some omnivorous or herbivorous fish species(grass carp Ctenopharyngodon idellus and gibel carp Carassius auratus auratus)in the traditional mussel-fish integrated system and feeding the fishes with formulated feed can enhance pearl yield and growth of mussel.Moreover,pearl yield of mussel is also affected by different fertilization methods(fertilized with fermented duck manure,or chemical fertilizer or both fermented duck manure and chemical fertilizer)when the novel mussel-fish integrated culture is used.In the present study,a 155-day experiment was conducted in land-based enclosures to evaluate the effect of different fertilization method on phytoplankton community in a novel mussel-fish integrated system.Three kinds of fertilizer,including DM(fermented duck manure),CF(chemical fertilizer)and DC(combination of fermented duck manure and chemical fertilizer),were experimented.Each treatment was three replicated,therefore,total 9 enclosures were used.Each enclosure was stocked with 20 mussel,15 grass carp,5 gibel carp,5 silver carp and 5 bighead carp.During the experiment,water samples were collected regularly from the enclosure,and species and biomass of phytoplankton and chlorophyll a(Chl.a)were determined.Results showed that the biomass of phytoplankton ranged from 2.1×10⁸ to 6.0×10⁸ cell/L in the enclosures.No significant differences were found in the species composition and dominant species of phytoplankton,Chl.a,biomass of phytoplankton and ratio of blue-green algae to phytoplankton in biomass among treatments DM,CF and DC.However,fertilization method significantly affected Shannon-Wiener's index and Pielou's index of phytoplankton.Phytoplankton community in the enclosures exhibited an apparent seasonal trend,and concentration of total nitrogen,ammonia and dissolved oxygen were responsible for the alteration of phytoplankton community.The results of the present study could not provide an explanation to the conclusion that pearl yield is higher in treatment DC than in treatments DM and CF.Further studies are needed to explore the linkage between pearl yield and phytoplankton community in integrated culture of H.cumingii and fishes.