Effects Of Nutrients And Three Omnivorous Fish Species On Phytoplankton Community In A Large Oligotrophic Reservoir

In order to understand the effects of omnivorous fishes on phytoplankton community at the beginning of eutrophication, we performed two enclosure experiments in Liuxihe Reservoir, an oligo-mesotrophic water, from September 10 to November 14 of 2009 and November 10 of 2010 to December 15 of 2011 respectively.In the first experiment, the added nitrogen, phosphorus or tilapia, five treatments were set as follows:1) control treatment (C):TN=0μg/L, TP=0μg/L, Fish=0 g/m3; 2) low nutrients (N, P) treatment (LN):TN=100μg/L, TP=10μg/L, Fish=0 g/m3; 3) tilapia treatment (F):TN=0μg/L, TP=0μg/L, Fish=2 g/m3; 4) low nutrients (N, P) and tilapia treatment (LNF):TN=100μg/L, TP=10μg/L, Fish=2 g/m3; 5) high nutrients (N, P) tilapia (HNF):TN=300μg/L, TP=30μg/L, Fish=2 g/m3. Each treatment had three replicates. Water quality variables and plankton were sampled and measured once a week (7 days). Nutrients were added after every sampling. The results showed that the abundance and biomass of phytoplankton increased markedly in all nutrient enriched treatments with the presence or absence of fish, and increased in fish treatments at the end of experiment (P<0.05). After adding low nutrients in the presence of fish (LNF), the abundance and biomass of phytoplankton increased markedly in the four weeks (P<0.05), but decreased at the last of week, the transparency tended to decrease and the phytoplankton species with a size< 30μm were increasing. In conlusion, after adding high nutrients, the density and biomass of larger size diatom and green alage increased in the fish group, but Peridinium was significantly reduced. At the end of the experiment, the density and biomass of phytoplankton only slightly increased in the control treatments. Bottom-up effects via nutrient excreted by fish are more important than top-down effects via grazing of fish, then promoting the growth of phytoplankton under the oligotrophic nutrient condition. Adding low nutrients in the treatments with fish (LNF) tilapia inhibited the biomass of the phytoplankton under the meso-eutrophic condition. However, in the enclosure of high nutrients addition, bottom-up effects via nutrients on phytoplankton was stronger than the top-down effects via grazing of tilapia. the phytoplankton species with a size larger than 30μm were multiplied due to escaping the grazing, resulting to a decrease in water transparency.In the five treatments of the second experiment, the added nitrogen, phosphorus, silver carp (Hypophthalmichthys molitrix), bighead carp (Aristichthys nobilis) and Tilapia nilotica were set as follows:1) control treatment (C),2) nutrients treatment(N): TN=625μg/L, TP=10μg/L,3) nutrients and silver carp treatment(NH): TN=625μg/L, TP=10μg/L, Silver carp=2 g/m3,4) nutrients, silver carp and bighead carp(NHA): TN= 625μg/L, TP=10μg/L, Silver carp=1.4 g/m3, bighead carp= 0.6 g/m3,5) nutrients, silver carp, bighead carp and tilapia treatment (NHAT):TN=625μg/L, TP=10μg/L, Silver carp=1.0 g/m3, bighead carp=0.6 g/m3 and Nile tilapia=0.4 g/m3. Water quality variables and plankton were measured once a week. Nutrients were added every four weeks. The results showed that the abundance and biomass of phytoplankton increased markedly in all nutrient enriched treatments with or without fishes, and increased in fish treatments at the end of experiment (P<0.05). After adding nutrients in the treatments with fish, the abundance and biomass of phytoplankton increased markedly during the experiments (P<0.05), the water transparency was decreasing and the phytoplankton species with a size less than 30μm were increasing. Adding nutrients with stocked Silver carp 2g/m3, the density and biomass of totall phytoplankton much higher than the control treatment (P<0.05). The significant increasing of biomass of Cyanobacteria, Chlorophyta and Bacillariophyta was induced by stocking Silver carp. Adding nutrients with silver carp and bighead carp, the density and biomass of totall phytoplankton higher than the control treatment (P<0.05), abundance and biomass of small phytoplankton with a size<10μm markedly increased. Adding nutrients with silver carp, bighead carp and tilapia, the density and biomass of totall phytoplankton higher than the control treatment (P<0.05). and abundance and biomass of Chroococcus sp. increased significantly. In conclusion, nutrients enrichment and three omnivorous fish species imposed a significant impact on the community structure of phytoplankton. Adding nutrients with fish, the abundance and bio mass of phytoplankton increased, the major of increased phytoplankton were the Cyanophyta with the small size and the large sized Bacillariophyta and the water transparency decreased.