| As the world’s third largest river estuary, the Yangtze River Estuary has receivedhuge amount of terrigenous organic carbon carried out by the Yangtze River runoffevery year, which has brought a great influence on the ecology and environment ofthe estuary and adjacent sea. Especially, the rapid development of economy in theestuarine delta and the construction of water conservancy projects in the upper andmiddle reaches of the watershed have exerted complicated effects on thebiogeochemical processes in the estuary. It has great significance to reveal the role ofshelf marginal seas in the global carbon cycling and clarifying the biogeochemicalprocesses in the Yangtze River Estuary that spatial-temporal distribution of dissolvedorganic carbon (DOC) and particulate organic carbon (POC), their transportcharacteristics and influencing factors were studied in the estuary. Based on the datafrom the seven cruises carried out in November2007,2009,2011and February, May,August, November2012, the spatial-temporal distribution of DOC and POC and theirseasonal and interannual variations were analyzed as well as their correlations withthe environmental factors. And the origin of POC and the contribution ofphytoplankton to POC were analyzed during different seasons in2012combined withC/N ratios and chlorophyll a (Chl a). The transport characteristics of organic carbonwere discussed by means of the runoff and sediment load into the estuary. The mainresults as follows:(1) The concentrations of DOC in the Yangtze River Estuary ranged from0.53mg/L to5.21mg/L in2012and the average content was (1.86±0.61) mg/L, theseasonal variation presented a trend of the highest values in autumn, followed bysummer and winter, and the lowest in spring. The spatial distribution of DOCdemonstrated that the higher values appeared in the near-shore zones and on the surface, the lower DOC was in the off-shore areas and on the bottom, which hadsubtle difference between different seasons. The relationship between DOC andsalinity was very complex, and presented distinct figures in different seasons. Therelation of DOC and apparent oxygen utilization (AOU) were stronger in spring andsummer than that in autumn and winter. DOC and oxygen-demanding organic carbonhad the same source in some degree as the favorable correlationship between DOCand chemical oxygen demand (COD), some of DOC may origin from the runoff andsewage discharge along the river. DOC and Chl a appeared positive correlation inautumn and winter, but weak negative correlation in spring and winter. DOC and POChad different relations in different regions of the estuary, they presented positivecorrelation in the inner mouth zones and near-shore areas, but negative correlation inoff-shore areas except summer.(2) The contents of DOC varied between0.66mg/L and12.40mg/L with anaverage value of (3.55±1.78) mg/L in autumn from2007to2012, and showed adescending trend in these years. DOC concentrations in the surface of the four yearspresented remarkable difference, which were similar to the bottom except2011and2012. The spatial distribution of DOC presented a trend of the higher values appearedin the near-shore and surface layer, but the lower appeared in the off-shore and bottomlayer, which indicated terrigenous inputs had a significant impact on DOC in theestuary. The spatial distribution of DOC in the four years had some differences. Therewere no prominent differences in DOC concentrations of the inner mouth zonesexcept the near-shore and off-shore areas. The poor correlations between DOC andAOU demonstrated biological activity played a negligible role in the distribution ofDOC. The ratios of oxygen organic matters in DOC were increased as thecorrelationship between DOC and COD enhanced from2007to2012. Thecontributions of phytoplankton production on site to DOC could be ignored becauseof the poor correlations between DOC and Chl a.(3) POC concentration in the estuary ranged from0.16mg/L to34.80mg/L in2012and the average content was (2.66±4.48) mg/L, seasonal variation presented thehighest values were in summer, followed by spring and autumn, and winter was the lowest. POC contents of summer in surface layers appeared prominent differenceswith spring, autumn and winter, and there were no significant differences betweenspring, autumn and winter as well as POC contents in the four seasons of2012inbottom layers. Spatial distribution of DOC present a trend of the higher valuesappeared in the coastal areas and the bottom layers, the lower appeared in theoff-shore zones and the surface layers, general trend of the distribution of POC indifferent seasons were basically identical. POC revealed non-conservative behavior inthe estuary. Suspended solid matter was the dominating influencing factors to POC.The main source of POC was terrestrial inputs, and organic debris was the mainexistence form of POC. Quantitative analysis showed the biomass of phytoplanktonmade small contribution to POC in the estuary.(4) The contents of POC varied between0.03mg/L and16.95mg/L with anaverage value of (2.30±2.77) mg/L in autumn from2007to2012, and the bottomwere higher than the surface. Interannual variation of surface POC concentration wasmore significant than bottom. POC contents in the inner-mouth zones and near-shorewaters were significantly higher than that of the off-shore. POC concentration in theinner mouth zones remained relatively stable, while in the near-shore and off-shorewaters showed significantly interannual changes. Runoff and sediment dischargeplayed the crucial roles on POC distribution in the estuary. The influencing areas ofrunoff were outlined in the inner mouth and near-shore waters, especially in themaximum turbidity zone. Similarly, the turbidity maximum zone and the north watersof the estuary were attributed to the main influencing regions of sediment discharge.Sediment discharge as the driving factor was dedicated to POC concentration in thewhole estuary waters. But, there were the diverse factors which impacted on POCconcentration in the partitioning region: deposition and resuspension of sedimentsdetermined the POC distribution in the inner mouth area, while in the near-shore,POC concentration shown high reliance on the terrigenous sediment input, andprimary productivity became main controlling factor of POC distribution in theoffshore.(5) DOC flux and POC flux in the estuary in2012were similar to the seasonal variation of the runoff and sediment discharge of the Yangtze River, meanwhile, theinterannual of DOC flux and POC flux in the autumn from2007to2012were alsosimilar to that of the runoff and sediment discharge of the river, indicating DOC fluxand POC flux were mainly controlled by the runoff and sediment discharge of theYangtze River, respectively. On the whole, DOC flux of the Yangtze River Estuaryappeared a trend of increasing from the year of2003, which may relative to the rapidgrowth of the organic pollutant leaded by the rapid development of economy. Thepercentages of POC in the suspended matter that transported to the estuary in the fourseasons were0.9%,0.7%,0.6%and0.4%, respectively. With the increase of TSMconcentration, POC%rendered a trend of index decrease, POC%growed quicklywhen TSM concentration lower than100mg/L,400mg/L,210mg/L and80mg/L inspring, summer, autumn and winter, respectively. DOC: POC ratios and TSM existedcollaborative changes in the estuary. When TSM contents were higher than117mg/L,123mg/L,222mg/L and335mg/L in spring, summer, autumn and winter,respectively, POC was the main expression form that transported into the sea,otherwise, it was DOC. |
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