Removal Efficiency Of Several HABs Species Using Modified Clay And Its Environmental Effects

For half a century,there has been a gradual increase in harmful algal blooms(HABs)species,scale,frequency and its impact on human activities.HABs has become a worldwide ecological problem.Modified clay(MC)treatment has been approved to be an in-situ method that can effectively control HABs.In the present study,the effects of MC on the transition of paralytic shellfish toxins(PSTs)in sedimentary environment and co-cultured shellfish were investigated during the process of eliminating simulated Alexamdrum tamarense bloom.The effects of MC on the water/benthic environment and the community structure were studied by tracking the field treatment of the Phaeocystis globosa.The removal efficiency and germination restriction of the Ulva microscopic propagules using MC were investigated by laboratory trials and ship-based experiments.Achievements as follows:1)The effect of MC on the accumulation and transformation of paralytic shellfish toxins(PSTs)in co-cultured bay scallop and in sedimentary environment was investigated by laboratory experiment.It is suggested that MC can effectively flocculate the A.tamarense cells in water column,which reduces the feeding of the co-cultured bay scallops on the algae cells,consequently reducing the accumulation of algal toxins in their tissue.At the same time,the bay scallops in the control group without MC assimilated 73.2% of the PSTs in the initial algae solution at 3 h.PSTs flocculated by MC can be degraded more rapidly than PSTs that are assimilated by shellfish.2)In order to investigate the effectiveness of field application of MC on the elimination of P.globosa colonies and the possible impact on the adjacent waters,the density of P.globosa colonies in the water column during the MC treatment,and several water quality parameters were investigated.In addition,several water environment parameters,sediment environmental indicators and biological community composition were investigated during the intermittent operation of the MC application for about three months.It is showed that the MC can effectively remove P.globosa colonies from water column and simultaneously reduce the dissolved phosphate and silicate concentration for a short time.On the longer time scale,the change trend of water environment parameters in the operating area is basically identical to that of the non-operating area.It's indicated that the water exchange is the main influencing factor of the water environment.Phytoplankton,zooplankton,benthic fauna,and benthic microbial community structure show certain monthly changes.Temperature may be the most critical environmental factors that cause these changes.AZTI's Marine Biotic Index(AMBI)was calculated by using the species and abundance information of benthic fauna in the survey area.The ecological environment quality of the surveyed sea area was evaluated.According to the evaluation criteria,C1,C2 and C3 stations were “slightly disturbed”,and C4 station was “undisturbed”.Multivariate-AMBI(M-AMBI)added the biodiversity and species number indicators on the basis of AMBI.The results of M-AMBI showed that the benthic ecological environment of the four survey sites is at “good status”.3)The effect of different concentrations of MC on the microscopic propagules of Ulva prolifera was studied by laboratory experiment.The results indicated that removal rate was boosted considerably with the increasing concentration of the modified clay.0.4g/L of modified clay can remove more than 80% of Ulva Prolifera microscopic propagules in a 4.8?105 cells/m L culture.The germination rate of microscopic propagules was further assessed after the removal experiment.It was found that modified clay can reduce the germination rate of Ulva Prolifera microscopic propagules,regardless of whether the culture was resuspended or not.And 1.0g/L of modified clay can significantly reduce the germination rate from 60% to <6.5%.Considering the difference between the laboratory simulation and the natural environment,this study carried out ship-based clay removal experiment at selected station(Ulva microscopic propagules density was 773.3 ± 85.9 ind./L)during the voyage of the South Yellow Sea in May 2016.>80% of the UMP was removed from the water column after MC application at a concentration of 0.1 g/L,and MC applied at a concentration of 0.5 g/L reduced the germination rate to 0.3%.There is still a need for a large number of laboratory and field experiments to further explore and validate the application cost of the method,the application mode and its effectiveness in the field condition and whether it has influence on the seaweed culture.