Production And Degradation Of Dissolved Organic Matter In Coastal Systems

Dissolved organic carbon(DOC)is one of the largest carbon pools in the ocean,equivalent to the CO2 stock in the atmosphere.Thus,subtle changes in the marine DOC inventory would potentially affect the balance between oceanic and atmospheric CO2,thereby affecting the global climate change.A better understanding of DOC production,consumption and transport,is vitally important to studying global carbon cycle.Coastal ocean is known to be the region that connects the land and ocean,in which the biogeochemical processes of DOM are very dynamic due to the the high degree of interplay among physical,biological and chemical processes.In this dissertation,we aimed to examine the bioavailability,production,degradation and transport of DOM in the coastal ocean.First,we chose the South Texas rivers,U.S.A.that entered the estuaries of the Gulf of Mexico to investigate the bioavailability of riverine DOM.Second,we chose the northern South China Sea shelf to investigate the dynamics and production of DOM on the continental shelf under the influences of complicated physical and biogeochemical processeses.At last,we chose the northern South China Sea basin to investigate the exchange of DOM between the marginal seas and the open ocean.Through these case studies,we aimed to better understand the biogeochemical processes of DOM along the river-estuary-coastal ocean continuum,including the characterization and quantification of the bioavailability,production,transformation and transport of DOM in the coastal ocean.River input is one of most important sources of DOM in the coastal ocean.Moreover,the bioavailability of riverine DOM is always related to the metabolism of bacterial and is considered to contribute to the hypoxia and eutrophication in the coastal ocean.We examined the bioavailability of DOM in the five South Texas rivers entering the estuaries of the Gulf of Mexico based on sampling in Jan.and Jul.2016.DOC and DON concentrations differed in the rivers,ranging 221.7?778.8 ?mol/L and 18.4?40.8 ?mol/L,208.5?567.6 ?mol/L and 26.9?44.9 ?mol/L in winter and summer,respectively.Nutrients concentrations were much higher in the polluted rivers than in the pristine rivers.Our incubation experiments indicated that 4.5?11.1%and 14.6?38%of DOC and DON were bioavailable during winter,both of which were higher than those(0%and 8.6%-15%)in summer,suggesting that DOM in the South Texas rivers contained the relatively high fraction of bioavailable DOM in winter.In contrast to the polluted river(San Antonio river),DON was the dominant input of bioavailable nitrogen in the pristine river(Mission river).Based on the analysis of total dissolved amino acids(TDAA)and its components in the rivers,we revealed that the contribution of TDAA to the bioavailable fraction of riverine DOM during winter,while riverine DOM was diagenetically altered during summer.This is consistent with our incubation results,indicating the influence of diagenetic processes in the rivers on the bioavailability of riverine DOM.The mass balance of dissolved nitrogen suggested that riverine bioavailable DON would be transferred to the forms of DIN and PON by estuarine bacteria.Our results indicated that most of the riverine bioavailable DON was decomposed to DIN during winter,while transferred to PON during summer,suggesting higher growth efficiency of bacteria in summer than in winter.Thus,our study also indicated the potentially different effects of riverine DOM bioavailability in the South Texas rivers on the metabolism of bacteria in the coastal ocean..Owing to abundant nutrients carried by the rivers and upwelling,continental shelf is always one of the most productive regions in the global ocean.However,the quantification of DOM production on the continental shelf remains difficult,due to the high degree interplay among physical,biological and chemical processes.Based on a cruise in summer 2008,we examined the dynamics and production of dissolved organic carbon(DOC)on a large continental shelf in the northern South China Sea,which is largely shaped by a river plume and coastal upwelling.The plume water extended from the mouth of the Pearl River Estuary to the middle shelf and was characterized by high DOC concentrations,while the upwelled water occupying the nearshore area featured low DOC concentrations.Biological production of DOC was observed in both the river plume and the coastal upwelling zones with different behavior between regions.The system appeared to be autotrophic in terms of DOC throughout the plume,while in the upwelling circulation,the metabolism of DOC was mixed trophic.Nevertheless,the integrated net DOC production rate of 11.5±6.9 mmol C m-2d-1 in the upwelling zone was comparable to that in the plume(7.1±7.0 mmol C m-2d-1).The net DOC production correlated strongly with net consumption of