| Plankton(phytoplankton and zooplankton)is an important part of the aquatic ecosystems.The structure of plankton communities is mainly affected by nutrients,light,temperature,hydrodynamics and fish.In the past,researches had focused on the effects of single factor on the structure of plankton communities.However,changes in the structure of plankton communities in nature are also affected by the combined effects of many environmental factors.This study explored the effects of disturbance patterns and nutrients on the structure of plankton communities.In this paper,field surveys in Meiliang Bay and East Taihu Lake was conducted to understand the annual changes of phytoplankton and zooplankton community structures in different habitats and their relationship with environmental factors.Based on field investigation,field simulation experiments were conducted to study effects of disturbance modes and nutrients on plankton in spring and summer.The main research contents and results are as followings:(1)Anniversary survey on changes in the structure of plankton communities in Lake Taihu(Meiliang Bay and East Taihu Lake)from Jul.2017 to Jun.2018.The survey results showed that the dominant species of phytoplankton in Meiliang Bay and East Taihu Lake were Microcystis.The annual average density and biomass of Microcystis in Meiliang Bay were significantly higher than that in East Taihu Lake,while the phytoplankton biodiversity was significantly lower than that in East Taihu Lake.The average annual microcystis density in Meiliang Bay and East Taihu Lake were 1.0×10~8 cells/L and 0.18×10~8 cells/L;the average annual biomass were 6.508 mg/L and 1.162 mg/L,respectively.In Meiliang Bay and East Taihu Lake,phytoplankton biodiversity index were 0.67 and 1.22(P<0.05),respectively.The average annual density of zooplankton(70 ind./L)and biomass(0.427 mg/L)in Meiliang Bay were higher than the average annual density of zooplankton(29 ind./L)and biomass(0.308mg/L)in East Lake Taihu.However,the dominant species of zooplankton in the two lake areas were different which was different from phytoplankton.The dominant species of zooplanktonin Meiliang Bay were mainly small size Polyarthra trigla(12 ind./L;17.73%)and Bosmina coregoni(11 ind./L;16.66%);while in East Taihu Lake,small crustacean zooplankton Bosmina coregoni(5 ind./L;15.88%)and Limnoithona sinensis(3 ind./L;11.06%)were dominant species.The survey also found that the degree of eutrophication in Meiliang Bay was significantly higher than that in East Taihu Lake.The average TN and TP in Meiliang Bay was 2.729 and 0.224 mg/L.While it was 2.027 mg/L and 0.118 mg/L in East Taihu Lake.In addition,the water transparency(WC)in Meiliang Bay(34.4 cm)was significantly higher than that in East Taihu Lake(27.2 cm).Correlation analysis results showed that TN,TP and Chla were significantly positively correlated in Meiliang Bay(P<0.05).The differences between plankon community structure in Meiliang Bay and that in East Taihu Lake may be caused by the combined effects of nutrients,especially nitrogen and phosphorus and water disturbance.(2)The effects of simulated disturbances and nutrients on plankton community structure in spring.The results of study found that,Chla in the continuous mixing groups with nutrient and intermittent mixing groups with nutrient were significantly higher than other treatment groups.Especially for continuous mixing and nutrient addition groups,the average cell density and biomass of phytoplankton were the highest among all treatment groups,which were 3.48×10~8 cells/L and 58.265 mg/L.During the whole experiment,the dominant species of phytoplankton mainly included Microcystis,Pseudodanabaena limnetica and Scenedesmus quadricauda.The dominant species of phytoplankton in intermittent disturbance plus nutrients groups was Microcystis(72.3%-97.0%).The phytoplankton community structure in the other treatment groups changed during the experiment.At the end of the experiment,the dominant species in no mixing and no nutrient group was Microcystis(59.76%),Pseudanabaena limnetica(19.02%),and Scenedesmus quadricauda(15.37%).The dominant species in intermittent disturbance nutrient addition groups was mainly Microcystis(73.98%)and Pseudanabaena limnetica(18.81%).No mixing groups with nutrient,continuous mixing groups with no nutrient