Surface Sedimentary Organic Carbon Fractions Off The Changjiang Estuary And Its Implications For Sedimentary Oxygen Consumption

In the Yangtze River estuary and its adjacent waters,the ecological health and the problem of low oxygen zone are receiving increasing attention due to the organic matter input from the Yangtze River basin runoff.However,studies on organic carbon fraction in sediments,especially on the differentiation of organic carbon activity status and dissolved oxygen utilization potential by means of chemical extraction,have rarely been reported.In this study,total organic carbon(TOC)in surface sediments of 26 stations was analyzed and determined based on the comprehensive survey data from the "Su Ru Fishery and Environmental Survey 08327" cruise in June 2019,and water extractable carbon(WEC),salt extractable carbon(SEC),and organic carbon in the sediments were extracted.The content of water extractable carbon(WEC),salt extractable carbon(SEC),microbial biomass carbon(MBC),and acid hydrolysable carbon(AHC)were analyzed and determined,and the spatial distribution characteristics and structure of each component were studied.The main sources of organic carbon were investigated by the ratio of total organic carbon(TOC)to total nitrogen(TN)(C/N),and the oxygen consumption characteristics of the sediment were estimated based on the active organic carbon fraction,revealing the distribution characteristics,main controlling factors and influence of the soluble(active)to insoluble(recalcitrant)organic carbon fraction.The main findings are as follows:(1)The TOC and TN contents of surface sediment samples ranged from 2.97 g/kg to14.63 g/kg and 0.06% to 0.14%,with mean values of 9.43±3.07 g/kg and 0.07±0.01%,respectively.The distribution of TOC in the Yangtze River estuary and adjacent waters tends to increase from north to south,with the highest content in the aquaculture area,and the same distribution pattern for TN,with a significant positive correlation between TOC and TN.The distribution of soluble organic carbon in sediment included WEC and SEC,and their contents ranged from 0.04 ~ 0.38 g/kg and 0.06 ~ 0.31 g/kg,with the mean values of 0.12 ± 0.10 g/kg and 0.16 ± 0.07 g/kg,respectively,and there was a significant positive correlation between WEC and SEC in sediment and TOC,which increased with the increase of total TOC.WEC and SEC were only a small fraction of TOC,with content distributions of 0.41%-3.54% and 0.60%-4.57%,respectively,and mean values of 1.31%± 0.81% and 1.79% ± 0.82%.there was a significant positive correlation between WEC and SEC,which are similar types of labile organic carbon(LOC).The size of sediment particle size has a role in influencing the content of LOC.Aquaculture activities can have an effect on the sedimentation of unstable organic matter fractions,and samples from aquaculture areas showed higher LOC with faster carbon turnover rates.Aquaculture activities further enhanced organic matter production through manure or pseudo-manure,and the higher SEC content and high SEC/TOC ratio in aquaculture areas indicated strong microbial activity in the sediments.The distribution of sediment MBC ranged from 0.32 ~ 1.76 g/kg,with a mean value of 1.58 ± 0.45 g/kg.MBC showed a clear decreasing pattern from the estuary to the sea,with a distribution pattern similar to that of lignin,one of the biomarkers of terrestrial origin.the distribution of MBC in TOC ranged from 2.38% ~ 58.26%,with a mean value of 14.19% ± 11.55%.MBC was significantly correlated with WEC and SEC,and not significantly correlated with AHC and TOC.The distribution of sediment AHC ranged from 0.67-8.11 g/kg with a mean value of 4.38 ± 1.95 g/kg.the AHC showed a seaward increasing pattern over the range of the study area.the distribution of AHC in TOC ranged from 22.63 ~ 62.97% with a mean value of 45.16% ± 11.25%.The content of AHC tends to be more stable than other carbon pools throughout the sediment.Among the four fractions,AHC accounted for the most content of the fraction.AHC showed a higher degree of positive correlation with WEC and SEC,and a lower degree of positive correlation with MBC.AHC differed from the composition of other organic carbon fractions and had a significant correlation with TOC.AHC determined the nature of the whole sediment OM.The distribution of AHC/TOC increased with the offshore distance.Also AHC is related to the aging of sediment OM,and the age pattern of sediment is very similar to the AHC/TOC pattern.(2)The C/N distribution of sediment organic matter in the Yangtze River estuary and its adjacent waters ranged from 2.8 to 17.1,with a mean value of 12.61 ± 3.52.The Yangtze River estuary received more organic matter from terrestrial sources,and higher levels of C/N were mainly found near the estuary.The source of sediment organic matter in the study area is influenced by a combination of marine organic matter and terrestrial organic matter mixing,and the fine-grained sediment area is also the area with higher C/N ratio.C/N is significantly and positively correlated with WEC,SEC,and AHC.the ratio of AHC increases with increasing sand content,while the ratio of WEC and SEC decreases with increasing sand content.TOC,WEC and SEC were all negatively correlated with sediment volume,and higher TOC and lower density sediments contained larger organic carbon active components.(3)WEC and SEC contents were higher when DO was lower in near-bottom waters,and a similar trend was observed for MBC,whereas this pattern was not observed for AHC.In the surface sediments,the WEC and SEC inventory within 4 mm of the sediment surface,was 1.46 g/m2 and the DO required to completely decompose this mobile and unstable OC was 0.15 mol/m2.It was assumed that the decomposition of the mobile and unstable fraction of carbon would result in anoxia in the bottom waters of the sediment,i.e.,the depth of anoxia caused by the complete decomposition of 4 mm of LOC in the sediment,ranged from 0.3 m to 4.7 m,with an average depth of 2 m.The contribution of sedimentary oxygen consumption to the AOU of anoxia occurrence was 10%(2/20),and the anoxia in the Yangtze estuarine waters was mainly caused by water column respiration,and the role of sedimentary oxygen consumption was small(Approximately 10%).