The Physical And Biological Modulation Mechanism Of Dissolved Organic Carbon Distribution In The Changjiang Estuary

Marine organic carbon is one of the most important carbon pools in the world.Its biogeochemistry processes are of great significance to the study of global carbon cycling.The estuary is the transition area from the freshwater end-member to the seawater end-member and the site where the carbon cycles on land and in the ocean interact with each other.The carbon cycle in estuaries is influenced not only by physical processes such as river discharge,tidal processes,and water mass mixing,but also by biotic and abiotic production,degradation and transformation.Under the influence of complex factors,it is of great significance to analyze the specific influence due to physical and biological factors,as well as the temporal and spatial variation and the progression mechanism thereof.At present,there are many studies on the composition and transformation of organic matter in estuary areas.However,the effect of tidal processes on organic carbon in different estuarine regions and the influence of urbanization near estuary areas have not been studied in detail.Likewise,inadequate are studies on the production and transformation of organic carbon under the influence of phytoplankton community structure succession in natural estuarine environment,especially their accurate characterization by numerical simulation.Such research contributes to the in-depth understanding of the carbon cycle in river-ocean interaction regions.With these questions in focus,this paper takes the Changjiang estuary as the research area,analyzing the spatial distribution of dissolved organic carbon(DOC)and identifying the key influencing factors thereof in different regions through the use of the three end-member mixing model.The composition and transformation of DOC and chromophoric dissolved organic matter(CDOM)under the influence of main physical processes(tidal processes and water masses mixing)in the Changjiang estuary from on-site measurements over different tidal periods are studied.The influencing range of dissolved organic matter(DOM)originating from the Huangpu River is traced through numerical simulation.A nutrient-constraint incubation experiment is conducted to investigate nutrient dynamics during a diatom-to-dinoflagellate succession in the late,low-nutrient phase of an algae bloom occurring in the Changjiang estuary and to evaluate its effects on carbon cycling.The diatom-to-dinoflagellate succession occurring in low-phosphate condition and its impact on organic carbon export are characterized and predicted quantitatively by means of numerical simulation.The scientific findings from this research are as follows:(1)The production of organic carbon under different water mass structures in the Changjiang River estuary in summer was quantitatively analyzed using the three end-member mixing model.Based on the salinity and temperature properties of the collected water samples,three end-members are identified,namely Changjiang fresh water,Taiwan Warm Current surface water and Taiwan Warm Current deep water.End-member mixing results indicate that in onshoure water with salinity less than 20,the net DOC variation(ΔDOC)is ca.9% of the observed DOC level in this area,whereas the net particulate organic carbon variation(ΔPOC)is ca.11% of the observed particulate organic carbon(POC)level in this area.This relatively low net organic carbon variation results mainly from physical processes with influence from the turbidity maximum zone.Biological production,on the other hand,is comparably insignificant.In offshore water with salinity greater than 20,ΔDOC amounts to 11% of observed DOC level and ΔPOC 51% of observed POC level.Nutrient consumption and organic carbon production are apparent and under the influence of significant biological processes.High chlorophyll fluorescence and POC% values suggest that the primary production regulates nutrients and organic carbon dynamics.Under different physical environment conditions,the production of organic carbon is closely related to water mass composition and environmental conditions.In the turbidity maximum zone,the desorption of particulate matter contributed to the addition of DOC.In the upwelling-influenced region,organic carbon production was strongly regulated by the nutrients supplied by the TWCDW except for the riverine silicates.In the stratified river plume region,nutrient uptake and OC production were controlled by the contributions of all three end-members(2)Tidal processes have significant effects on the distribution and transformation of dissolved organic matter(DOM)in the Changjiang estuary.The observation during the spring and neap tides in the south branch of the Changjiang estuary showed there were no significant differences in concentrations of DOC and CDOM in different tidal periods.The overall distribution of DOC and CDOM decreased with the increased salinity,and the outer areas exhibit gradually lower concnetrations than the inner areas.In the upstream of the freshwater-saltwater interface with salinity less than 1,due to microbial degradation in sediments and DOM released from pore water influenced by strong mixing processes,the concentration