Abstract In order to explore the regular pattern of community succession of zooplankton and the influencing factors in the middle eutrophic reservoir， a systematic investigation was made to the zooplanktons in Baguishan Reservior from February to November in 2016， including the diversity， abundance， biomass of zooplanktons and environmental factors. The water eutrophication level was evaluated by using trophic state index （TLI）， and Redundancy Analyses （RDA） was employed to build dynamic changes of structure of protozoa， rotifer and crustacean. Finally， a total of 40 samples were collected. A total of 96 species of zooplankton were identified， including 41 species of protozoa， 34 species of rotifers， 12 species of cladocerans， and 9 species of copepods. TLI varied from 42.8 to 48.9 with an annual average of （45.07±2.02）， indicating that reservoir belonged to mesotrophic water body. Furthermore， ShannonWiener biodiversity （H′） and Pielou evenness（J） showed that the water quality of the Reservoir was βpollution pattern. The results of RDA showed that the zooplankton community structure changed significantly with months. Chlorophyll a （Chl.a）， water temperature （WT） and ammonia nitrogen （NH3N） were the main factors influencing the community of protozoa， and Chl.a， WT， pH， NH3N and the ratio of NH3N to total phosphorus （TP） were important factors to rotifer community. The changes of crustacean community could be better explained by the factors of TP and WT， pH， and the ratio of NH3N to TP.
　　Key words Zooplankton； Community secession； Baiguishan Reservoir； Redundancy Analyses
　　Zooplankton is the hub in the water ecosystem connecting phytoplankton with fish and other highlevel animals. It is the intermediate link of material circulation and energy transfer in the ecosystem[1-2]. The dynamic changes of its community structure can directly reflect the response of ecosystem to the environment[3-4]. A large number of studies have been conducted at home and abroad on the zooplankton community structure and the indicative role in biological monitoring[5-7]. The zooplankton community structure， changes in abundance， and tolerance to water body nutrient conditions can all be used as important indicators for lake and reservoir water quality monitoring[8-9]. Therefore， exploring the changes in species composition and community structure of zooplankton in lakes and reservoirs with different nutrient levels has important implications for studying the structure of water ecosystems， evaluating water quality， and fisheries. 　　Baiguishan Reservoir is located in the southwestern suburb of Pingdingshan City， Henan Province. It is an inland freshwater reservoir in the main stream of Shahe River of the Shayinghe River system in Huaihe River Basin. It is a drinking water source for local residents and a water source for industrial and agricultural use. It is also an important fishery resource. The purpose of this paper was to explore the relationship between the annual changes of zooplankton community structure and the physical and chemical properties of water in Baiguishan Reservoir， so as to provide some theoretical and practical bases for the protection of zooplankton diversity protection， reasonable utilization of fishery resources as well as the control and protection of water environment in Baiguishan Reservoir.
　　Materials and Methods
　　Distribution of sampling sites
　　In this study， 4 sampling sites were set up in Baiguishan Reservior （N33°44′， E113°09′）， namely， S1 （33°43′5.50″N， 113°13′22.28″E）， S2 （33°43′29.90″N， 113°10′11.63″E）， S3 （33°45′5.39″N， 113°10′4.14″E）， and S4 （33°44′13.96″N， 113°09′5.97″E）. Samples were collected from February to November 2016.
　　Sampling method
　　From February to November 2016， the samples were collected at the end of each month according to the "Water Quality Sampling Technical Guidelines" （HJ 4942009）. First， 1 L of qualitative samples were collected on the surface of the water with No. 25 plankton net （mesh diameter of 0.064 mm）， and after fixed in 4% formaldehyde solution， the samples were observed and identified under a Nikon eclipse 80i biomicroscopy[10-12]. As for the collection of the quantitative samples of protozoa and rotifers， 1 L of water was taken respectively at the 0.5 and 1.5 m of the reservoir using a 1 L plexiglass water sampler， after mixing， the collected water was fixed in Lugols solution and then placed still for 24-36 h to precipitate， and then concentrated to 10 mL. Afterwards， 0.1 mL of phytoplanton chamber was used to count the number of protozoa， and 1 mL of plankton chamber to count the number of rotifers. The counting was performed twice under the NiKon E200 biomicroscope， and the average values were taken. For the collection of the quantitative samples of crustaceans， 5 L of water was taken respectively at the 0.5 and 1.5 m of the reservoir using a 1 L plexiglass water sampler， after processing with No. 25 plankton net for filtration， collection and fixation， the samples were brought back to the laboratory for concentration and counting. The volumetric method was used to calculate biomass. 　　Physicochemical factors included total phosphorus （TP）， total nitrogen （TN）， ammonia nitrogen （NH3N）， dissolved oxygen （DO）， permanganate （CODMn）， chlorophyll a （Chl.a）， water temperature （WT）， pH and transparency （SD）. Their determination was based on the Lake Ecological Survey Observation and Analysis[13].