and continuous mixing group with nutrient were mainly Pseudanabaena limnetica,which proportions were 94.41%,75.61%,and 90.52%,respectively.Among the all groups,the phytoplankton biodiversity of continuous mixing and nutrient addition groups was the highest,being 1.63.The proportions of Cladoceran and Copepod in the undisturbed group were significantly higher than that in the disturbed group,but rotifers remained the dominant species in all treatment groups(61.19%-99.73%).The zooplankton biodiversity of the different treatment groups were similar without significant differences(P>0.05).The study showed that mixing and nutrient addition was beneficial to the growth of phytoplankton.Continuous mixing and nutrient addition was the best in promoting the growth of phytoplankton.Intermittent mixing and nutrient addition was beneficial to maintaining the dominant position of Microcystis.The study also showed mixing was beneficial to rotifers,but was infaus for copepods and cladocerans.(3)The effects of simulated disturbances and nutrients on plankton community structure in summer.Different from the results of the spring simulation experiment,the average density of algal cells in intermittent mixing and nutrient addition groups was 1.7×10~8 cells/L,which was higher than those in no mixing and continuous mixing group with nutrient.However,the average biomass of phytoplankton in continuous mixing and nutrient addition groups(39.406mg/L)was significantly higher than that in other treatment groups,and Chlorophyta contributed the largest biomass,accounting for 76.54%of the total biomass.Chla in continuous mixing and nutrient addition group was always the highest among all treatment groups.At the beginning of the experiment,the dominant species of phytoplankton was Microcystis(93.30%).The structure of phytoplankton community in no mixing and continuous mixing group with nutrient changed during the experiment.At the end of the experiment,in no mixing and nutrient addition groups,Microcystis accounted for 54.72%,Pseudanabaena limnetica accounted for 32.10%.In continuous mixing and nutrient addition group,Microcystis accounted for 54.85%,Pseudanabaena limnetica accounted for 38.38%.In intermittent mixing and nutrient addition group,Microcystis was always the dominant species(73.47%-93.77%),and Pseudanabaena limnetica(20.19%)appeared only on the 10th day.At the end of the experiment,the proportion of Microcystis was 81.97%,and Pseudanabaena limnetica accounted for 15.61%in intermittent mixing and nutrient addition group.The phytoplankton biodiversity indiex in continuous mixing group with nutrient(2.13)were significantly higher than that in no mixing and nutrient addition groups(1.56),and that in intermittent mixing and nutrient addition group(1.37)(P<0.05).Continuous mixing and nutrient addition group was beneficial to the accumulation of zooplankton and biomass.The average density and biomass of zooplankton in continuous mixing and nutrient addition group were the highest,which were 5404 ind./L and 23.637 mg/L,respectively;the lowest in no mixing and nutrient addition group were 1811 ind./L and 17.687 mg/L,which were significant differences(P<0.05).It is similar to the results of the spring simulation experiment,the proportion of cladoceran in the no mixing and nutrient addition group was higher than that in the mixing group.Rotifers accounted for the majority(85.02%-97.36%)in all treatment groups,but the main dominant species were different.The dominant species was Synchaeta pectinata(877 ind./L;48.42%)in the no mixing and nutrient addition groups;Brachionus capsuliflorus(1677 ind./L;31.04%)in the continuous mixing and nutrient addition groups,and Synchaeta pectinata(1026 ind./L;38.23%)in the intermittent mixing and nutrient addition groups.The zooplankton biodiversity index of the continuous mixing and nutrient addition groups was the highest The study results showed that mixing and nutrient addition was beneficial to the growth of phytoplankton,and continuous mixing and nutrient addition was beneficial to increase the biomass of Chlorophyta.The study results also showed that intermittent mixing and nutrient addition was beneficial to maintaining the dominant position of Microcystis,and mixing was beneficial to rotifers,which was not conducive to copepods and cladocerans. |
