Distributions Of Organic Carbon And Suspended Particulate Matter In The Changjiang (Yangtze) River Estuary In Response To The Extreme Flood Occurring In The River Basin

The Changjiang（Yangtze River）,the third river around the world,transports large amounts of terrestrial materials to the oceans every year.As the largest marginal sea in China,the biogeochemical behaviors of biogenic elements in the Changjiang Estuary are quite complex owing to the multiple influences from the strong physical,chemical and biological processes that occur in these important areas.Extreme climate events may also change the fluxes and compositions of terrestrial materials transported by Changjiang,but fewer studies reported it.In order to trace the influence of extreme flood events on the distribution and variations of biogenic elements in the Changjiang Estuary,based on the two flood season cruises in July 2016,the non-catastrophic flood cruises in flood season in 2019 and the cruises in July（during the flood event）and August（after the flood event）in 2020,we analyzed seasonal variations,spatial distributions and the relationships among the biogenic paremeters.With the background of climate change,we explore the responses of river-dominated marine ecosystem to the large amounts of terrestrial materials inputs during extreme flood events.In addition,according to the characteristics of the flood,the flood in the Changjiang basin in 2016is defined as regional flood,and the flood in 2020 is basin flood.The main results of thesis are as follows:（1）Distribution of POC and DOC in the Changjiang Estuary and the adjacent area in response to the extreme flood.Through the systematic observation of two cruises in July 2020 and August 2020,the concentrations of SPM,POC,and DOC were measured in the surface samples collected at all stations.We compared the results of the above two cruises with those obtained in July 2019.Our results showed that the flood event in July 2020 greatly elevated the concentrations of SPM,POC and DOC in the Yangtze which were ready to be transported out to the adjacent oceans.However,the influence of the flood on the adjacent area had a hysterisis.In the pelagic oceans with higher salinities,concentrations of SPM,POC,and DOC generally showed the highest values in July2020,when compared with the other two cruises in July 2019 and July 2020.During the August 2020 cruise,the three parameters indicating the chemical properties of organic matter,i.e.,POC（%）,POC/DOC（mol/mol）and POC/PN（mol/mol）,also showed the strongest terrestrial signals among the three cruises conducted in the study area.（2）Sources and dynamics of SPM in the Changjiang Estuary and the adjacent area in response to the extreme flood.Before and after the peak highest discharges appeared in the lower Changjiang（at the Datong station）,two cruises were conducted continuously in July 2016,covering the area from the Changjiang freshwater end-member station（Xuliujing）to the estuary,and then to the adjacent East China Sea shelf.During the two cruises,the water samples of 75 and 87 stations were respectively collected.The main purpose of the two cruises was to increase the knowledge about the distributions and variations of SPM in the estuarine and shelf areas,as well as its compositions and properties,how they responded to the extreme flood event.The salinity contours at the surface were pushed much more seaward by the extreme flood during the second cruise.The parameter of SPM concentration,obtained by seawater filtration and always used to quantify terrestrial SPM contents in seawater,generally showing higher values during the second cruise that was influenced by the flood.During both cruises,LISST was used to measure in situ SPM total volume,mean size,and size spectrum.Before the flood event,the SPM in the whole study area was generally dominated by large-size particles.The extreme flood successfully brought more small-size and terrestrial-derived SPM into the whole study area.Probably owing to the sunlight sheltering effect of these intruded terrestrial SPM fluxes,the phytoplankton primary production and biomass growth,at the surface of the pelagic stations outside the river mouth,were greatly depressed during the second cruise.As a result,the increased LISST-derived SPM total volumes haven’t been observed in most stations during the second cruise,which was quite different from the variation of SPM concentration obtained by seawater filtration.This study suggests that the variability of SPM compositions and properties in estuarine and coastal seawater,as well as the structure,function,and status of the related marine ecosystem,which could be significantly modified by the extreme flood event occurring in the Changjiang basin.（3）Distribution of the parameters related to marine ecosystems in the Changjiang Estuary and the adjacent area in response to the extreme flood.During the July 2016 cruise,an extreme regional flood event occurred in the Changjiang basin.During the same period,two conservative cruises were conducted in the estuary and the adjacent shelf area.The two cruises just covered the period before and after the arrival of the flood（judged by the surface salinity distributions）.POC（%）in SPM,δ¹³C（‰）in POC and chlorophyll-a concentrations were measured both at the surface and bottom layers at each surveyed station during the two cruises,in order to elucidate the influence of flood events on the related marine ecosystems.Compared to same parameters in the first cruise which the seawater was not affected by the flood in quite large scale,more terrestrial materials were transported into the whole study area by the big flood during the second cruise.It was verified by the overall decreased POC（%）values measured in the surface seawaters during the second cruise.The extra terrestrial SPM outputs during the second cruise decreased the seawater transparency and sunlight penetrating,which might further depress the phytoplankton primary productivity and biomass in seawater.Thus,the measuredδ¹³C values and the chlorophyll-a concentrations at the surface generally showed lower values during the second cruise,and at the same time,the coverage area with higherδ¹³C and/or higher chlorophyll-a concentrations decreased.Before the arrival of the flood in the first cruise,the areas with higherδ¹³C values and/or higher chlorophyll-a concentrations at the surface could be found in the area including both northern and southern stations.On the contrary,with elevated Changjiang discharge in the second cruise,the stations with higher values ofδ¹³C and/or higher chlorophyll-a concentrations could generally only be found in the northern part of the study area,which was in the same transport direction of the intensified Changjiang Diluted Water（CDW）.The above result suggested that the extra terrestrial materials（such as nutrients）transported by the CDW could still help stimulate the local phytoplankton production and biomass even with the overall high turbidity in seawater to some degree.This study may further help elucidate the coupling mechanism between the extreme flood event occurring in the river basin and the corresponding status,structure,and dynamics of the marine ecosystem in the adjacent estuarine and coastal areas.