| The Caohai Wetland in Guizhou is a typical karst plateau wetland,which is seriously polluted by heavy metals in sediments due to the influence of mining smelting activities.Under the influence of uneven seasonal rainfall,the Caohai water level has obvious seasonal variation,which leads to the alternations of"flooding-drying-flooding"in the sediments around the wetland,forming a seasonal flooding area.Under the regulation of water level,the spatial distribution of aquatic plants in the Caohai wetland was significantly different.The seasonally flooded area was widely distributed with upstanding plants,while the permanently flooded area was abundant with submerged plants,which resulted in a significant spatial difference of sediment organic matter.The content of sediment organic matter in the permanently flooded area was significantly higher than that in the seasonally flooded area.Previous studies have found that the mercury（Hg）content of sediments in permanent flooding areas is significantly higher than that in seasonal flooding areas,but the methylmercury content of sediments in seasonal flooding areas is significantly higher than that in permanent flooding areas.This spatial heterogeneity is thought to be related to differences in organic matter and its humic acid components.However,it remains unclear whether dissolved organic matter（DOM）,which is the most active organic matter,plays an important role in regulating mercury bioavailability,methylation and spatial heterogeneity of bioenrichment in Caohai Wetland sediments.Therefore,in this study,the karst plateau lake（Caohai,Guizhou）was taken as the research object,and the seasonal and permanent flooded areas of Caohai were taken as the research area.Combined with field investigation and indoor simulation experiments,the following research was mainly carried out:1)Using Fourier transform infrared spectroscopy,UV-visible absorption spectroscopy and three-dimensional fluorescence spectroscopy,combined with parallel factor analysis（PARAFAC）and modified Stern-Volmer model,the structure and spatial characteristics of DOM and different molecular weight components of sediments in different regions were studied,as well as the adsorption differences of mercury ions.2)A microcosmic soil culture experiment was established to reveal the effects of DOM and its different molecular weight components in sediments from different regions on mercury methylation and methylmercury release in sediments by exogenous input of DOM and its different molecular weight components in sediments from seasonal and permanent flooded areas.The main results are as follows:The concentrations of dissolved organic carbon（DOC）and colored DOM（CDOM）in sediments of seasonally flooded area(32.776±11.233 mg/L and71.666±33.567 m-1)were significantly lower than those of permanently flooded area（148.911±39.409）mg/L and 15.253±10.247 m-1)（p<0.05）.The relative contribution of CDOM to DOC was higher in permanently flooded areas(CDOM/DOC,0.465±0.098 L mg·C-1m-1)than in seasonally flooded areas(0.432±0.127 L mg·C-1m-1).The E2/E3 values of DOM in the seasonally flooded area（7.39±3.81）were significantly lower than those in the permanently flooded area（9.79±4.03）（p<0.05）.For different molecular weights of DOM,the relative contribution of a355 to DOC in the high molecular weight components（30 k Da～0.45μm）(0.73 and 1.02 L mg·C-1m-1)of seasonal and permanent flooded sediments were higher than that of the low molecular weight components（<30 KDa）(0.60 and 0.49 L mg·C-1m-1).In addition,the humification index（HIX）of the low molecular weight component（1.98）was higher than that of the high molecular weight component（0.66）in the sediment DOM in the permanently flooded area,but the humification degree of the high molecular weight component（3.24）was stronger than that of the low molecular weight component（1.95）in the seasonally flooded area.Two humus-like components C1（UVA humus）and C3（UVC humus）and a protein-like component C2（tryptophan）were analyzed in sediment DOM by the PARAFAC model.Humic component C1 is the main component of DOM in the seasonally flooded area,accounting for 52～61%,and protein-like component C2 is the main component of DOM in the permanently flooded area,accounting for39～79%.The mercury ion adsorption experiments of different DOM components showed that the adsorption capacity of DOM on mercury ions in sediment from seasonal flooded areas was significantly higher than that in permanent flooded areas.The complexing ability of humus like components（log K value is 2.05±0.22）in sediment DOM of seasonal flooding area is stronger than that of permanent flooding area（log K value is 1.58±0.25）,while the complexing ability of protein like components mainly occurs in sediment DOM of permanent flooding area（log K value is 1.06±0.34）.Among the high molecular weight and low molecular weight components of sediment DOM,the adsorption capacity of mercury ions by different molecular weight components of sediment DOM in seasonal flooding areas is significantly higher than that in permanent flooding areas.The maximum adsorption capacity Qm of high molecular weight and low molecular weight components of sediment DOM in seasonal flooding areas is 30.41 and 29.88,respectivelyμThe maximum adsorption capacity Qm of high molecular weight and low molecular weight components of DOM in permanent flooded sediment is 23.36 and 8.79,respectively.The humus like components with high humification degree have stronger binding power with Hg（II）;The binding constant log K value of protein like components varies among samples with different molecular weight fractions.Protein like components undergo complexation in low molecular weight components of permanent flooded sediment DOM and high molecular weight components of seasonal flooded sediment DOM.The high molecular weight components of permanent flooded sediment DOM and low molecular weight components of seasonal flooded sediment DOM did not fit the equation according to the Stern Volmer model.Adding different regional sediment DOM and its molecular weight components to artificially prepared sediment,the spectral index results of soil culture experiments showed that compared to the permanent flooded sediment DOM control group（0.50±0.002-1.45±0.07）,the BIX value of overlying water in the seasonal flooded sediment DOM control group significantly increased（0.97±0.34-1.47±0.005）,with an overall increase of 14.35%.Compared with the high molecular weight components of DOM in seasonal flooded sediment,the low molecular weight components overall increased the BIX value of overlying water by 9.07%.The SUVA254value of overlying water in the seasonal flooded sediment DOM control group(1.13±0.007-3.87±0.37 L mg·C-1m-1)was lower than that in the permanent flooded sediment DOM control group(1.02±0.20-3.95±0.66 L mg·C-1m-1),with an overall decrease of 8.03%.Compared with the high molecular weight components of DOM in seasonal flooded sediment,the low molecular weight components overall reduced the SUVA254value of overlying water,with a decrease of 6.67%.The results of soil culture experiments showed that compared to the permanent flooded sediment DOM control group（0.36±0.11-0.62±0.19 pg/ml）,the dissolved Me Hg（DMe Hg）in the overlying water of the seasonal flooded sediment DOM control group was significantly increased（0.46±0.11-1.08±0.36 pg/ml）.Compared to the high molecular weight components of DOM in seasonal flooded sediment,the low molecular weight components overall increased the DMe Hg concentration in overlying water.On the 25th day,the seasonal flooding zone sediment DOM low molecular weight component treatment group increased by 75.83%compared to the high molecular weight component treatment group,while the permanent flooding zone sediment DOM low molecular weight component treatment group increased by110.75%compared to the high molecular weight component treatment group.Compared with the permanent flooded sediment DOM control group（0.20±0.03-0.39±0.01 ng/g）,the soil Me Hg of the seasonal flooded sediment DOM control group was significantly increased（0.15±0.04-0.85±0.23 ng/g）.The soil Me Hg of the low molecular weight component treatment group of the seasonal flooded sediment DOM was higher than that of the high molecular weight component treatment group,an increase of 12.23-62.78%. |
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