| In order to explore the effects of in situ biological treatment on the plankton structure of tilapia ponds,experiments were conducted in the base ponds of Guangxi University from April to October 2018 on the ecological cultivation of tilapia,silver carp and Bighead carp,and Myriophyllum spicatum floating-bed.Eight experimental ponds were divided into four groups.The stocking density of tilapia was 1500 tails per mu.Group A,B and C were treated with 9:1,1:9 and 1:1 for silver carp and Bighead carp,respectively.At the same time,the floating bed of Myriophyllum spicatum with 10%pond area was placed.Group D was the control group.Qualitative and quantitative studies of plankton in monitoring ponds were conducted once a month during the experiment.Water quality was tested and nutritional status was evaluated.The results of plankton community structure,abundance,dominant species,functional group analysis and water quality nutritional status evaluation were as follows:1.Total characteristics of plankton structure:(1)A total of 168 speciesof 7 phyla of phytoplankton(including varieties),82 species of Chlorophyta,34 species of Gymnophyta,24species of Cyanophyta,22 species of Diatomaceae,3 species of Dinoflagellate,2 species of Xanthophyta and 1 species of Cryptophyta were detected in this experiment.The phytoplankton types of treatment group and control group were Chlorophyta-Cyanobacteria type.(2)Twenty-one functional groups of phytoplankton were detected in this experiment:C,D,N,P,MP,T,TC,S1,S2,SN,X3,X1,Y,F,G,J,K,Lo,M,W1,W2.The main functional groups were Chlorophyta,Cyanophyta and Diatomaceae,among which the functional groups of Chlorophyta were the most,followed by Cyanophyta.(3)79 species of zooplankton belonging to 3 classes and 2 classes were detected in this experiment.Among them,there are 44 species of Rotifers,6 species of Copepods,12 species of Cladocera,10 species of Sarcopoda and 7species of Ciliata.Rotifers are the main zooplankton species,followed by Cladocera.2.Population structure characteristics of phytoplankton:(1)Species quantity and similarity:In treatment group A,B and C,147 species of phytoplankton were detected in 7phyla.Among the groups A>C>B,the similarities of species structure between A and B,A and C,B and C were 0.742,0.724,0.787,respectively,but there were no significant differences in species quantity and structure(P>0.05);in control group D,121 species of phytoplankton were detected in 7 phyla.In control group D,the species number of phytoplankton was the least.There was no significant difference between groups(P>0.05).(2)Abundance and relative abundance:The ranges of phytoplankton abundance in treatment group and control group were2.390×106-2.465×10~7cells/L and 2.743×10~5-1.659×10~7cells/L,respectively.The average values were from low to high in group B(1.450×10~7cells/L)>group A(1.272×10~7cells/L)>group C(1.141×10~7cells/L)>group D(1.028×10~7cells/L),and there was no significant difference among groups(P>0.05).The overall variation trend of phytoplankton abundance is gradually increasing with the process of cultivation.The relative abundance of Chlorophyta was the highest in the treatment group,with the average relative abundance of 52.664%,54.369%and 51.890%in group A,B and C,followed by that of Cyanophyta,with the average relative abundance of 25.135%,23.197%and 26.241%in group A,B and C,respectively.The results of control group were similar.The average relative abundance of Chlorophyta and Cyanophyta were 49.544%and 25.390%respectively.The relative abundance of cyanobacteria in group A,B and C decreased by 23.652%,12.979%and 13.919%from the highest value to the last sampling,respectively.The relative abundance of cyanobacteria in group A,B and C decreased by 9.718%in the control group.The results showed that the combined treatment of floating bed and bighead carp could effectively inhibit cyanobacteria.(3)Dominant species,dominance degree and Shannon-Weiner index:There were 38 dominant species in treatment group,17dominant species in chlorophyta,11 dominant species in cyanophyta,5 dominant species in diatomaceae,3 dominant species in gymnosperma and 1 dominant species in cryptophyta.There were 25 dominant species in the control group,11 species of Chlorophyta,8 species of Cyanophyta,4 species of Diatoma,1 species of Xanthophyta and 1 species of Cryptophyta.The dominant species are concentrated in the phylum Chlorophyta and Cyanophyta.The dominant species of silver carp and bighead carp in the stocking group and the post-treatment group before and after the floating bed placement did not change significantly compared with the control group.However,the dominance degree of cyanobacteria species decreased.Treatment group A,B and C decreased by 0.067,0.168 and 0.102 respectively from the highest value to the last sampling,while control group D decreased by 0.083.The results showed that the floating bed could optimize the population structure and inhibit the growth of cyanobacteria.Shannon-Weiner index ranged from 4.468 to 5.189 in the treatment group and from 3.684 to5.199 in the control group.Statistical tests showed no significant difference among the groups(P>0.05).Shannon-Weiner index was more than 3,which indicated that the pond water was light polluted or non-polluted,suggesting that the structure of phytoplankton community was stable and the pond water had high stability.