of DOC and CDOM varied over a wide range and were greater in spring tide than that in neap tide.In the downstream of the freshwater-saltwater interface with higher salinity,the concentrations of DOC and CDOM were near the conservative mixing line,exhibiting no significant differences between the spring and neap tides.Due to the different sources of DOM,the properties of CDOM were quite different in the North Channel and the South Channel.DOM in the North Channel was mainly controlled by Changjiang River,and the CDOM was characterized with a high concentrantion of humic-like components,while the CDOM in the South Channel was characterized with a high protein-like components composition and high DOC concentration due to the pollutant discharge from the Huangpu River.The distribution of DOM from the Huangpu River in the Changjiang estuary is investigated by releasing a tracer at the mouth of the Huangpu River.The results indicate that the DOM from the Huangpu River primarily affect the South Channel,with the South Passage being influenced more than the North Passage.Strong tidal process and lateral water transport which occur during spring tide may force a part of the DOM originating from the Huangpu River out of the South Passage over the North Passage,resulting in the influence from DOM from the Huangpu River on the North Passage being more significant during spring tides than during neap tides.The DOM from the Huangpu River becomes quickly dulited by the large Changjiang River discharge.An analysis on the carbon flux at the north and south passages indicates that the concentration of DOC from the Huangpu River at the exit of the South Passage is respectively 14.25% ± 3.12% and 17.55% ± 4.13% the DOC in water masses in Huangpu River in spring and neap tides,whereas these values for the North Passage are 4.36% ± 1.20% and 5.93% ± 3.97% respectively.(3)The organic carbon dynamics during a diatom-to-dinoflagellate succession were characterized by a nutrient-constraint incubation experiment.The incubation successfully induced a transition from a diatom bloom dominated by Skeletonema costatum to a dinoflagellate bloom dominated by Prorocentrum donghaiense.Dissolved inorganic phosphorus(DIP)constitutes the primary nutrient constraint in this succession process.In the diatom-dominate stages,the consumption rates of dissolved silicate(DSi)and dissolved inorganic nitrogen(DIN)are related to the DSi/DIN structures in the water and decrease with the decay of diatoms.The production of organic carbon is regulated by phytoplankton communities and nutrient structures.In the later phases of the incubation,the exhaustion of available nutrients gives rise to a higher POC ratio in the organic carbon produced in the diatom phase than in the dinoflagellate phase.DOC production is related to nutrient structure.The concentration ratio of phytoplankton carbon biomass to chlorophyll a is affected by both the growth phase of phytoplankton and nutrients,being higher in decay phase and under low nutrient concentrations but exhibiting no significant differences under different phytoplankton community structures.As the succession progresses,the biological availability of DOM increases as a result of the interaction of phytoplankton community structure,nutrients and microorganism community.Compared to the growth phase of phytoplankton,DOM released in decay phase have a higher protein-like composition,although the composition does not vary significantly among different community structures.(4)Based on the culture experiment,an ecological box model applied in DIP-limited condition was established to characterize the phytoplankton succession process and the resulting organic carbon output.Based on existing model-based research,the measure of direct utilization of dissolved organic phosphorus(DOP)by phytoplankton and the dynamic carbon-to-phosphorus ratio(C/P)is introduced,which contributes to a good simulation of the diatom-to-dinoflagellate succession process in the natural environment.Numerical experiments indicate that when DIP is inadequate,the direct utilization of DOP significantly affects phytoplankton growth.The introduction of this parameter adequately represents the growth characteristics of phytoplankton in low-DIP conditions.When in a low phosphorus concentration,phytoplankton adjust their internal C/P depending on the actual phosphorus concentration in the water mass as a means of adaptation.This model is able to represent the diatom-to-dinoflagellate succession in a realistic environment and can furthermore exhibit the C/P value in phytoplankton under different nutrient conditions.This is superior to traditional models which struggle to represent the growth of diatoms and dinoflagellate in low-phosphorus conditions and estimate phytoplankton carbon export.This paper’s study on the DOM in Changjiang estuary reveals that the mixing of organic matter from land and ocean in the estuary is influenced by physical processes such as water transport,water mass compopsition and tidal processes as well as biological processes of phytoplankton.The combination of chemical analysis and numerical simulation has provided an in-depth and quantified understanding of carbon flux in estuaries and its controlling factors.