　　Data analysis
　　BergerParker species dominance index （D）[14]， Shannon-Wiener diversity index （H′）[15]， Pielou species evenness index （J）[16]and trophic state index （TLI）[17]were used respectively to calculate the dominance degree of species， evaluate the planktonic diatom biodiversity， determine the water pollution status， and assess the degree of eutrophication according to the recorded methods. A species was determined as the dominant species of a month if it had D>0.1 in at least 2 samples. The standards were as follows： H′>3， clean； 1-3， medium pollution； 0-1， heavy pollution； J>0.5， clean； 0.3-0.5， medium pollution； 0-0.3， heavy pollution； TLI<30， oligotrophic； 30-50， mesotrophic； over 50， eutrophic.
　　In the SPSS 19.0， Pearson correlation analysis was used to analyze the correlation between physical and chemical factors and biological indicators.
　　In order to give more weights to species with less abundance， log（x+1） conversions were made to the species abundances of protozoa and rotifers， so as to better explain the changes of zooplankton community[18].
　　Redundancy Analyses （RDA） based on linear regression were used to study the effect of environmental factors on zooplankton community structure changes[19-20]. First， the main physicochemical factors measured at 4 sampling sites in Baiguishan Reservoir in different months were used as explanatory variables. There may be highly autocorrelated variables， and therefore the variables with coefficients of expansion larger than 20 were excluded. Second， the abundance of zooplankton with D ≥ 0.1 in at least 3 sampling sites was selected as the response variable. Therefore， 10 environmental variables （WT， TP， TN， NH3N， DO， CODMn， SD， pH， Chl.a， NH3N/TP and TN/TP） and 38 species of zooplankton （16 species of protozoa， 9 species of rotifers and 13 species of crustaceans） were selected for analysis. The above statistical analysis was completed in CANOCO 4.5.
　　Results and Analysis
　　Physical and chemical factors and water quality assessment of Baiguishan Reservoir
　　The diversity index （H′） varied from 2.08 to 3.05， with an average of （2.53 ± 0.23）. The maximum value appeared in sampling site S1 in August， and the minimum value appeared in S3 in April. Overall， Baiguishan Reservoir was in the βmesosaprobic zone. The evenness index （J） ranged from 0.43 to 0.53， with an average of （0.49±0.04）， which also indicated that the reservoir was in the βmesosaprobic zone. The range of trophic state index （TLI） was （42.8-48.9） and the annual average was （45.07±2.02）. According to the evaluation criteria， the reservoir was in the mesotrophic level. 　　The contents of nutrient salts varied a lot （Table 1）， and TP changed the most significantly with the lowest value of 0.01 mg/L and the highest value of 0.45 mg/L.The TP concentration was high from February to June， with an average value of （0.22 ± 0.08） mL， and （0.02 ± 0.01） ml in July-November. The content of TN was （0.63-3.78） mg/L， and NH3N was （0.01-3.00） mg/L. The ratios of nutrient salts also had great changes. The average TN/TP in from February to June was （6.76 ± 0.26） mg/L， while the average from July to November was （64.2 ± 0.15） mg/L. The concentration of nutrients as well as their proportions also showed great changes， which inevitably affected the growth of planktonic algae， thereby affecting the structure of the zooplankton community.
　　Species composition of zooplanktons
　　A total of 96 species of zooplankton were identified in the survey， including 41 species in 32 genera of protozoa， 34 species in 14 genera of rotifers， 12 species in 6 genera of cladocerans， and 9 species in 3 genera of copepods. In general， protozoa and rotifers were the main components of zooplankton in Baiguishan Reservoir， and there were fewer species of Cladocera and Copepoda. From the perspective of time variation （Table 2）， the number of protozoa species had no significant changes， and it was the largest in July and least in November； the species of rotifers fluctuates greatly， and the number was the largest in August and the least in November； the number of crustacean species had no big change throughout the year.
　　The species with D ≥ 0.1 in at least 3 sampling sites in each month was determined as the dominant species （Table 2）. Species of Trichocerca had a certain degree of dominance throughout the year. Species of Difflugia and Polyarthra were in the dominant states in spring， while Tintinnopsis wangi Nie was mainly dominant in summer， and the dominant species of crustaceans varied greatly from month to month. The predominant Stribilidium gyrans， Stribilidium gyrans， Polyarthra trigla Ehrenberg， Polyarthra remata Skoricov， and Keratella cochlearis Rotatoria were the indicative species of mesotrophic water body[21-22].