(4)There were differences in the number of species,abundance and dominance of Cyanophyta among different proportions of silver carp and Bighead Carp mixed culture treatments.Group A>group C>group B,phytoplankton abundance group B>group A>group C,and the decline of Cyanophyta dominance in group B>group C>group A.Statistical tests showed no significant differences among groups(P>0.05),but the larger the proportion of silver carp,the more obvious of the inhibition of phytoplankton.3.Structure and environmental characteristics of phytoplankton functional groups:(1)Structure and succession characteristics of functional groups:21 in treatment group and 19 in control group;19,18 and 20 in A,B and C groups,respectively.The functional groups with frequency of 100%in the treatment group and control group were C,D,P,MP,SN,X1,Y,F,J,Lo,M and W1.Phytoplankton functional groups C,X1,J and Lo in the treatment group and C,X1 and J in the control group were higher than 10%of the average relative abundance.The representative functional groups with average relative abundance greater than 5%in treatment group were F and M,while those in control group were D,MP,Lo and M.The dominant functional groups C,J,X1,Y,MP,Lo,F,M with dominance greater than 0.1 in the treatment group were the main dominant functional groups in the experimental pond,while those in the control group were C,J,X1,Y,MP,Lo and W1.The composition of functional groups in the treatment group was different from that in the control group.The representative functional groups in the treatment group were mainly chlorophyta,while those in the control group were more Cyanophyta and diatomaceae.During the experiment,the main functional groups were Chlorophyta,Cyanophyta and Diatomaceae.During the monitoring period,the changes of functional groups were small,and the basic functional groups were stable in the treatment group,while the succession of functional groups in the control group was unstable.During the experiment,the dominance degree of Cyanobacteria-dominated functional groups decreased.The treatment group A,B and C decreased by 24.979%,13.995%and 14.007%respectively from the highest value to the last sampling,while the control group decreased by 9.872%.This indicated that in situ biological treatment could reduce the risk of cyanobacteria blooms in this experiment.(2)Environmental characteristics and changes of main dominant functional groups:The environmental characteristics of main dominant functional groups of phytoplankton in treatment group and control group were interpreted,and the results showed that the pond environment was mixed eutrophic water body;The results of single index surface water environmental quality assessment show that the water quality of ponds is stable and there is some organic pollution,make the water eutrophic.The nutritional status evaluation results showed that the water quality of treatment group and control group was medium-eutrophic state,which accorded with the environmental characteristics of main dominant functional groups.There was no significant difference between the treatment group and the control group in interpreting environmental characteristics,but the results of nutritional status evaluation showed that there were differences.Group A was eutrophic in June and August,group B was eutrophic in July,August and October,and group C was eutrophic in the remaining months,while group C was eutrophic in April,July,August and October.Compared with traditional phytoplankton analysis methods,the environmental characteristics indicated by phytoplankton functional groups are consistent with the nutrient status of pond water quality,which can better reflect the changes of ecological functions of water body.4.Population structure characteristics of zooplankton:(1)Number and similarity of species:73 species were detected in the treatment group,and the number of species A>C>B.The similarity between A and B,A and C,B and C were 0.537,0.597 and 0.593,respectively.The similarity of species among the treatment groups was higher.47 species were detected in the control group,and the similarity index with group A,B and C were 0.500,0.508 and 0.574,respectively.The similarity between the two groups was low.Rotifers were the main zooplankton species in the treatment group and the control group,but there was no significant difference between the control group and the treatment group(P>0.05).(2)Abundance and relative abundance:The variation range of zooplankton abundance in treatment group was1.04×10~2-6.208×10~3ind./L,and that in control group was 5.06×10~2-2.926×10~3ind./L.There was no significant difference among groups(P>0.05).The relative abundance of rotifers in the treatment group was the largest,and it was relatively stable in the later stage,which maintained above 90%.The relative abundance of copepods increased and rotifers decreased in the later period of the control group.(3)Dominant species:Rotifers were the dominant species in the treatment group,and Brachionus was the dominant species.Cladocera and Copepods did not appear after floating bed was placed.The dominant species in the control group were Rotifers,but Cyclops mesophylla appeared widely in each month.The dominant species appeared in most months,and Cladocera also appeared in the dominant species.These results indicate that silver carp and bighead Carp inhibit the survival of large zooplankton such as Copepods and Cladocera,and that the biological plankton bed can improve the survival pressure of zooplankton,especially rotifers,and increase their dominance.(4)The results of mixed culture of silver carp and bighead carp in different proportions showed that the bigger the proportion of bighead carp in group A,B and C,the fewer the species of zooplankton,suggesting that the inhibition of bighead carp on large zooplankton increased with the increase of the number,but the statistical test showed no significant difference(P>0.05).In conclusion,under this experimental condition,compared with monoculture tilapia,the combined treatment of Myriophyllum spicatum floating-bed and different proportion of silver carp and bighead carp can improve the diversity of plankton species in tilapia pond,inhibit the species and quantity of cyanobacteria,improve the survival pressure of zooplankton and stabilize water quality.Compared with the analysis of phytoplankton species diversity,the analysis of phytoplankton functional groups can objectively indicate the water quality,it can more objectively indicate the water quality and reflect the changes of water ecological functions.However,there was no significant difference between the treatment group and the control group and between the treatment group.The mechanism of phytoplankton structure and water quality in Tilapia ponds controlled by the combination of Foxtail algae floating bed and silver carp and bighead carp needs to be further studied. |
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