Effects Of Nutrient Enrichment, Filter-Feeding Fish And Zooplankton On Phytoplankton Community

It is now well recognized that nutrient and predation would play the key roles in phytoplankton structure and biomass, but most studies are conducted mainly in the temperate regions, less in the tropics and subtropics. In order to understand the trophic cascade effects of nutrient enrichment, filter-feeding fish and zooplankton on phytoplankton community in tropical and subtropical reservoirs, we performed three large enclousure experiments in Liuxihe Reservior, an oligo-mesotrophic reservoir, located in South China from 2008 to 2009. The enclosures were filled with about 85 m3 water from the reservoir. The trophic cascade effect of the nutrient enrichment, silver carp(Hypophthalmichthys molitrix,[val.]),bighead carp(Aristichthys nobilis) and Leptodora kindti on the phytoplankton community in the enclosure were examined and analysed.The first experiment was carried out with 21 large enclosures from April 24 to June 7,2008 (49 d,7weeks). It was designed with 7 treatments of nutrient enrichment and stocking silver carp. The experiment included 1)the nutrient additions of three different levels including low (P concentration 30μg/L) (LN), medium (P concentration 60μg/L)(MN) and high (P concentration 90μg/L)(HN)conditions, with a N/P(mass ratio) of 10/1 in all treatments;2)the above three levels of nutrient enrichment conditions with silver carp 4g/m3 stocked(LNF,MNF and HNF); 3)controls without nutrient enrichment and fish stocking(C). There were three replicates for each treatment. The response of phytoplankton to nutrient enrichment was analyzed through the nutrient added treatments and the controls. The trophic cascade effects of the filter-feeding fish on the phytoplankton were also investigated and analyzed between the only nutrient enriched treatments.The results showed that the Secchi disk depth in both nutrient enrichment treatments and silver carp+nutrient addition treatments descreased, and it was higher in the enclosures without fish than in the enclosures with fish. The total nitrogen(TN) in the water column decreased,while the total phosphorus(TP)increased with the presence of fish. Chlorophyll a concentration in nutrient enrichment with and without fish stocking increased, and it was higher in the enclosures stocked fish than the only nutrient addition. Nutrient enrichment resulted in a decrease in phytoplankton diversity, and fish stocking resulted in an increase. Nutrient enrichment and fish stocking both sustained high total phytoplankton biomass depending on nutrient concentrations. Responses of algal groups, such as Cyanobacteria, Chlorophyta and Cryptophyta, were similar in the nutrient enrichment enclosures with and without fish, but the biomass of diatoms (Bacillariophyta) increased solely in the nutrient enrichment with fish. Cyanobacteria, Bacillariophyta, Chlorophyta and Cryptophyta biomass was higher in the enclosures with fish than in the enclosures without fish, but Chrysophyta biomass and Dinophyta biomass were on the contrary. The structure of the phytoplankton community in the enclosures treatments with and without fish shifted from an initial dominance of Dinophyta-Bacillariophyta-Chlorophyta phytoplankton pattern to Bacillariophyta-Chlorophyta-Cryptophyta, and finally to Chlorophyta-Bacillariophyta-Cyanobacteria, while the dominant species shifted to inedible species or the species with rapid growth rate. The phytoplankton size structure and species composition in the enclosures of erichments with and without fish varied during the experiments, but the variability would depend on the nutrient concentrations. Nutrient enrichment only resulted in the dominance of large sized groups, especially increase in the relative biomass of>30μm fraction, while nutrient enrichment with fish reaulted in small size groups became dominant, especially increase in the relative biomass of<10μm fraction.These were because that the enclousure treatments only enrichment were mainly regulated by Daphnia geleata while the enclousure treatments enrichment with fish were regulated by silver carp and nutrient at the same time, so the trophic cascade would be different with the treatments.The second experiment was carried out in 21 large enclosures from December 26 to November 28,2008 (32d,5weeks). The experiment was designed with nutrient enrichment and bighead carp. The experiment included:1)the nutrient additions with three levels including low (P concentration 10μg/L)(LN), medium(P concentration 30μg/L)(MN) and high (P concentration 50μg/L)(HN)conditions, with a N/P(mass ratio) of 10/1 in all treatments; 2)the above nutrient enrichment conditions(LN and MN) with 3g/m3 bighead carp stocked(LNF and MNF); 3)controls without nutrient enrichment and fish stocked(C); 4) controls without nutrient enrichment but only with fish stocked(CF). There were three replicates for each treatment. The results showed that the Secchi disk depth in both nutrient enrichment treatments with and without fish stocking decreased, to the various extents regulated by the