Distribution, Sources, And Decay Of Size-fractionated Particulate Organic Carbon In The Changjiang Estuary Based On Water Elutriation

Knowledge of the sources and fate of riverine particulate organic carbon (POC) is essential to the research of global carbon cycling, and studying the transport of POC from the perspective of size fractionation is helpful to better understand the transformation of OC, provide a new breakthrough point for the study of biogeochemical processes of estuarine OC, and enrich the theory of biogeochemistry of estuaries. In this study, surface and bottom suspended particulate matters (SPM) were collected from the Changjiang Estuary in March 2012, July 2013 and March 2014, and then separated into different size fractions using the water elutriation method. POC contents, atomic element ratios (N/C), stable carbon isotopic abundance (δ13C), and lignin composition, as well as the laser particle size composition of total SPM (TSPM) and mass distribution of these size-fractionated particles were determined in order to study the seasonal and depth variation of distribution, sources, and decay of OC along the salinity gradients in the Changjiang Estuary. Followings are the main results:In general, the highest concentration of TSPM appears at salinity 5-20, and the TSPM in bottom water are higher and coarser than those in surface in most stations. Within the studying period, concentrations of TSPM in July (flood season) are lower than those in March (dry season), and particles in flood season are relatively finer. However, TSPM in the inner estuary and offshore area of different seasons are very similar. The laser particle size analysis of TSPM indicates that the<32 um fractions are the major composition of TSPM, accounting for over 85%, and the particle contents gradually decreased with the increase of particle size. According to the mass distribution of size fractionated particles based on water elutriation, the contents of 16-32μm fraction are highest among all samples with an average of 63%. The contents of>63 μm fraction gradually reduced with the increase of salinity. The contents of<16 μm fractions reduced while the 16-32 μm fraction increased near the maximum turbidity zone, likely due to the flocculation of the small particles. In general, there is no significant difference of mass distribution of size fractions between the surface and bottom SPM. The contents of>16 μm fractions are relatively higher in bottom than in surface, while the contents of<16 μm fractions are lower. In particular, the>63 μm fraction contents in bottom are much higher than those in surface.The distribution of POC has obvious time, space and size differences in the Changjiang Estuary. Compared with the dry season, size-fractionated particles in the flood season have higher TPOC content and N/C ratios, lower As content and partially negative 813C. In the vertical distribution, POC contents in the surface layer are lower than those in the bottom, in contrast to the As contents. There are no significant differences between the surface and bottom layers for the N/C ratios and the δ13C values of size fractions. POC contents (%) are lowest in sites of the maximum turbidity zone compared with other sites, while POC concentrations (mg/L) are highest, indicating that POC concentrations are greatly influenced by the TSPM concentrations, the N/C ratios and δ13C values increase whereas A8 values decrease in all sampling seasons with the increase of salinity, indicating the increase of marine OC input and decrease of terrestrial OC contribution along the salinity gradients. POC contents gradually decrease with the increase of particle size, however,>32 μm fractions have relatively higher POC contents than 16-32μum fractions in flood season.POC mainly concentrate in fine particles (<32 μm) in the Changjiang Estuary, which have lower A8 content, higher N/C ratio, and positive 3C in general, indicating higher contribution of marine OC in these fractions. However, similar to the bulk particles, the δ13C values of fine particles, especially the<8 μm and 8-16 μm fractions in surface layer of the inner estuary in flood season are even more negative, indicated that OC in these fractions may also be influenced by soil OC, freshwater phytoplankton and/or pollutant materials. Coarse particles (>32 μm) have lower OC contents, smaller N/C ratios, depleted 13C and higher A8 contents, indicating relatively high contribution of terrestrial OC. During transport along the estuary, POC contents in all size fractions have lowest values in the maximum turbidity zone, while Σ8 and A8 contents are relatively higher, indicating possible addition of terrestrial OC in the maximum turbidity zone, which may be attributed to the resuspension of benthic sediments. Both the C/V (Cinnamyl to Vanillyl phenol ratio) and S/V (Syringyl to Vanilla phenol ratio) values of lignin phenols in different size-fractionated particles are small, indicated that the lignin tissues in SPM of the Changjiang Estuary is a mixture of angiosperms and gymnosperms, and the smaller the particle size, the greater the contribution of angiosperm grass tissues.With the increase of particle size, the lignin degradation parameters, such as (Ad/Al)v (Vanillic acid to Vanillin), P/(S+V) (p-hydroxyl to the sum of Syringyl and Vanillyl phenols) and 3,5-Bd/V (3,5-dihydroxybenzoic acid to Vanilla phenol) all decrease gradually, indicating high level of degradation of lignins in finer particles. For<8 μm fractions, the (Ad/Al)v values were higher than 0.3,3,5-Bd/v ratios greater than 0.1, and P/(S+V) ratios greater than 0.39, indicating the lignin in these fractions are in medium level of degradation. (Ad/Al)v and P/(S+V) both have significant positive correlations with 3,5-Bd/V, indicating that lignins are influenced by different decay mechanisms during transport.