dissolved inorganic carbon(DIC)and inorganic nutrients,suggesting that the net DOC production was highly coupled to net community production(NCP)in both the plume and upwelling zones.Both regimes had similar DOC/NCP partitioning,with 19-27%of NCP in the plume and 24-26%of NCP in the upwelling zones converted to DOC.A positive correlation was also found between POC and net DIC consumption,with higher POC production in the upwelling zones where large phytoplankton prevailed.Most NCP removal occurred through POC sinking and/or the diffusion and horizontal transport of DOC.The materials exchange between the marginal seas and the open ocean is very critically important for the constraint of the global carbon budget.More importantly,the open ocean is often characterized by the low nutrients and DOM bioavailability,while the marginal sea is characterized by relatively high nutrients and DOM bioavailability.Thus,such exchanges between the marginal seas and open ocean may potentially alter the DOM biavailability,and then influence the metabolism of bacteria therein.Based on a large and high quality dataset of total organic carbon(TOC,an approximation of dissolved organic carbon)collected from three cruises in spring,fall and winter in 2009-2011,we examined the distribution of TOC and its seasonality in the oligotrophic regime of the northern South China Sea(NSCS)as well as its exchanges with the West Philippine Sea(WPS)in the Northwest Pacific Ocean through the Luzon Strait,the only deep channel linking the South China Sea(SCS)and the Pacific Ocean.Wu Kai(2013)for the first time compared the profiles of TOC between the West Philippine Sea(WPS)and the South China Sea(SCS),and concluded that the surface distribution of TOC in the SCS was mainly influenced by the Kuroshio intrusion.In this dissertation,we further quantified the impact of Kuroshio intrusion on the inventory of TOC and the biological production of TOC in the upper 100 m of the northern SCS.Moreover,following the sandwich structure of water exchange through the Luzon Strait,we conducted a first order estimation of the TOC transport fluxes based on the reported cross strait volume transport.The TOC transport flux was-107.1±54.6,54.7±15.0 and-16.4±13.1 Tg C yr-1 at the upper,intermediate and deep layer,respectively.Note that the positive sign means that the flux was from the SCS to the WPS.The SCS is featured by higher DOC production,the exchange of these fluxes with the open ocean interior where DOC would have experienced more degradation would have important implications for both the microbial community in the ocean interior and overall carbon cycle in the SCS.We examined the bioavailability of Kuroshio-derived DOM in the SCS based on the cruise in Nov.2014.Our data confirmed that TOC concentration in the Kuroshio is significantly higher than that in the SCS.The incubation results indicated that the degradation rate of Kuroshio DOC was faster in the experiment groups innoculated with the SCS bacterial than those innoculated with the Kuroshio bacterial,suggesting that the Kuroshio DOC bioavailability increased in the SCS.Meanwhile,our incubation experiments also found that the biodegradtion of Kuroshio DOC was significantly enhanced under the conditions of high level nutrients,suggesting that relatively high nutrients in the SCS might be one of the factors caucing the quick degradation of Kuroshio DOC.The other factors such as the bacterial community structure are necessary to be considered.Therefore,the enhanced biodegradation of Kuroshio DOC would have many implications for the carbon and nutrients cylces in the SCS.In summary,through the case studies in the South Texas rivers,USA and the northern South China Sea,we aimed to improve our understanding on the biogeochemistry of DOM along the river-estuary-coastal ocean continuum.First,based on the incubation experiments and DOM component analysis,we indicated that the seasonal variation of riverine DOM bioavailability in the South Texas rivers,which was associated with the phytoplankton production and bacterial degradation processes in the rivers.Second,although the distribution of DOC on the northern SCS was mainly modulated by the mixing of water masses sourced from the Pearl river plume,upwelled subsurface water and offshore surface water,we semiquantitatively estimated the biological production of DOC on the shelf using the three end-member mixing model.According to the "sandwich" structure of water exchange between the SCS and the WPS,we for the first time estimated the net exchange flux of DOC between the SCS and the WPS using the historical and published water flux through the Luzon Strait and the isopycnal mixing model.More importantly,based on the incubation experiments,we found that the bioavailability of Kuroshio-derived DOM was increased in the SCS,which may influence the carbon and nutrients cycle in the SCS.