The Effects Of Modified Clay On The Typical Harmful Algae And Turbot Embryos

The frequent occurrence of harmful algal blooms?HABs?poses great threats to the fishery,aquaculture,tourist and public health.The removal efficiency of clay to various microalgae has been improved greatly after surface modification.As a result,the modified clay technology is now regarded as the most promising method for HABs'mitigation.Previous studies usually focus on the removal efficiency of modified clay to the vegetative cells of microalgae.However,for some harmful algae,cells in special stages of their life history is crucial for the bloom.The effects of modified clay on these special cells is lack of research.The microscopic propagules of Ulva prolifera are regarded as one of the factors that seed the green tides.The effects of different modified clay on the removal and germination of microscopic propagules were investigated to provide references for typical green tides'control.For the dinoflagellates,cyst plays an important role in the initiation and termination of blooms.The Alexandrium pacificum,a typical toxic cyst-producing dinoflagellate,was investigated to assess the effects of modified clay flocculation to the cysts and paralytic shellfish toxins?PSTs?.In addition,to assess the safety of using modified clay in the spawning and nursery grounds of fishes,the survival and hatching rate of turbot Scophthalmus maximus embryo were investigated as well as the growth and development of newly hatched larvae.The periodic results are as follows.Different modified clays were used to remove microscopic propagules.The results indicated that the removal efficiency increased with the rising amount of modified clay and modifier.AS modified clay was more effective than PAC modified clay,under the same concentration and modifier/clay ratio.Moreover,the germination of microscopic propagules could be inhibited completely under appropriate modified clay concentration.To investigate the effects of modified clay on the cyst formation and germination during HABs'mitigation,bloom of A.pacificum was simulated in the lab,and the different concentrations?0.2,0.4,0.6,0.8 and 1.0 g/L?of modified clay were added for the bloom control.The results showed that modified clay could remove A.pacificum effectively,with higher concentrations of modified clay exhibited higher removal efficiencies.The vegetative cell density was limited in a long period with an appropriate concentration of modified clay,and the residue cells did not show a persistent growth.Finally,the resting cyst density in groups with 0.2,0.4 g/L modified clay was slightly higher than the control,while resting cyst density in groups with more than 0.4 g/L modified clay was less than control.The resting cyst formation rate was not increased after modified clay addition.To assess the differences in germination ability,thirty resting cysts in experimental and control groups were randomly pipetted out after 90 d of modified clay addition.The results showed that the germination rate of resting cysts decreased from 68%to 12.4%after adding modified clay,which means the addition of modified clay may be unfavorable for the resting cyst germination.The effects of modified clay on the paralytic shellfish toxins?PSTs?in the water column and bottom flocs were also studied.Modified clay was able to reduce the PSTs of the environment within 24 h,with 0.8 g/L led to the PSTs content in the water column lower than the detection limit.Turbot?Scophthalmus maximus?embryos were used as a model to assess the effect of modified clay on this sensitive stage of fishes.The 24 and 48 h LC₅₀?median lethal concentrations?of the modified clay were 1.70 and 1.65 g/L,respectively,and the safe concentration was 0.5 g/L,which was much higher than the modified clay concentrations used in mitigating HABs.Modified clay did not significantly influence the hatchability of turbot embryos,but when the concentration exceeded 0.50 g/L,the deformity rate of newly hatched larvae increased significantly.The total length,specific growth rate?SGR?and yolk sac absorption rate of larvae reached their peaks at 0.50g/L and then gradually decreased as the concentration of modified clay increased.Based on the previous researches,this study further investigated the effects of modified clay on the dinoflagellate cyst,toxins,and non-target organisms.The results showed that appropriate amount of modified clay would not leave more cysts in the sediment to act as“seed”,and were able to control the PSTs in 24 h.Modified clay with the concentration used in fields would not cause any adverse effects on the survival and development of the tested fish embryos.This study is expected to provide references in fully understanding the environmental effects of modified clay in HABs'mitigation.