Effect Of Filter-feeding Silver Carp (Hypophthlinmicthys Molitrix) On The Phytoplankton Community Structure In The Polyculture Ponds Of Freshewater Fish

Experiments about polyculture of grass carp, silver carp and common carp are conducted in ponds of Freshwater Fishery Research Institute of Shandong Province from 2009 to 2010 using the enclosure approach. The dynamics of phytoplankton community structure, phytoplankton size structure and seston structure are studied by microscopy; HPLC pigment data are compared with microscopy to analyze the phytoplankton community structure and the use of HPLC pigment analysis in freshwater ponds is also discussed; the relations between phytoplankton fatty acid compositon and phytoplankton community structure, nutrition status are analyzed by using diagnostic fatty acid, respectively. Attentions are focused on the filtration effect of silver carp.Polyculture of silver carp can suppress the Cyanobacteria bloom and make phytoplankton size smaller, contributions of <5μm, 5²0μm, >20μm are about 69%, 24%, 7% to total phytoplankton biomass, respectively; Zooplankton are dominated by protozoa and rotifer and zooplankton biomass is reduced by silver carp. In group GC without silver carp, Cyanobacteria bloom occurrs, contributions of three size strcutures to total phytoplankton biomass are about 30% and zooplankton are dominated by copepods and Cladocera. Suspended particulate matter(SPM)concentration ranges from 18.89 mg/L to 116.27 mg/L and is 52.49 mg/L on average. The contribution of particulate organic matter(POM)to SPM averages 50.08%. Positive correlation(R²=0.188,P<0.001)is found between POM and SPM indicating the quality of SPM. Optimum stocking denstity is 0.58 ind·m^-2, 0.69 ind·m^-2 and 0.19 ind·m^-2 for grass carp, silver carp and common carp respectively.HPLC pigment analysis provides accurate separations of main pigment markers. class-specific phytoplankton biomass and corresponding pigment concentrations are correlated significantly such as alloxanthin with Cryptophytes biomass, lutein with Chlorophytes biomass. Significant difference is present between groups exclusively in August and September while insignificance in October. Silver carp can suppress the appearance of Cyanobacteria species in August and it takes a decreasing effect on the Cyanobacteria growth from August to October. Grass carp and common carp have failed to control phytoplankton biomass and algal bloom but common carp may play a complementary role in suppressing the growth of Cyanobactetia to a certain extent. The high content of chlorophyllide a indicates the importance of chlorophyll a degradation. Fucoxanthin-containing algal and alloxanthin-containing algal contribute most to similarity and dominate in silver carp group while contribute most to similarity zeaxanthin-containg algal dominate in group of grass carp and common carp group. The role of HPLC pigment analysis for assessing phytoplankton community structure should be considered as a strong complement to microscopic enumeration.Phytoplankton fatty acid compositon reflects the phytoplankton dynamics and the filtrate effect of silver carp. The diversity of fatty acid compositon between groups and within groups indicates the complication of phytoplankton compositon. BAFA, SFA, MUFA and PUFA represent for 3.40-10.23%, 30.06-51.32%, 10.25-34.68% and 14.20-56.29% of total fatty acid, respectively. C13:0, C16:0, C18:1n9, C16:1n7, C18:2n6, C18:3n6, C18:3n3 and C20:5n3 contribute a considerable percentage to the corresponding fatty acid taxa, respectively, which indicates the nutrient status in the ponds. Group of grass carp monoculture and group of polyculture of grass carp and silver carp cluster in the MDS ordination,respectively. C16:0 and C18:1n9 contribute most to similarities within groups in July and August while C18:3n3,C18:3n6 and C20:5n3 contribute a high percentage besides C16:0 in September and October. C20:5n3, C18:3n3 and C18:3n6 contribute most to dissimilarities between groups. Chlorophytes and Euglenophytes which contain abundant C18:3n3 and C18:3n6 contribute more relatively in group G compared with group GS from July to September, while the case is opposite in October. Nutrient status of ponds is changed by the filtration effect of silver carp.