Formation and Heavy Metal Enrichment of Epiphytic Biofilm in Cynodon dactylon in the Water Level Fluctuation Zone of the Xiangxi River
A hanging bottle experiment was conducted to simulate the formation process of Cynodon dactylon biofilm at different water depths(0.2,5,and 10 m below the water surface)in the flooding stage of the water level fluctuation zone(WLFZ).Field samples of Cynodon dactylon epiphytic biofilms were collected at different water levels(175,165,and 155 m)in the flooding stage of the WLFZ.The composition and characteristics of the epiphytic biofilms and the enrichment characteristics of four heavy metals(Cr,Cu,Pb,and Cd)in Cynodon dactylon in the Xiangxi River WLFZ of the Three Gorges Reservoir Area were exam-ined.The results demonstrated that Cynodon dactylon attached to the biofilm within 7 days and fully formed within 28 days after being flooded at water depths ranging from 0.2 m to 10 m.The variation in illumination due to different water depths was identified as the key factor influencing biofilm growth.During the flooding period,the average relative biomass(RB),chlorophyll(Chl),and extracellular polymeric substance(EPS)of the Cynodon dactylon biofilm were 1 112.05±651.02,0.81±0.45,and 22.47±9.35 mg/g,respectively.The biofilm formed on the surface accounted for approximately 1.1 times the wet weight of Cynodon dactylon.The average content of Cr,Cu,Pb,and Cd in the Cynodon dactylon biofilm was 74.94±17.18,20.46±2.61,14.25± 3.08,and 0.57±0.10 mg/kg,respectively.which was more than 10,000 times higher than the corresponding concentrations in the Xiangxi River water.The enrichment of Cr,Cu,Pb,and Cd by the Cynodon dactylon biofilm per unit area was 23.08,5.78,4.02,and 0.16 mg/m2,respectively.In comparison to the national average atmospheric deposition flux of the same heavy metals,the content of Cr was found to be similar,whereas the content of Cu,Pb,and Cd was approximately one-fourth.During the flood-ing stage of the WLFZ,a biofilm of comparable biomass was formed on the surface of Cynodon dactylon,which accumulated heavy metals from the water.However,after the WLFZ dried up,the biofilm would detach and enter the soil,potentially becom-ing an important source of heavy metal pollution in the WLFZ.
water level fluctuating zoneCynodon dactylonbiofilmheavy metal
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