　　As for the abundance level （Fig. 2）， the abundance of zooplankton was the highest in July and was the lowest in November， showing a trend of increasing first and then decreasing throughout the year. For all species， the abundance of protozoa was the highest in July and the lowest in November， being 27 110 and 6 210 ind./L respectively； the rotifer abundance was the highest in October and the lowest in February， being 3 463 ind/L and 459， respectively； the abundance of crustaceans （including nauplii） was highest in June and lowest in February. The abundances of protozoa and rotifers were the main body of zooplankton abundance in Baiguishan Reservoir. 　　As for the biomass level （Fig. 2）， zooplankton biomass was the highest in May， the lowest in February， and the change was great throughout the year. Protozoan biomass was the highest in August and was the lowest in November， being 0.63 and 0.1 mg/L respectively. The biomass of rotifers were the highest in May， the lowest in February， 1.05 and 0.09 mg/L， respectively， and the biomass of crustacean was the highest in June and the lowest in February， 0.74 and 0.03 mg/L， respectively.
　　Relationship between spatiotemporal variation of protozoa community structure and environmental factors
　　Detrended correspondence analysis （DCA） was performed on protozoa， rotifers， and crustaceans of the Baiguishan Reservoir， obtaining singlepeak response values. However， the gradient length （SD） values were all smaller than 3， so Redundancy Analysis （RDA） was selected.
　　The relationship between protozoan changes and environmental factors was shown in Fig.3a. D. globulosa， a dominant species of protozoa， could be found all through the year， and it showed strong correlation with NH3N/TP. The number of T. wangi was the largest in August and September， and pH had a significant effect on the number. S. velox was the most in July and September， which had a positive correlation with Chl. a. The speciesenvironment correlation coefficients of axis 1 and axis 2 of RDA were 0.857 and 0.899， respectively， indicating that there were strong correlations between environmental factors and the protozoan community. As shown in Fig. 3b， almost all of the samples collected in March， April， October and November fell to the second quadrant， when NH3N， TN and TP were the limiting factors； samples of February， May and June were in the third quadrant， negatively correlated with NH3N/TP， pH， and CODMn； samples collected in July， August， and September were in the first and fourth quadrants， which showed significantly positive correlations with WT， pH and Chl.a.
　　Relationship between spatiotemporal variation of rotifer community structure and environmental factors
　　The rotifer community structure fluctuated greatly. The relationship between rotifers and environmental factors was shown in Fig. 4a. K. cochlearis were mainly concentrated in July， presenting a strong correlation with WT. T. lophoessa and T. elongate were mainly concentrated in March and October， having good correlations with NH3N/TP. In the RDA results， the speciesenvironment correlation coefficients of axis 1 and axis 2 of RDA were 0.893 and 0.870， respectively， indicating that there were strong correlations between environmental factors and the rotifer community. As shown in Fig. 4b， almost all of the samples collected in February-May were in the third and fourth quadrant， presenting strong positive correlations with TP and DO； samples of June and July were in the second quadrant， when WT was the major limiting factors； most of samples collected in August-November were in the first quadrant， and CODMn， Chl.a， and NH3N/TP had a great impact on them. 　　Relationship between spatiotemporal variation of crustacean community structure and environmental factors
　　The speciesenvironment correlation coefficients of axis 1 and axis 2 of RDA were 0.810 and 0.766， respectively. The relationship between crustacean and environmental factors was shown in Fig. 5a. Naupiar larvas were the young of crustaceans， and they could be found in each of the investigated month. The number was great in June， August， September and October， which presented significantly positive correlations with Chl. a and CODMn. WT was the limiting factor for B. longirostris and D. brachyurum， and NH3N/TP was an important affecting factor for T. kawamurai and O. mohammed， while D. cucullata showed strong correlations with TP. As shown in Fig. 5b， water temperature and DO were the most important influencing factors affecting the zooplankton community.
　　Correlation analysis
　　As shown in Table 3， the protozoan biomass was extremely significantly correlated with the biomass and abundance of phytoplankton， P<0.003 and P<0.000 1， respectively. The abundance of rotifers also showed extremely significantly positive correlations with phytoplankton biomass and abundance， P<0.000 1. However， there was no correlation between planktonic algae and crustaceans in terms of biomass and abundance. The diversity index H′ and evenness index J presented significant positive correlations with the zooplankton biomass and abundance. The reason was that a large number of zooplankton species were suitable for their growth， and therefore， there would be more species. No good correlation was found between TLI and zooplankton. The abundance and biomass of protozoa and crustaceans， and the abundance of rotifers， all had very significant positive correlations with water temperature， indicating that water temperature had a great impact on zooplankton community.
