Analysis Of Zooplankton Community Structure And Fish Production Potential In Tian-e-Zhou Oxbow And Hewangmiao Oxbow Of The Yangtze River

As the national and provincial ex situ reserves of the Yangtze finless porpoise in the Yangtze River basin,Tian-e-Zhou and Hewangmiao Oxbows play an important role in the protection of the Yangtze finless porpoise.The Yangtze finless porpoise feeds on small fish or large and medium-sized fish larvae,and zooplankton is an important food for fish,which is very important for maintenance of the two Oxbows fish resources.Zooplankton,as the primary consumers in the aquatic ecosystem,feed on phytoplankton,bacteria,organic debris,etc.,they are preyed on by other large aquatic organisms such as fish and shrimp,and as one of the food organisms for finless porpoises,zooplankton can play the role of material transfer and energy conversion in the food chain,and can regulate and control the excessive growth of algae and bacteria in the water,maintain the fish resources in the former waters,and promote the growth of the population of Yangtze finless porpoise.Based on the survey data of two Oxbows zooplankton from 2021 to 2022,this study analyzed the community structure of the two Oxbows zooplankton and its relationship with environmental factors,and estimated the fish production potential provided by the two Oxbows zooplankton.On the one hand,under the downward predation pressure of filter-feeding fish,the effects of different upward effects of phytoplankton,physical and chemical properties of water on the temporal and spatial distribution of zooplankton in the two Oxbows were analyzed,so as to evaluate and compare the community structure of zooplankton in water.On the other hand,the current stock of zooplankton was statistically analyzed to estimate the fish production potential of small fish as the food species of finless porpoise,and to provide scientific theoretical guidance for protecting the survival and reproduction of finless porpoise population,healthy development and reaching the maximum environmental capacity of finless porpoise.The results of the study are as following:1.In 2022,a total of 133 species of zooplankton belonging to 67 genera were detected in Tian-e-Zhou Oxbow,slightly higher than 126 species belonging to 67 genera in Hewangmiao Oxbow,and lower than 165 species belonging to 87 genera in Tian-e-Zhou Oxbow in 2021.Among them,18 species of zooplankton of 10 genera were detected in Tian-e-Zhou Oxbow in 2022,lower than the 27 species of 15 genera in Hewangmiao Oxbow,it is also lower than the 20 genera and 29 species of Tian-e-Zhou Oxbow in 2021.Protozoa and rotifers are the main zooplankton species in the two Oxbows,and the dominant species are mainly protozoa,especially the dominant species of the zooplankton in Tian-e-Zhou Oxbow in 2022 are protozoa.In2022,the number of protozoan species in Tian-e-Zhou and Hewangmiao Oxbow was49 species in 23 genera and 49 species in 24 genera,respectively,which were lower than that in 2021,which was 64 species in 37 genera;in 2022,the number of species of rotifers in the two Oxbows was 66 species in 33 genera and 59 species in 31 genera,respectively,the number of species of rotifers in Tiane’zhou was slightly higher than that in Hewangmiao,but both were lower than that in Tiane’zhou in 2021,which was 71 species in 33 genera.During the investigation and study period,the large zooplankton in the water bodies of the two Oxbows,namely cladhorn and copepods,were less.In 2022,the cladhorn species in Tian-e-Zhou Oxbow and Hewangmiao Oxbow were 6 genera 10 species,and the copepods were 5 genera 8 species and 6 genera 8 species,respectively.The Hewangmiao Oxbow was slightly more than Tian-e-Zhou Oxbow.In 2021,the number of cladechords and copepods in Tian-e-Zhou Oxbow was 16 species in 9 genera and 14 species in 8 genera,respectively,which was higher than those of the two Oxbows in 2022.Difflugia sp.,Strobilidium sp.,and Tintinnopsis sp.were the dominant species with high occurrence frequency and dominance in the two Oxbows.The seasons with more dominant species in the two Oxbows were summer and autumn,in addition to protozoa,other dominant species will appear more,and the number of dominant species in spring and winter is relatively small.2.The annual average zooplankton density in 2021 and 2022 of Tian-e-Zhou Oxbow was 2145.49 ind./L and 4099.01 ind./L,respectively,and the annual biomass was 3.06 mg/L and 0.75 mg/L,respectively.The annual zooplankton density in 2022 of Hewangmiao Oxbow was 3761.35 ind./L,which was slightly lower than that in 2022 of Tian-e-Zhou Oxbow.The biomass was 1.08 mg/L,slightly higher than Tian-e-Zhou Oxbow in 2022.In 2022,the density of zooplankton in Tian-e-Zhou Oxbow was the highest in spring,slightly lower in autumn and winter was the lowest.In Hewangmiao Oxbow,the density in autumn was the highest,the lowest in winter.Tian-e-Zhou Oxbow was the highest and lowest occur in winter and