Study On The Distribution And Degradation Of Dissolved Organic Matter In The Yangtze Estuary In Winter

Dissolved organic matter (DOM) is one of the important carbon pools in the aquatic environment, which plays an important role in the global carbon cycling. The Yangtze river is the longest river with the intricate hydrological environment in China, so study on the DOM in the Yangtze estuary in winter could provide a better understanding of the global carbon cycle. The optical absorption and fluorescence properties of chromophoric dissolved organic matter (CDOM) and the concentration of dissolved organic carbon (DOC) in the surface water of the Yangtze estuary were investigated. Moreover, the photodegradation and microbial degradation of DOM were studied to better understand the dynamics of organic carbon in the Yangtze estuary.The results showed that the temperature and salinity ranged from 5.7-12.7 ℃ and 0.5～33.4, respectively. The ratio of total nitrogen and phosphorus concentration was 35:1, which showed phosphorus was the limit factor in the reasearch area.a355 ranged from 0.31～2.16 m-1 with the mean value of 1.00±0.64 m-1 in the reasearh area. The significant negative linear correlation between CDOM and salinity (R2=0.9546) indicated that the conservative behaviour of CDOM during the mixing process of fresh water and seawater. The negative relationship between SUVA254 and salinity suggested that the aromatic of terrestrial CDOM was higher than marine CDOM. The relationship between SR and salinity followed the simple mixing model, which suggested CDOM was mainly controlled by physical mixing process. The distributions of fluorescent components of chromophoric dissolved organic matter of the Yangtze estuary were determined by fluorescence excitation emission matrix spectra combined with parallel factor analysis. Six individual fluorescent components were identified by PARAFAC, including three terrestrial humic-like components C1 [280/320 nm], C3[235(285)/336 nm], C6[225(275)/310 nm], marine biological production component C2[235(310)/428 nm], C4[270(310)/464(386) nm], C5[250/468 nm]. The conservative behaviours of the three humic-like components showed that terrestrial was the main source of humic-like fluorescent components. DOC concentration ranged from 0.94-2.22 mg/L with the mean value of 1.56±0.39 mg/L. The significant negative linear correlation between DOC and salinity (R2=0.8048) demonstrated that the conservative behaviour of DOC and Yangtze runoff was the main source of DOC. The relationship between CDOM and DOC (R2=0.8841) was fitted by using the multiple linear regression method showed that CDOM was a tracer for terrestrial DOC.The photodegradation rate of CDOM was enhanced with increasing temperature showed that temperature plays a positive role in promoting CDOM photochemical degradation. The activation energy of the station C1 and C7 was 8.62 KJ/mol and 9.69 KJ/mol, respectively. It only changed of DOC concentration of station C7 during 48 h showed that the DOC of station C7 was easier to degrade than others. The alkaline condition of photodegradation of CDOM degradation was relatively high, which showed CDOM was easy to degrade under the condition of alkaline. However, the degradation of DOC of station B5 and A4-8 were degraded under the acid condition, which showed the structure of CDOM and DOC were different during the degradation progress. Choosing three salinity gradient (0.5,15.9,32.7) stations to study on microbial degradation process of DOM. The lower salinity of the sample, the smaller change of CDOM and DOC after incubated a period of time showed that the terrigenous high molecular organic matter was difficult to degradation.