Effects Of The Hatching Of Resting Eggs On The Interactions Between Zooplankton And Toxic Cyanobacteria

Dormant eggs are a survival strategy for zooplankton to survive in an adverse environment,and their hatching will lead to more genotypes and increase the genetic diversity of the population.The hatching individuals,which may provide sufficient raw materials for the resistance evolution of zooplankton to toxic cyanobacteria,have an important effect on the interaction between zooplankton and cyanobacteria.This study,which focuses on the incubation of dormant eggs,explores the role of dormant eggs in population evolution and community succession from three angles:small-scale evolutionary experiments,the mesoscale enclosure experiment and the field historical population reconstruction.The experimental results were as follows.Firstly,this study explored the effect of hatching from dormant egg on the evolutionary process of resistance to toxic cyanobacteria in Daphnia pulex population in the small-scale evolutionary experiment.In this experiment,the dormant egg density gradients were set up,and after 2 months of mixed food with 25% toxic Microcystis aeruginosa and 75% Chlorella pyrenoidosa,life table experiments were used to test the resistance of D.pulex population to cyanobacteria with different dormant eggs density.The results showed that the average life span,intrinsic rate of increase,net reproduction rate and generation time of all treatment groups under good food(100% Chlorella pyrenoidosa)conditions were higher than those under mixed poor food(50% toxic Microcystis aeruginosa + 50% Chlorella pyrenoidosa)conditions.The average life span,intrinsic rate of increase,net reproduction rate and generation time of the D.pulex population without feeding cyanobacteria were significantly lower than those with feeding cyanobacteria under the poor food condition,respectively.When under the good food conditions,the difference of the average life span,intrinsic rate of increase,net reproduction rate and generation time of the D.pulex population without feeding cyanobacteria,comparing those withfeeding cyanobacteria,was not significant,respectively.With the increase of dormant egg density,the resistance of D.pulex population which fed toxic cyanobacteria showed a change of first droping and then rising,and this may be explained by which the D.pulex clones selected from the D.pulex population fed cyanobacteria for 2months were relatively low tolerance to cyanobacteria.However,The resistance of D.pulex population with feeding toxic cyanobacteria is stronger than which of D.pulex without feeding cyanobacteria.Secondly,this study also explored the effects of hatching from dormant egg on the resistance evolution of zooplankton dominant species and community structure successionin in mesoscale enclosure experiments.In this paper,the dormant egg density gradients were set up,and the resistance of the dominant population,Simocephalus vetulus(medium-size cladoceran)and Chydornus sphaericus(small-size cladoceran),to toxic cyanobacteria was tested by using life table experiment and population growth experiment in the mesocosm after 2 months mixed food feeding with 25% Microcystis aeruginosa and 75% Chlorella pyrenoidosa.The environmental parameters of water bodies in different treatment groups were measured daily,and species identification and count of the zooplankton samples were carried out.The results showed that there was no significant difference in resistance between the dominant population,respectively.The average life span,intrinsic rate of increase,net reproduction rate and generation time of S.vetulus under good food conditions were higher than those under poor food conditions,while the population growth rate of Chydornus sphaericus under good food conditions was also higher than that under poor food conditions.In the enclosure ecosystem,the community structure of zooplankton changed dynamically with time and remained relatively stable.In each group,the copepods and cladocerans were dominant taxonomic groups,and the total density of zooplankton showed a tendency to increase firstly and then decrease,and the species richness of zooplankton increased firstly and then remained dynamicallystable.At the same time,the increase of species richness reflected the hatching of dormant eggs in sediments.The dominant large-size cladocerans were replaced by the small-size cladocerans over time.Finally,the evolutionary process of the resistance of zooplankton in Dianshan Lake to Cyanobacteria was also performed by using dormant eggs.The Dianshan Lake sediment core samples were collected in the field and sliced in the laboratory,and then stored 4 ? in the refrigerator.The dormant eggs in sediment were sieved through water and hatched in the constant temperature incubator,and the resistance of common hatching species to toxic Microcystis aeruginosa was tested by population growth experiment when the individual hatching becomes a population.The results showed that the numbers of hatching species per column was similar in different Dianshan Lake sediment cores,but the numbers of hatching per species was different.The S/V column had hatchings each column,while the G column sample's deeper section did not have any hatchings.The common cladocerans species of the three sediment cores was Ceriodaphnia cornuta.The column layers of 6-10 cm,21-25 cm and 26-30 cm in Dianshan Lake correspond to the period of 1999-2008,1969-1978 and 1959-1968,respectively;and the Ceriodaphnia cornuta clones in three periods had different resistance to toxic cyanobacteria.The population growth rate under good food condition was higher than that under poor food condition.Under the poor food condition,the population growth rate of Ceriodaphnia cornuta showed an increasing trend over time.The resistance of Ceriodaphnia cornuta to toxic cyanobacteria showed an increasing trend over time.In addition,the resistance of Ceriodaphnia cornuta to cyanobacteria had obvious clones difference in the same period.To sum up,hatching from dormant egg can provide an evolutionary basis for the resistance evolution of zooplankton to cyanobacteria,which can lead to more genetic variation and enable the population survive in an adverse circumstances.At the same time,hatching from dormant egg can supplement the zooplankton community,whichis beneficial to maintain the stability of community structure.In addition,using dormant eggs in sediments to track the adaptive process of zooplankton in natural selection is a powerful tool for studying rapid evolution.