　　Conclusion and Discussion
　　Composition characteristics of zooplanktons in Baiguishan Reservoir
　　According to the 10month survey， a total of 96 species of zooplankton are identified and analyzed. The number of zooplankton species is the largest in August of 59， and the least in November of 28. The density of plankton is the highest in July， and the least in November， 29 501 and 6 968 ind./L， respectively. One of the characteristics of community structure of zooplankton in Baiguishan Reservoir is that the proportion of protozoan abundance is the highest in all zooplankton abundances， followed by the abundance of rotifers， and least was found in crustaceans. In terms of biomass composition， rotifers take up a large proportion， and the biomasses of protozoa and crustaceans are relatively high in June， July and August， which is similar to the composition ratios of zooplankton species in the known mesotrophic lakes and reservoirs[23-24]. The reason for this is that protozoa are small and have a fast growth rate， so the species number of protozoa is greater than that of rotifers and crustaceans， and the biomass is low. The individual biomass of rotifers is smaller than that of crustaceans but the number is much greater than that of crustaceans. Therefore， the total biomass of rotifer is higher than that of crustaceans in some months. 　　Relationship between zooplankton community structure and environmental factors
　　The zooplankton community structure is different in the rivers， lakes and reservoirs with different nutritional levels. Even in the same reservoir， zooplankton community structure is different in different seasons and at different sites[25]. There are many reasons for this difference， not only related to physical and chemical factors in water， including temperature， pH， and nutritional status， but also related to the impact of other biological groups， such as the effect of cyanobacterial toxins[26]， the feeding preference of ticks and toads to zooplankton[27]， and the abundance and biomass of algae fed by zooplankton.
　　In this study， the results of RDA showed that the community structure of protozoa， rotifers， and crustaceans changes significantly with month， and the community structure is quite different in different months. WT is one of the major factors affecting the abundance and biomass of zooplankton[20]. As shown in Fig. 3， 4 & 5， WT is the main factor affecting the zooplankton community in Baiguishan Reservoir. Pearson correlation analysis shows （Table 3） that there are very significant correlations between the WT and the abundance and biomass of protozoa， rotifers， and crustaceans （except for rotifer biomass， P<0.01）. Zooplankton abundance and biomass are related to their food sources， such as algae， bacteria， and organic detritus[28]. At the same time， the change of community structure is closely related to the fluctuation of nutrients. According to the Redundancy Analyses， TP， NH3N， and NH3N/TP are also the main influencing factors of zooplankton community changes in Baiguishan Reservoir. They can affect the abundance and biomass of zooplankton which feed on phytoplankton by affecting the abundance and biomass of phytoplankton. Similarly， Sun et al. also found that the concentrations of nitrogen and phosphorus were the main factors affecting the protozoa in Xixi Wetland[29].
　　Among the dominant species， P. trigla， T. kawamurai， S. gyrans as well as T. lophoessa and T. elongate of Trichocerca were all positively correlated with TP， NH3N， and NH3N/TP， which is consistent with the results that they are considered to be indicative species of mesotrophic water bodies.
　　Relationship between zooplankton and water quality
　　In zooplankton， because of their fast reproduction and short cycle， protozoa and rotifers can sensitively reflect the changes in water[30]. P. trigla， P. remata， K. cochelearis， D. brachyurum and species of Trichocerca， which are the indicative species of mesotrophic water bodies， are the dominant species among the zooplankton community in Baiguishan Reservoir. The evaluation of the eutrophic degrees of water according to the density of zooplankton density[31]shows that the water body is in the mesotrophic state. 　　The results of the ShannonWiener diversity index and Pielou evenness index for all plankton show that the reservoir is in the level of βmesosaprobic level， and oligopolluted or clean level in some months. However， the results are slightly different from the diversity evaluation of protozoa， rotifers， and crustaceans. It may be related with the different order of magnitudes. The TLI evaluation results suggest that the water is in mesotrophic status， which is slightly different from the indicative function of zooplankton. The reason is that the water quality is close to eutrophic level， together with appropriate temperature， making it possible for the mass propagation of the indicator species which indicate mesotrophic water bodies.
　　To sum up， Baiguishan Reservoir is in the mesotrophic state， but it is close to the eutrophic level. Therefore， it is necessary to strengthen the control and monitoring of the degree of nutrition as a source of drinking water in order to be able to take preventive measures in advance to protect water quality. At the same time， this study further demonstrates that zooplankton and its community structure play an important role in the monitoring and evaluation of water quality， and contributes to the change of the zooplankton community structure with the month in the mesotrophic reservoir.
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