nutrient additon levels. Fish presence increased total nitrogen (TN) while decreased total phosphorus (TP). Chlorophyll a concentrations increased in nutrient enrichment treatments with and without fish, and it was higher in the enclosures with fish than in the only nutrient enrichments. Nutrient enrichment and stocking fish resulted in increase in phytoplankton diversity. And phytoplankton diversity was higher in the nutrient enrichment with stocking fish than in those with nutrient enrichment. Nutrient enrichment and stocking fish sustained high phytoplankton total biomass. Nutrient addition led to the increase of Chlorophyta,Bacillariophyta,Cyanobacteria and Cryptophyta,but the decrease of Dinophyta and Chrysophyta.On the other hand,addition of fish caused biomass increase of green algae, dinoflagellates,cryptophyta and chrysophytes,and decrease of diatoms and cyanobacteria,but the magnitude of the changes depended on the level of nutrients. The structure of the phytoplankton community in the treatment enclosures with and without fish stocking shifted from an initial pattern dominated by Bacillariophyta-Dinophyta-Chlorophyta to the final dominance of Bacillariophyta-Chlorophyta-Cyanobacteria, while the dominant species shifted to inedible species or the rapid-growing species. The phytoplankton size structure and community composition in the enrichments with and without fish stocking changed during the experiments, but the the extent depended on the nutrient concentration. Nutrient enrichment resulted in small size groups becoming dominant.Particulary,the low enrichments would increase the relative biomass of 10-30μm,while the medium and higher enrichments could increase the relative biomass in<10μm and >30μm fraction. Low enrichment with fish resulted in large size groups become dominant, especially increased the relative biomass in>30μm fraction, while in medium nutrient enrichments, smaller sized components become more dominant, especially cells of<30μm fraction. The predation pressure of zoophytoplankton to the phytoplankton was higher in the treatment enclosures without fish than the treatments enclosures with fish,sucn as the treatements without fish were mainly controlled by cladocear and nutrient,but the treatements with fish were mainly regulated by rotifera,bighead carp and nutrient,so theses would make the phytoplankton response different.The third experiment was carried out in 15 large enclosures from January 21 to April 17, 2009 (85 d). Leptodora kindti and bighead carp were used as predator treatments. The experiment included:1)the control (C).2) with 2.0 ind/m3 Leptodora kindti (L).3)with 1.1 g/m3 bighead carp (F).4) with 2.0 ind/m3 Leptodora kindti and 1.1 g/m3 bighead carp (LF). There are more than three replicates for each treatment. The response of phytoplankton to Leptodora kindti was analyzed through the Leptodora kindti added treatments and the other treatments. The trophic cascade effects of the filter-feeding fish on the phytoplankton were also investigated and analyzed between different treatments. Phytoplankton diversity in FL treatment was the hith3w5, and it was higher in the enclosures with fish than in the enclosures without fish, while the diversity in L and C treatments was similar. Total phytoplankton biomass was the most in FL, and was the least in the L treatment. Phytoplankton biomass increased in F and FL.Addition of Leptodora kindti would resulted in decrease in biomass of Cyanobacteria, Bacillariophyta, Chlorophyta, but improved Dionphyta and Chrysophyta biomass. The structure of the phytoplankton community in the enclosures treatments with Leptodora kindti and fish stocking shifted from an initial pattern with dominance of Bacillariophyta-Chrysophyta-Dinophyta to the final dominance of Bacillariophyta-Chlorophyta-Dinophyta, while the dominant species shifted to inedibile species or the species with fast-growing. The phytoplankton size structure and species composition in the enclosures with Leptodora kindti and bighead carp varied during the experiments, large sized groups became dominant. Fish stocking and Fish stocking+Leptodora kindti treatments would increase the relative biomass of>30μm fraction, and reduce the relative biomass of<30μm fraction. Leptodora kindti increased the relative biomass of 10-30μm fraction, but decreased the relative biomass of<10μm and>30μm fraction. Daphnia geleata in treatments without fish was more than treatments with fish,while copepods and rotifera were on the contrary. Leptodora kindti can prey rotifera and some small sized cladocera. So the different predation pressure would led to different trophic cascade to the phytoplankton.Our experiments indicated that phytoplankton community structures were regulated by both nutrients and predation. The interaction of nutrient and fish would also play a key role. However, the trophic cascade effects on phytoplankton community would mostly depend on the nutrient concentration, and the species and density of the predators. Consenquently, it is important to find out and understand the interaction between predators and nutrients.