| Submerged macrophytes can absorb large amounts of nutrients from water body.They can also increase the transparency of water body by removing pollutants,increasing dissolved oxygen,and preventing the suspension of solid particles.In addition,some allelochemicals secreted by submerged macrophytes can effectively inhibit the growth of algae.Blue-green algae is one of the dominant algae species that cause water eutrophication problems.Inhibiting the growth of blue-green algae has thus become one of the key issues in phytoremediation of polluted water environment.Screening of tolerant submerged macrophytes,investigation of their inhibitory effects on blue-green algae growth,and analysis of related factors affecting COD,NH3-N,TN,TP,SS,pH,and DO can provide useful data and information for development of water pollution control technology.Identification of allelochemicals produced by different submerged macrophytes and determination of their types,contents,and allelopathic effects are of high significance with both theoretical and practical values for ecological remediation of eutrophic water bodies.In the present study,six local submerged macrophyte species widely distributed in Hunan Province,Myriophyllum verticillatum,Vallisneria natans,Hydrilla verticillata,Ceratophyllum splendens,Potamogeton crispus and Elodea Canadensis,were selected as phytoremidiation species.The blue-green algae was taken as the target species of which the growth needs to be inhibited.Under laboratory conditions,a series of designed pilot experiments were carried out to investigate the inhibitory effects of the selected submerged macrophytes on the growth of blue-green algae,their purification effects on water body and related factors that affected the treatment efficiency.Based on the results from the comparison tests among individual species,the best four submerged plants were chosen and applied in further combination trials in which two species were cultivated together to test their integrated effect.Samples of culture water were taken from different treatments.By concentration and separation techniques,GC-MS analysis was performed using the culture water samples to identify the chemical substances secreted from different submerged macrophytes.The potential allelopathic effects of the identified substances were further investigated.The main results are summarized as follows:1)The blue-green algae growth inhibition rates by the six tested submerged macrophyte species were following the order:Vallisneria natans(33.13%)>Hydrilla verticillata(27.77%)>Myriophyllum verticillatum(25.98%)>Ceratophyllum demersum(22.98%)>Potamogeton crispus(1.22%)>Elodea Canadensis(-28.92%).Among the tested plants,Vallisneria natans,Hydrilla verticillata,Myriophyllum verticillatum and Ceratophyllum demersum were found to the best species with respect to their high and stable levels of pollutant tolerability and removal rates as well as their ability to depress the planktonic algae growth rate.These species were shown to have high application values for in-situ ecological restoration of polluted water environment.2)The planktonic alga growth inhibition rates were found to be significantly higher in all two-submerged macrophyte species group treatments than in single species plots.The inhibition rates were ranked as follows:(Ceratophyllum demersum+Hydrilla verticillata)>(Myriophyllum verticillatum+Vallisneria natans)>(Ceratophyllum demersum+Vallisneria natans)>(Myriophyllum verticillatum+Hydrilla verticillata)>(Hydrilla verticillata+Vallisneria natans)>(Myriophyllum verticillatum+Ceratophyllum demersum)>single species.Differences were observed regarding the removal of different pollutant types.For removal of NH3-N and SS,as well as for enhancement of DO level,all two-species groups performed much better than single species.The differences in COD removal rates,however,were found to be inconsistent among tested species combination and single species plots.For removal of TN and TP,two-species groups did not perform as well as single species.In general,species combination enhanced the integrated pollutant removal capacity,and the cooperated cultivation of Ceratophyllum demersum and Hydrilla verticillata was shown to be the best combination.3)A total of 56 types of compounds were detected from the culture water samples collected from five submerged plant species treatments.The determined compounds were mainly organic acids,organic lipids and organic alcohols,and a few present in small quantities were phenols and ketoness.The highest area-percentages of organic acids were detected in samples from Myriophyllum verticillatum,Vallisneria natans,Elodea Canadensis and Hydrilla verticillata plots while the highest area percentages of ketones and lipids were from Ceratophyllum demersum plot.Organic acids secreted from the five submerged macrophytes were found to be the main compounds responsible for inhibition of algae growth.4)The main allelochemicals detected in the culture water samples were succinic acid(succinic acid),citric acid and phenol from Myriophyllum verticillatum;lactic acid,citric acid,sebacic acid,stearic acid(octadecanoic acid),lauric acid and phenol from Vallisneria natans;azelaic acid,p-hydroxybenzoic acid,trans ferulic acid and palmitic acid from Elodea Canadensis;trans aconitic acid,citric acid and ferulic acid from Hydrilla verticillata;and citric acid,oleic acid,butyl acetate and diisobutyl phthalate from Ceratophyllum demersum. |
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