Study Of Crustacean Zooplankton Community In Tangpu Reservoir Affected By Upstream And Downstream Effects

The relationship between crustacean zooplankton and(planktivorous)fish has not been well clarified in bigheaded-carps-dominated reservoirs.Previous research is often obtained from studies in temperate and/or shallow lakes.The limitation of these studies is that crustacean zooplankton lack either large grazers such as the bigheaded carp or the vertical migrating space for zooplankton to avoid predation by planktivorous fish.Reservoirs are more and more critical drinking water sources for human consumption.Still,they are facing the increasing challenge of eutrophication resulting from the evergrowing nonpoint pollution,so a better environmental management of reservoirs will rely on a better understanding of the relationship between zooplankton and fish in bigheaded carp-dominated reservoirs.Therefore,we investigated the structure and variation of zooplankton and studied the relationship between zooplankton and fish in Tangpu reservoir,Zhejiang province,China.Our results showed that:1.The total number of zooplankton species showed a trend that was less in spring and winter than in summer and autumn,and was the lowest in winter;the density of cladocera > copepods > nauplius in spring,density of copepods >cladocera > nauplius in summer and autumn,and density of nauplius > copepods >cladocera in winter.2.The structure of zooplankton is often affected by various environmental factors.In Tangpo Reservoir,RDA analysis showed that water temperature(WT)and total phosphorus(TP)were the two factors that had the most significant positive effects on zooplankton overall,while NPUE,chlorophyll a(Chl-a)and total nitrogen(TN)also had positive effects on the available patterns of zooplankton.Water temperature in Tangpu Reservoir was higher in summer than in autumn than in spring than in winter,TN,TP and p H showed a trend of higher in summer than in spring,and total nitrogen,total phosphorus,p H and chlorophyll showed a positive correlation with water temperature,and a positive correlation between these physicochemical factors.There is a positive correlation between chlorophyll a and zooplankton biomass,and chlorophyll concentration shows a trend of summer > spring > autumn > winter,and dendrobatids are mainly influenced by the phytoplankton trophic cascade in spring(as food),and the two peaks of nodular larvae in summer and winter in Tangpu Reservoir can be seen as the reserve biomass of dendrobatids in winter and the common reserve biomass of copepods and dendrobatids in summer.biomass.The effect of zooplankton on phytoplankton was not significant compared to other physicochemical factors.Chlorophyll concentration had an effect on the species composition and density of crustacean phytoplankton in a suitable or stable water environment,but the correlation between the density and species composition of crustacean phytoplankton and chlorophyll was not significant compared to other physicochemical environmental factors when the ambient temperature was highly variable and the precipitation was irregular with large water turnover.This suggests that the effect of phytoplankton on zooplankton is a bottom-up fixation effect,while the effect of zooplankton on phytoplankton is a top-down weak effect.4.By comparing the vertical distribution of zooplankton and NPUE we found that fish were mainly distributed in the upper column.In contrast,zooplankton are in the deeper water column,so the divergence of the vertical distribution of zooplankton and fish will greatly reduce the predation pressure of fish on zooplankton.Thus,zooplankton can feed on phytoplankton and fish detritus or feces in a deeper water column.5.The results of the crustacean zooplankton survey in Tangpo Reservoir from January to November 2021 showed that the water quality of Tangpo Reservoir was clean most of the time,but spring was the lowest among the four seasons from the perspective of diversity index and evenness index,and only from this perspective it was seen that the water body in spring was more nutritious than other seasons,but the richness index was higher in spring than other seasons,and the results of microscopic examination showed that the density of branchial horns was the highest in spring,and from the total In terms of the total number of species,the number of species in spring was less than that in summer and autumn.the crustacean zooplankton species were more prosperous and dominated by larger species;the biomass higher in this planktivorous carps dominated deep reservoirs than in many carp-less-dominated reservoirs or in shallow lakes.We also found that zooplankton and fish had a differential vertical distribution,i.e.,zooplankton occurred deeper and fish shallower in many months of the year.This could well explain why zooplankton was not reduced and not dominated by small species in bigheaded carp-dominated reservoirs.Thus,we concluded that in planktivorous carp-dominated deep reservoir ecosystems,zooplankton might not compete directly with planktivorous carps for phytoplankton by distributing or migrating to a deeper water column;they may survive and thrive on the phytoplankton in a deeper column.The detritus resulted from the feces of planktivorous fish and thus might be able to improve water quality with planktivorous fish coordinately.