spring,respectively,in 2021.In 2022,the seasonal changes of biomass of the two Oxbows were similar,with the highest and lowest occur in autumn and winter,respectively.In2021,the biomass of Tian-e-Zhou Oxbow had slightly different changes,with the highest and lowest occur in spring and winter,respectively.The density and biomass of zooplankton near the elk Reserve in the upper reaches of Tiane’zhou Oxbow,the waters near Qiantou in the middle reaches of Tiane’zhou Oxbow and the estuary are higher.The density of zooplankton is the highest at the estuary and the corner of the Oxbow in the upper reaches of Hewangmiao,which gradually decreases with the flow direction,while the biomass is the highest in the lower reaches.3.In 2021 and 2022,the mean values of Shannon-Wiener（H’）diversity index were 3.46 and 3.13,respectively,and varied in the range of 2.37-4.27 and 2.44-3.79.In 2022,the variation range and mean values of Hewangmiao Oxbow were higher than those of Tian-e-Zhou Oxbow,3.10～4.35 and 3.72,respectively;The two-year Marglef（D）richness indices of Tian-e-Zhou Oxbow were 2.96 and 2.87,respectively,with a range of 2.04 to 4.83 and 1.95 to 3.80.In 2022,the variation range and mean of Hewangmiao Oxbow were also higher than that of Tian-e-Zhou Oxbow,with a range of 2.33 to 4.80 and a mean of 3.47.Pielou（J）evenness index in 2021 and 2022 is0.54～0.74 and 0.48～0.7,and the mean value of 0.65 and 0.62 is lower than Hewangmiao,which is 0.63～0.73 and 0.7.Shannon-Wiener（H’）and Pielou（J）in autumn were the highest in the whole year,while Marglef（D）had little seasonal change,and spring and winter were lower than summer and autumn.In 2022,H’,J and D of Hewangmiao were higher than that of Tian-e-Zhou Oxbow,and the species richness of Hewangmiao was higher than that of Tian-e-Zhou Oxbow.In 2021,all the diversity indices of Tian-e-Zhou Oxbow were higher than those of 2022,during the study period from 2021 to 2022,the zooplankton richness of Tian-e-Zhou Oxbow declined.4.According to the Pearson correlation analysis,multiple stepwise regression analysis and redundancy analysis of the number of species,density and biomass of zooplankton and 12 physicochemical factors in water,it can be seen that the main environmental factors affecting the community structure of Tian-e-Zhou and Hewangmiao Oxbow zooplankton include:WT,DO,NH₄⁺-N and PO₄^3--P.In addition,Tian-e-Zhou Oxbow also include NO₂^--N and COD_Mn,while Hewangmiao Oxbow also include p H and Chl.a.The number of species,density and biomass of zooplankton in the two Oxbows were positively and negatively correlated with WT and DO,NH₄⁺-N,respectively.The species number,density and biomass of Tian-e-Zhou Oxbow were positively and negatively correlated with PO₄^3--P and NO₂^--N,respectively.And there was a negative correlation between Hewangmiao and PO₄^3--P.According to the results of multiple stepwise regression analysis,we can see that WT and NO₂^--N were the main influencing factors in the regression equation of the number of zooplankton species in Tian-e-Zhou Oxbow,WT,COD_Mnand TN were the main influencing factors in the density regression equation,and WT was the main influencing factors in the biomass regression equation.WT,Chl.a and NH₄⁺-N were the main influencing factors in the regression equation of zooplankton species number,Chl.a,NO₂^--N and COD_Mnwere the main influencing factors in the density regression equation,and WT and Chl.a were the main influencing factors in the biomass regression equation.The redundancy analysis results showed that:The distribution of zooplankton density in Tian-e-Zhou in 2021 was mainly affected by WT,COD_Mn,NO₂^--N and PO₄^3--P,among which COD_Mnwas positively correlated with axis 1,and NO₂^--N and PO₄^3--P were positively correlated with axis 2.The density of zooplankton in Tian-e-Zhou Oxbow was positively correlated with axis 1.The distribution of zooplankton density in Tian-e-Zhou in 2022 was mainly affected by COD_Mn,WT and DO.COD_Mnand WT were positively correlated with axis 1,DO was negatively correlated with axis 1,and WT was positively correlated with axis 2.The density of dominant species of zooplankton in Tian-e-Zhou Oxbow was negatively correlated with axis 1.The distribution of zooplankton density in Hewangmiao in2022 was mainly affected by p H,Chl.a and WT,wherein p H,Chl.a and WT were all positively correlated with axis 1,p H and WT were positively correlated with axis 2,and Chl.a was negatively correlated with axis 2.The density of dominant species of zooplankton in Hewangmiao was mostly positively correlated with Axis 1.5.The annual average zooplankton biomass of Tian-e-Zhou Oxbow in 2022 was0.75 mg/L,lower than that of 2022,3.06 mg/L,and lower than that of Hewangmiao Oxbow in 2022,1.08 mg/L.In 2022,the fish production potential provided by zooplankton in Tian-e-Zhou Oxbow is 36.95 tons,far lower than that of Tian-e-Zhou Oxbow in 2021（226.38 tons）,and lower than that of Hewangmiao Oxbow in 2022（96.68 tons）.