On The Sources,Distribution,and Transport Of Dissolved Organic Carbon In The Esturine And Shelf Area Of China Seas

Dissolved organic carbon(DOC)is one of the largest reactive carbon reservoirs in the ocean,similar in scale to atmospheric CO2,which mediates the microbial food web and interacts with climate on both short and long timescales.The coastal ocean is known to be a highly productive and dynamic regime which connects terrestrial and oceanic systems featuring extremely dynamic organic carbon cycling.It has been a long-standing challenge to constraining the sources,transformation,and transport of DOC in coastal settings,yet this knowledge is essential for understanding the role of coastal ocean in global carbon cycle and climatic change.Taking the Pearl River Estuary(PRE),Northern South China Sea(NSCS)shelf and East China Sea(ECS)as targeted areas,this dissertation investigated DOC distributions,seasonality,and controls along the estuary-coast-deep ocean continuum.Using stable and radioactive carbon isotopic tracers(?13C and ?14C)and employing an end-member mixing model,we aimed to identify various DOC sources,differentiate biological mediation from physical mixing,and(semi)quantify its inter-and cross-shelf transport in the estuarine and shelf systems off China.Carbon isotopes have been widely used to elucidate sources and transformation of organic matter in estuarine systems.Here we presented a comprehensive dataset of ?13C and ?14C of both DOC and particulate organic carbon(POC)in the PRE,as well as in the adjacent bottom hypoxic zone on the NSCS shelf in summer of July,2017.?13CDOC and ?13CPOC in the surface waters varied within a range of-27.2-23.9‰ and-29.9-22.5‰.The 14C age of DOC and POC was 390-2260 and 225-2260 years,respectively.?13CDOC and ?13CPOC simultaneously increased along the salinity gradient,while ?14CDOC generally decreased and ?14CPOC increased from the river water to the seawater.?13C and ?14C signatures of the river water end-member indicate that the Vverine DOC and POC entering the PRE was mainly composed of decadal-aged terrestrial organic matter through deep soil erosion.In the surface waters of lower PRE,identified DOC addition beyond the conservative mixing between river water and seawater was evidenced by both DOC concentrations and carbon isotopic compositions.Both ?14CPOC and%POC(the percentage of POC in TSM)in the bottom hypoxic zone to the west of the PRE indicate that the oxygen-consuming organic matter was largely derived from in-situ phytoplankton production stimulated by high nutrients in the upper water.Based on isotope mass balance calculation,the contribution of terrestrial POC to the non-fossil POC in the PRE was equal to that of in-situ produced POC.Moreover,a three end-member mixing model shows that an additional source of DOC with enriched 14C had been introduced into the hypoxia,which likely resulted from degradation,dissolution,or fragmentation of non-fossil POC according to the isotope mass balance calculation.As a consequence,the remineralization and transformation of sinking biogenic POC,formed by eutrophication-induced in-situ production,was mainly responsible for the oxygen depletion in the summer bottom hypoxic zone on the NSCS shelf off the PRE.The distribution and seasonality of DOC on the NSCS shelf were studied based on a large data set collected from four cruises in spring(May-June 2011),summer(July-August 2012),autumn(October-November 2010),and winter(November-December 2008).Surface DOC concentrations generally decreased offshore from the inner to the outer shelf during all four seasons.The highest surface values of?117?mol L-1 were observed nearshore in summer suggesting high DOC supplies from the Pearl River plume and other local small river inputs,whereas the lowest surface values of?62?mol L-1 were on the outer shelf in winter due to entrainment of DOC-poor subsurface water under strengthened vertical mixing.Vertical profiles generally displayed decreasing DOC concentrations with depth during all four seasons.However,in subregions intensely influenced by winter coastal downwelling,opposite distribution patterns were observed.The coastal downwelling delivered higher surface DOC to the mid-shelf deep waters from the inner shelf,and induced higher DOC in depth than in shallow waters.Combined with net DOC production likely stimulated by high nutrients in the CCC,the total inter-shelf DOC transport by the CCC was estimated to be(10.3±6.4)x1011 g C on the NSCS shelf,which is approximately one order of magnitude higher than the Pearl River input in winter.In a transect extending from the inner shelf near the PRE mouth to the outer shelf,the DOC inventory in winter was estimated to be(1.200±0.005)x109 g C,down from(1.245±0.003)×109 g C in autumn.This winter depression most likely resulted from DOC export out of the shelf to the deep slope and/or basin area through intensified winter downwelling,which would generate a cross-shelf DOC transport of 3.1×1012 g C over a large shelf area.In addition to the variable physical controls,net biological production of DOC was(semi)quantified in both the river plume(2.8±3.0?mol L-1)and coastal upwelling(3.1±1.3?mol L-1)in summer.We demonstrated that the NSCS shelf had various origins of DOC including riverine inputs,inter-shelf transport,and in situ production.Via cross-shelf transport,the accumulated DOC would be exported contributing to the carbon sink in the NSCS.The distributions and seasonality of DOC in the ECS were also examined based on a large data set collected from four cruises in spring(May-June 2011),summer(August 2012),autumn(November-December 2010),and winter(December 2009-January 2010).Surface DOC concentrations were significantly higher inshore than offshore and increased northward along the shelf.The highest surface values of?175?mol L-1 were observed in the Yangtze River plume water in spring due to both terrestrial inputs and biological production.The lowest surface values of?59?mol L-1 were observed at the shelf break in winter resulting from the enhanced wind-induced mixing and the upwelling of Kuroshio subsurface water.By the combination of temperature,salinity,and DOC concentrations,we successfully identified different water masses in the ECS and further illustrated the seasonal variability of DOC therein.DOC through the entire water column on the slope displayed a significant negative relationship with potential density(?t),suggesting the primary control of physical mixing.In the upper layer,however,obvious variations in DOC concentrations at each ?t indicated that other factors,such as biological activities and photochemical processes,would play a role.Compared to DOC profile distributions in the SCS and the Kuroshio Current,an excess DOC was observed above 600 m in the ECS and was probably a consequence of transformation from POC or horizontally transported organic carbon from the shallow shelf.Based on volume transport derived from a three-dimensional ocean circulation model and field measured DOC concentrations,we estimated a net DOC export of 10.6 Tg C yr-1 across the 200 m isobaths out of the ECS shelf,which is comparable to the global average DOC flux from the marginal sea to the open ocean.In summary,both physical and biological processes play important roles in regulating DOC sources,distribution,and transport in the estuarine and shelf systems of China Seas.While rivers input both abundant organic carbon and nutrients,the shoreward transport driven by coastal upwelling and bottom intrusion of boundary current brings offshore deep water with higher nutrients yet lower DOC to shallow shelves.The rich nutrient supply would stimulate biological production on the continental shelves,which would produce more organic carbon than they can respire.On the other hand,the seaward transport driven by coastal downwelling and cross-shelf transport in the boundary current system delivers DOC to the deep and the open ocean,which would escape from rapid microbial degradation and be stored at greater depth for a longer period of time suggesting an effective carbon sink in the framework of the continental shelf pump.As regional work is highly sensitive to global scale estimate of DOC budget,regional studies as demonstrated in this dissertation with seasonal and enough spatial sampling coverage that couple physical dynamics and biogeochemistry are of vital importance.Moreover,the stable and radioactive carbon isotopes were successfully applied to identify organic matter sources and alteration in the hypoxic zone,which is conducive to organic matter management and policy making in mitigating hypoxic conditions.