Spatial Distribution And Source Identification Of CDOM In Wetlands And Rivers Around The Bohai Bay

Chromophoric dissolved organic matter is an important part of dissolved organic matter in water.It participates in many key biogeochemical processes such as material cycles and biological activities of elements(carbon,nitrogen,phosphorus and silicon)in the world,and plays an important role in the changes of chemical forms of trace elements and the transport of pollutants in rivers,wetlands and other water bodies.In order to investigate the spatial distribution characteristics and fate of CDOM in wetlands and rivers around central part of the Bohai Bay,the CDOM in surface water and bottom water of the Beidagang wetland(Beidagang Reservoir,Guanqi Lake and Swan Lake),surface water in the Duliujian River and the Jiedijian River as well as their estuary areas was studied using UV Visable absorption spectroscopy and three-dimensional fluorescence spectroscopy(3D EEMs)technology,combined with parallel factor analysis(PARAFAC).The spatial distribution characteristics,sources and influencing factors of CDOM in water samples in summer were discussed.The results can provide scientific basis for environmental protection and management of wetlands and rivers in this region.The main conclusions are as follows.(1)Results of UV absorption spectrum parameters showed that the aromaticity of humus continuously increased and the average molecular weight of CDOM continuously decreased during the migration of CDOM from the river to the coastal sea,indicating that it was mainly related to the photochemical effect and microbial degradation during the processes of terrestrial CDOM entering the sea.The variation in aromaticity of humus in the surface water of wetland was larger than that in the river,while its variation in the bottom water of wetland was similar to that in the river.The average molecular weight of CDOM in the surface water of wetland was larger than that in the bottom water,indicating that there were more humus components with small molecular weight in the bottom water.(2)Results of fluorescence spectrum parameters showed that the sources of CDOM in the rivers were influenced by endogenous and terrestrial inputs,and the endogenous CDOM is dominated in the river water.The influence of land-based input and humification degree of surface water in wetland were larger than those in the rivers,but weresmaller than those in the bottom water of wetland.In addition,the sources of CDOM in the river samples were similar with those of DOC,but it did not show this phenomenon in the wetland,indicating that the source differences between DOC and CDOM in the wetland were more complex than those in river samples.(3)PARAFAC grouping analysis can be used to distinguish the spatial distribution characteristics and different effects of CDOM in different sections of the river,which can be regarded as a simple,reliable and effective method to identify sources and distribution characteristics of CDOM fluorescence components.Results of PARAFAC analysis showed that five types of fluorescent components were detected inof the rivers and wetlands in this study,i.e.terrestrial humus like(Component 1),phytoplankton derived protein like or tryptophan like(Component 2),phytoplankton derived microbial humus like(M like,Component 3),synthetic tryptophan like(Component 4)and autogenous fulvic acid like(C like,Component 5).Component 1 and Component 2 were detected in the Duliujian River and the Jiedijian river;Component 3 was detected in the lower reaches of both rivers,and the fluorescence intensity showed that Component 3 was highly unstable during the process of river water entering the sea.Only Component 4 was detected in the north side of the downstream of Duliujian River,which was also detected in the polluted rivers in previous studies.Component 5 was detected in the upper and the middle reaches of Jiedijian River,respectively,indicating that these two sections were less affected by seawater reverse irrigation and had more active microbial activities.In addition,the emission wavelength of Component 1 fluorescence peak in the north side of the downstream of Duliujian River showed an obvious red shift,which indicated that the input of industrial wastewater had a certain impact on the molecular weight and molecular structure of CDOM.Component 1 and Component 2 were detected in all wetlands in the study areas,and the fluorescence intensity in the wetlands was significantly higher than that in rivers.Component 3 was detected only in the surface water of Beidagang Reservoir,and the fluorescence intensity of Component 1 in the surface layer of the reservoir was significantly lower than that in the bottom,which may be due to the disturbance of external water diversion and the release of organic matter from the reservoir sediment as an endogenous endmember to the bottom water.Inthe study areas,the proportion of protein like components in the total fluorescence components in rivers was larger than that in wetlands,which was due to the combined effects of summer biological decomposition and anthropogenic input;while the proportion of humus like components in wetlands is larger than that in rivers,which was due to the decay of algae and vascular plants,having an impact on the distribution of CDOM in wetlands.(4)The salinity was one of main factors controlling the distribution of DOM,and theanthropogenic input as well as the damming physical interception also had important impacts on the distribution characteristics of DOM.The salinity was negatively correlated with DOC content as well as fluorescence components(Component 1 and Component 2),while Component 1 and Component 2 were positively correlated with TN and TP.Due to the damming at the estuary,the CDOM content,molecular weight and humic aromaticity in the downstream of Duliujian River were different from those in the coastal sea.However,the CDOM content,molecular weight and humic aromaticity in the downstream of Jiedijian river without damming is similar to those in the coastal sea,indicating that the damming can produce physical interception effect and cause hydrodynamic changes such as water collision,thus affecting the distribution characteristics of DOM.However,the damming has a less influence on the content of Component 2 than that of other components.