Biogeochemistry Of Chromophoric Dissolved Organic Matter And Spatiotemporal Variations Of Suspended Particulate Matter In The Changjiang (Yangtze River) Estuary And The Adjacent Sea

Abstract:It is quite difficult to analysis the non-conservative behaviors of Chromophoric Dissolved Organic Matter?CDOM?and the dynamics of Suspended Particulate Matter?SPM?in areas like the Changjiang Estuary,where the influence of dominant physical processes is enormous.This paper takes the Changjiang Estuary and the adjacent sea as the research areas,biogeochemical behavious of CDOM and spatiotemporal variations of SPM in the Changjiang Estuary has been analyzed,it is conducive to elucidate the behaviors of biogeochemical compounds and the dynamics of SPM in estuarine areas,further studies about the interrelation between CDOM and SPM are still needed,the main results are as follows:?1?Seasonal variation of CDOM at freshwater end-member of the Changjiang EstuaryFrom November 2016 to October 2017,surface water samples were collected in areas near Xuliujing.Absorbance spectra and fluorescence Excitation-Emission Matrix spectra?EEMs?were measured in CDOM samples.The concentration of DOC,the relative concentration of CDOM(reflected by a₂₅₄),the CDOM overall aromatic degree(reflected by SUVA₂₅₄),and the molecular weight(reflected by S_275–295)all fell within the range of natural freshwater bodies.Compared with some freshwater bodies such as rivers and lakes,the proportion of CDOM in DOC was higher,and the CDOM overall aromatic degree was higher;the compositions of FDOM were uniform during the study period.Although the dry season and the flood season showed significant differences in runoff,and the seasonal changes in DOC fluxes also reflected the significant impact of runoff,there was no significant correlation between DOC,CDOM concentration and runoff.?2?Biogeochemical behavious of CDOM in the Changjiang EstuaryIn March and July 2016,CDOM samples were collected in the Changjiang Estuary and the adjacent sea.Absorbance spectra and fluorescence EEMs were measured in those samples and parallel factor analysis was conducted in the obtained EEMs in which three humic-like components and one protein-like component were identified.All parameters of DOC,?₂₅₄54 and the four fluorescence components showed the significantly negative correlations with salinity,suggesting the physical mixing was the most important factor that controlling the distribution and variation of those compounds.The CDOM overall aromatic degree(reflected by SUVA₂₅₄)and molecular weights(reflected by S_275–295)decreased from land to sea.In July,the high Changjiang runoff,rarely occurred in the history,brought the Changjiang Diluted Water?with salinity<31?overlying onto the surface of the whole study area,which provided the opportunity to study the non-conservative behaviors of CDOM within the Changjiang Diluted Water.The concentrations of DOC and CDOM in 13 pairs of water patches,with similar salinity values but quite different transport distances?times?from the river mouth,were compared and it was found that the Changjiang Diluted Water showed a significant sink effect on?₂₅₄54 and all the three humic-like components.However,the sink or source effects on DOC and the protein-like component were not significant.The most?Ex/Em?_maxax blue-shifted humic-like components showed the highest average degradation rate,followed by the two more red-shifted humic-like components,?₂₅₄,DOC and the protein-like component.All CDOM compounds were more conservative in March than in July.This study provides a new method to study and even quantifying the non-conservative behaviors of biogeochemical compounds in the large river estuarine areas,where the physical processes always played the dominant role on marine biogeochemistry.?3?Spatial distribution and seasonal variation of SPM in the Changjiang EstuaryThe Changjiang Estuary is a typical high-turbidity estuary,and SPM in the Changjiang Estuary and its adjacent sea has large variation ranges of mass concentration,and shows quite active and complicated behaviors there.SPM at 99and 89 stations in this area was studied using OBS and LISST instruments in July2015 and March 2016,respectively,and parameters of turbidity,beam attenuation coefficient,floc total volume,mean size,and size spectrum of SPM were obtained from the two instruments.SPM mass concentration at surface,middle,and bottom layers of all stations were obtained after filtration,and?¹³C?‰?values of Particulate Organic Carbon?POC?,?¹⁵N?‰?values of Particulate Nitrogen?PN?,and molecular ratios of POC/PN in SPM at typical stations were also obtained.The three parameters of turbidity,beam attenuation coefficient,and floc total volume all showed significant positive correlations with SPM mass concentration.At freshwater end-member of the Changjiang Estuary,the mean sizes of SPM at surface layers were slightly smaller than bottom layers in the flood season,under the influence of plankton,the mean sizes of SPM at surface layers were larger than bottom layers at seawater end-member.In the dry season,the mean sizes of SPM transported by the Changjiang freshwater were much larger and the floc total volume were much higher,larger SPM mean sizes were generally found at surface rather than at bottom in the study area.The SPM contents with similar size spectra could be further differentiated by their significantly different?¹³C and?¹⁵N values.In March,the Changjiang-originated SPM?with larger size,and lower density?transported as far as the eastern edge of the northern part of the study area.However in July,the Changjiang-originated SPM?with smaller size,and higher density?would deposit rapidly near the river mouth.According to the vertical profile of the typical stations SPM floc size spectrum derived by LISST,the SPM transported by the Changjiang freshwater performed a distinct seasonal variation at the stations strongly affected by the Changjiang Diluted Water,and the SPM often varied with depth.In the offshore stations,SPM were mainly dominated by plankton,these signals were discontinuous with depth.