Effects Of Nutrient Enrichment And A Filter-feeding Fish [Silver Carp, Hypophthalmichthys Molitrix (Val.)] On Plankton Community In A Subtropical Oligo-mesotrophic Reservoir, South China: A Large-scale Enclosure Experiment

In temperate lakes and reservoirs,the members of genus Daphnia are common and often dominate in zooplankton community.Beacause they can graze phytoplankton heavyly,they are considered as keystone species for significant trophic cascades interaction and successful biomanipulation.In tropics and southern subtropics,the genus Daphnia is often absent or owns very low abundance in the freshwater.Filter-feeding fishes are common and suggested to have significant effects in structuring zooplankton and ecosystems.However,few in situ experiments have been done to further testify this idea.In order to understand the ecological mechanisms of Daphnia distribution and evaluate the strength oftrophic cascade effects in tropical and southern subtropical freshwater water,and also to recognize the effects of the filter-feeding fish on plankton community under different nutrient levels,we first reanalyzed zooplankton community in a large oligotrophic reservoir: Xingfenjiang Reservoir,and then carried out a large-scale enclosure experiment with volume of 85 m~3 in Liuxihe Reservoir,an oligo-mesotrophic water body located in the same catchment, from April to June,2008.The experiment consisted of two parts:1.adding nutrients to enclosure with three gradients,namely,low nutrient level(P concentration:30μg/L),medium nutrient level (P concentration:60μg/L) and high nutrient level(P concentration:90μg/L),with a N/P(mass ratio) of 10/1 in all treatments.2.adding filter-feeding fish with a biomass of 4g/m~3 to each enclosure in addition to the above-mentioned three nutrient levels.There are three replicates for each treatment and 21 enclosures in all.The response of plankton to nutrient enrichment was analyzed through the nutrient enriched treatments and controls.Top-down effects of filter-feeding fish on plankton were analyzed between the fish addition and only nutrient enriched treatments. In the Xingfengjiang Reservoir,zooplankton was mainly composed of eurythermal species. The zooplankton were dominated by copepods,such as Phyllodiaptomus tunguidus, Mesocyclops thermocyclopoides and Tropocyclops bopingi.Daphnia was observed,but cladocerans were mainly composed of Bosmina longirostris,Diaphanosoma orghidani and Ceriodaphnia cornuta.Bosmina longirostris had the highest abundance in the cladocerans,and its dominance was probably related to the species' adaption to oligotrophic condoition and being free of invertebrate predation.Rotifer's dominant species were Keratella cochlearis and Chromogaster sp..The body lengths of the zooplankton in the reservoir range from 50μm to 1400μm.The largest dominant species was Phyllodiaptomus tunguidus with an average length of 1190μm.In December,perhaps due to the Neosalanx taihuensis Chen's heavy predation,the abundance and biomass of Phyllodiaptomus tunguidus were less than in the other sampling periods.The dominance of Phyllodiaptomus tunguidus in the reservoir were probably related to it's low threshold food concentration and efficient filtering mode in comparison with Daphnia galeata.In viewpoint of the specific composition,the zooplankton has a typical identification of oligotrophic water bodies in the Xinfengjiang Reservorir.In the large enclosure experiment,the abundance and biomass of phytoplankton increased significantly in all nutrient enriched treatments in the presence or absence of fish.The difference was not remarkable between treatments with fish and treaments without fish.In the nutrient enriched treatments without fish,diatoms and peridinium dominance in biomass at the beginning shifted to diatom dominance,then to Cryptophyta,and finally to Ankyra Fott,which belongs to Chlorophyta.The dominant species had similar alteration in the the treatments with fish,but shifted to Chlorella vulgaris as the dominant species finally.Phytoplankton in the nutrient enriched treatments without fish were mainly composed of species less than 10μm at the beginning,and were dominated by those species larger than 30μm in the last two weeks.However,in the treatments with fish,the phytoplankton species with a size larger than 10μm were decreasing as the experiment going on while the species less than 10μm becoming more dominant and reached peak at the end of the experiment.In the treatments without fish,the abundance and biomass of zooplankton increased remarkablely in all three nutrient enriched levels.However,fish addition only had slight effects on the abundance of zooplankton but decreased their biomass significantly. In the treatments without fish,nutrient enrichment remarkably enhanced both abundance and biomass of Phyllodiaptornus tunguidus and Daphnia geleata,but silver carp decreased the abundance and biomass of cladocerans,especially Daphnia geleata.There was no significant effect on Phyllodiaptomus tunguidus.Abundance of rotifers did not show significant changes within the nutrient enrichments,but increased in the treatments with fish.In control treatments,the dominant species shifted from Daphnia geleata to Phyllodiaptomus tunguidus and its copepodites.These results indicate that Daphnia could become the dominant species in water body without fish or with low fish biomass in the tropics and subtropics.Filter-feeding fish does not decrease the effect of trophic cascade effect during eutrophication of freshwater in these regions. Therefore,the ecological effects of filter-feeding fish in tropical oligo- and mesotrohic water bodies should be paid enough attention for ecosystem management.