Effects Of Ecological Stocking Silver Carp And Bighead Carp On Nitrogen And Phosphorus Cycle Of Aquatic Ecosystem In Changshou Lake

Changshou Lake is located in the eastern part of the Changshou district of Chongqing,which is the largest artificial freshwater lake in southwest region,also the national AAAA tourist attraction and an important freshwater fish stocking base in Chongqing.In recent years,Changshou Lake has implemented an ecological stocking mode which uses silver carp?Hypophthalmichthys molitrix?and bighead carp?Aristichthys nobilis?as its main stocking objects.And it aims to increase the fish yield through the ecological stocking of filter-feeding fish,and also to improve the water quality.Although silver carp and bighead carp can participate in the nutrient cycling of water,it is also worth exploring further that weather a large number of silver carp and bighead carp play a role to maintain the water balance or to harm the water environment.Therefore,the study of silver carp and bighead carp action on the nitrogen and phosphorus cycling in water ecosystems will benefit for fishery ecological environment research as well as extention and application for ecological stocking models.In order to understand the role of silver carp and bighead carp on nitrogen and phosphorus cycle in Changshou Lake,the present study first conducted water quality analysis methods to monitor physical and chemical indicators of water and the plankton in March,2016 to June,2017 in Changshou Lake and the water samples were collected from five sampling sites.Then 52 silver carp and 54 bighead carp were catched in Changshou Lake to measurement the nitrogen and phosphorus excretion rate in situ and contents of nitrogen and phosphorus in fish body in four seasons from October,2016 to June,2017.Finally,from the perspective of chemometrics,quantitative analyzed the effect of silver carp and bighead on nitrogen and phosphorus cycles.The main results are as follows:?1?Physicochemical indexes and water quality in Changshou Lake.Water temperature of Changshou Lake was 19.88�5.84?;the average water depth wea 14.40�3.67m;the average DO was 5.38�1.23mgL^-1;the average pH was 7.77�1.10;the average TN was 0.83�0.21mgL^-1;the average TP was 0.057�0.020mgL^-1;the average NH₃-N was 0.102�0.086mgL^-1;the average NO₃-N was 0.032�0.020mgL^-1.And the water quality indicators conform National Environmental Quality Standards for Ground Surface Water and basically meets the class?,the water quality is good.The assessment of water quality by the Shannon-Weiner diversity index,Pielou evennessindex and Margalef species richness index show that the water quality of Changshou Lake is generally mild pollution or no pollution,and the water quality status is relatively good.?2?Analysis of the effects of silver carp and bighead carp on nitrogen and phosphorus cycling in Changshou Lake.?1?In situ measurement of nitrogen and phosphorus excretion rate on silver carp and bighead carp.The study assessed the excretion rate of 52 silver carp and 54 bighead carp in 4 times,and constructed a regression equation between specific body weight excretion rate and temperature.The results showed that the specific body weight excretion rate increased with temperature.The formula for specific nitrogen and phosphorus excretion rate and temperature of silver carp is:LogEn=18.972�Log?T?-46.369;LogEp=32.906�Log?T?-81.347;The formula for specific nitrogen and phosphorus excretion rate and temperature of bighead carp is:LogEn=15.247�Log?T?-37.181;LogEp=33.850�Log?T?-83.638.Under average water temperature 19.88?in Changshou Lake during 2016-2017,the N/P excretion rates of silver carp were 0.9731?g N L^-1d^-11 and 0.2422?g P L^-1d^-1,and bighead were 0.6425?g N L^-1d^-1 and 0.1749?g P L^-1d^-1 respectively.?2?Comparison of nitrogen and phosphorus excretion rate between silver carp and bighead carp with zooplankton.According to the estimation of zooplankton excretion rate formula,the excretion of N/P in zooplankton is 83.023?g N L^-1d^-1 and 1.456?g P L^-1d^-1.Compared with the excretion rate of silver carp and bighead carp,the excretion rate of zooplank is much greater than the excertion of silver carp and bighead carp.Therefore,the loading of nitrogen and phosphorus caused by silver carp and bighead carp is much less than zooplankton.?3?Comparison of nitrogen and phosphorus excretion rate between silvercarp and bighead carp with nitrogen and phosphorus contents required for primary production of algae.The nitrogen and phosphorus content required for primary production of algae were 81.21?g N L^-1d^-1 and 2.53?g P L^-1d^-1,respectively.On the other hand,the nitrogen and phosphorus excretions of silver carp and bighead carp account for 1.19%,0.79%of N amount,and 9.57%,6.91%of P amount required by algae primary production in Changshou Lake.The content of nitrogen and phosphorus excreted by silver carp and bighead carp account for the small proportion of nitrogen and phosphorus required for primary production of algae.The results showed that the excretion of silver carp and bighead carp was not the main reason for algal blooms and eutrophication.?4?The nitrogen and phosphorus stoichiometry results of silver carp and bighead carp and the contents of nitrogen and phosphorus taken out from water by silver carp and bighead carp.According to determinating the nitrogen and phosphorus stoichiometry characteristics of silver carp and bighead carp,the results show that the N content of the silver carp is 9.52�2.35%,the P content of silver cap is 3.64�0.55%,and the N:P is 5.79.The N content of the bighead carp is 9.50�2.20%,the P content of the bighead carp is 3.69�0.39%,and the N:P is 5.70.Therefore,in combination with the annual catch of silver carp and bighead carp,silver carp is 1785.51 t and bighead carp is1224.72 t.The amount of N/P taken out by marketable silver carp and bighead carp was up to 60.61 t and 23.31 t in Changshou Lake from 2016 to 2017.In summary,the ecological stocking in Changshou Lake has a small contribution to the nitrogen and phosphorus of water,and it takes away a large amount of nutrients held in the fish body by regularly catching fishery products.,Therefore,within a certain ecological capacity,the ecological stocking can reduce the nitrogen and phosphorus loading of the water and ease the eutrophication trend.And also can play a role in improving the water and achieving the purpose of water conservation fishery.