Effects Of Organic Carbon Source And Fish On The Structure Of Planktonic Crustaceans’ Community

Planktonic crustacean plays a key role in energy flow and material cycling and biomanipulation in lake ecosystems. Organic carbon source and fish are the major factors to influence the development of planktonic crustaceans. In this study, we collected sediment cores in Lake Fuxian, analyzed the nutrient concentration, abundance of planktonic crustaceans and carbon stable isotope ratios of dominant species via analyzing the microfossils of cladocera. Combined with the results of dating, we discussed the effects of organic carbon source on community structure of planktonic crustaceans. In order to study the up-down effects between fish and planktonic crustacean community in tropical lake, we monitored the water quality and planktonic crustacean communities before and after biomanipulation, also analyzed the fish sample after the restoration in Jida South Lake.The main results are as follows:(1) From 1954 to 2011, with the increase in nutrient concentrations of lake Fuxian, ome species such as Daphnia longispina gradually disappeared, Chydoridae sphaericus and Alona rectangular appeared and increased. With the decrease of the contribution of allochthonous carbon sources(Allo-C) to the planktonic crustaceans’ food, the food source of Bosmina spp. has changed, zooplankton rely more and more on autochthonous carbon sources(mainly phytoplankton).(2) After the manipulation in Lake Nanhu, the total nitrogen, total phosphorus, chlorophyll a and loss on ignition concentrations were significantly decreased, transparency increased. The abundance and biomass of cladoceran increased significantly after the biomanipulation. Higher density of large cladocerans Daphnia pulex were appeared after the initial appearance at the beginning of the biomanipulaiton. The abundance and biomass of copepod did not change significantly. Biodiversity and the body length of planktonic crustacean increased. The abundance of Daphnia pulex decreased quickly when fish biomass increased, and increased again when fish biomass was reduced later.The results showed that the reduction of contribution of Allo-C to the planktonic crustaceans’ food, fish and the eutrophication of lake are the main factors on the community of planktonic crustaceans. The proportion of contribution of Allo-C to Lake Fuxian decreased gradually, it is likely due to the increased phytoplankton production caused by eutrophication and the reduction of Allo-C input from the catchment. Such changes mainly caused by the increased pollution, the loss of vegetation in the catchment and the increasing intensity in the development of the watershed.Our study also showed that fish are the key factor controlling the population of large zooplankton in tropical shallow lakes. As controlling fish to maintain high densities of large-sized zooplankton is difficult, it is unlikely to achieve a significant improvement in the restoration of tropical lakes using biomanipulation.