| Mangrove ecosystem is a special kind of wetland ecosystem located in the intertidal zone of the coast.Because of its unique physiological characteristics,mangrove ecosystems often become the sink of heavy metals and other pollutants.In this paper,the spatial distribution of total heavy metals and their available status in the sediments of mangrove wetlands in Zhangjiang Estuary were studied systematically using in situ and traditional sampling methods,and the level of heavy metal pollution was evaluated using the index of geoaccumulation and the potential ecological risk index.At the same time,the seasonal dynamics and spatial distribution of DOM in sediments were explored using three-dimensional fluorescence spectroscopy and parallel factor analysis techniques.The correlation analysis revealed the correlation between typical environmental factors,heavy metals and DOM in sediments.The main conclusions are as follows:1.The influence of rhizosphere behavior of mangrove plants on typical environmental factors was very complicated.The rhizosphere effects of plants significantly reduced the pH of rhizosphere sediments.The relationship between the pH of each spot was:rhizosphere<bulk soil<mudflat.On a vertical gradient,pH and salinity decreased with depth(except for A.marina).Root activity of plants increased the content of organic carbon in the rhizosphere sediments.Except for A.corniculatum community,the contents of organic carbon in sediments of all communities showed rhizosphere>bulk soil>mudflat,and the differences were significant.On the vertical gradient,the TOC content in the surface layer is low,reaching a maximum at 10-30 cm.The total organic nitrogen(TN)content in the rhizosphere sediments were significantly higher than that in the mudflat,and the TON content was highest in the sediments of the Spartina alterniflora communities.The C/N ratio of the sediments in mangrove forests was significantly higher than that of the mudflat outside the forest,which was favorable for the storage of organic carbon.2.DGT could accurately evaluate the bioavailability of heavy metals in mangrove sediments.And these four heavy metals were slightly polluted.The relationship between the total amount of heavy metals was:Zn(111.30 mg/kg)>Pb(71.93 mg/kg)>Cr(64.35 mg/kg)>Cu(32.68 mg/kg).The content of heavy metals in the sediments of mangrove forests was higher than that in the mudflat.The distribution characteristics of Cr,Cu,Zn,and Pb in the sediments of three mangrove species were Res.>Oxi.>Red.>Exc..The relationship between the available contents of heavy metal elements measured in DGT in the wet season was:Zn(4.10 ?g/L)>Cu(2.15 ?g/L)>Cr(2.03?g/L)>Pb(0.68 ?g/L).The content of heavy metals in the dry season was significantly higher than that in the wet season.The regularity of the available states of each element in the surface sediments was:Zn>Cr>Cu>Pb.The content of available heavy metals in the surface layer is relatively high.As the depth increases,the available content will decrease firstly and then increase,with the maximum value appearing around 10-20 cm.The evaluation results of the geoaccumulation index and potential ecological index show that all four heavy metals belong to ligh pollution.In the dry season,the Pb in the K.obovata community was moderately polluted.3.There were significant differences in the fluorescence characteristics of DOM in sediments in different seasons.The average content of DOC in sediments in the wet season was 2.16 mg/L,and the DOC content in the rhizosphere sediments of plants was significantly higher than that in the outside of the forest.The content of DOC in the dry season was higher than the wet season(5.67 mg/L).On a vertical gradient,the DOC content was highest at a depth of 10-30 cm.EEM-PARAFAC results showed that the fluorescence intensity of C1 and C2 components was higher in the four components during the wet season,indicating that both terrestrial and marine sources input affected the composition of DOM in mangrove sediments at the same time.During the dry season,the DOM fluorescence intensity was highest(366.67 R.U.)in the sediments and the lowest in the mudflat(160.40 R.U.).The fluorescence intensity of C1 component was the highest,indicating that the human activities in the dry season had the strongest effect on sediment DOM.In addition to the community of A.marina,the HIX of all the other communities showed a consistent pattern:rhizosphere<bulk soil<mudflat.It was shown that the root activity of the plants during the wet season reduced the degree of humification.The BIX of sediment DOM in mangrove forests during the wet season was higher than that in the sediments outside of the forest,indicating that mangrove plants promoted the production of new components.4.The bioavailability of heavy metals was influenced by both environmental factors and rhizosphere effects.In the wet season,pH,S were negatively correlated with heavy metals,and positively correlated with TOC,TON,C/N,P,and salinity.Various environmental factors had different effects on heavy metals in different seasons.There was a significant negative correlation between the extractable state of thsee four metals in the sediment and pH.TOC was positively correlated with the Exc.and Red.,and negatively correlated with the Oxi..The S element was significantly negatively correlated with the Exc.and Oxi.,and positively correlated with the Red..On the vertical gradient,each extracted heavy metal was negatively correlated with salinity,and TOC was significantly correlated with the three forms of heavy metals in all sediment layers,and was significantly positively correlated with the exchangeable Cr,Zn,and Pb.,indicating that TOC significantly increases the content of exchangeable heavy metals in sediments.The DGT available Cr in wet season was negatively correlated with pH and S,and was positively correlated with C/N.On the vertical gradient,the DGT available Cr was significantly negatively correlated with TOC,TON,and C/N.The available Cr,Cu,Zn,Pb and C1 components and the total fluorescence intensity showed a significant positive correlation.The total amount of Cr in sediments at the wet season was significantly negatively correlated with A250/A365 and A240/A420,and was positively correlated with the FI value,indicating that microbially derived humus could promote the enrichment of Cr in sediments. |
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