Surface Co₂ In The Yellow Sea And Buoy Co₂ In The South Atlantic Bight

Although the coastal oceans play a disproportionate important role in the ocean carbon cycling, an affirmative conclusion whether they are sinks or sources relative to atmospheric CO2 still could not be achieved due to their high spatial and temporal variability. Accordingly, a large number of CO2 studies in the coastal oceans are initiated based on the traditional shipboard investigations as well as the autonomous buoy CO2 measurement. Therefore, surface water CO2 in the Yellow Sea (YS) together with buoy CO2 in the South Atlantic Bight (SAB) was studied to further complement the China coastal CO2 study and to set an example for the buoy CO2 study, which will be soon conducted in China coastal oceans.Data used in the dissertation are from nine cruises conducted in the YS and the buoy in the SAB. During each cruise, surface water CO2 partial pressure (pCO2) and associated physical and biogeochemical parameters (including temperature, salinity, wind, chlorophyll a and dissolved oxygen etc) were collected. In this dissertation, several scientific problems are attempted to be resolved:(1) what are the main pCO2 controlling processes in the YS? (2) is the YS a sink or source of atmospheric CO2? (3) does the continental shelf pump (CSP) hypothesis that was proposed in the East China Sea operate in the YS? (4) does the high resolution CO2 data collected from the buoy have significant implications in assessing the pCO2 controlling processes and estimating air-sea CO2 fluxes? Below are the results and conclusions obtained:There are significant seasonal and spatial variations in the YS surface pCO2.The nearshore area in the southern Yellow Sea (SYS) was almost always oversaturated with respect to the atmospheric CO2. In contrast, the central SYS in winter and fall was oversaturated, while in spring (April and May) was undersaturated. In summer (July), surface pCO2 was undersaturated in the Changjiang Diluted Waters (CDW), and oversaturated in the area beyond the reach of Changjiang water. In the northern Yellow Sea (NYS), surface pCO2 in summer was oversaturated over the entire study area with respect to the atmosphere. In contrast, in the other three seasons,pCO2 distribution was complicate. In fall, the central NYS was undersaturated, and the remaining NYS was oversaturated. In winter, the nearshore area was generally undersaturated with respect to the atmospheric CO2, while the area affected by the Yellow Sea Warm Current (YSWC) was oversaturated. In spring, undersaturated pCO2 area only occurred in parts of the central NYS and Lubei coastal area, whereas the remaining area was oversaturated.Surface pCO2 controlling processes in the YS also varied with seasons and regions. In the SYS, the oversaturated p9CO2 in the nearshore area was mainly caused by the terrestrial input and perennial vertical mixing (including the resuspension of mud in the old Huanghe estuary). While in the central SYS, the controlling processes changed seasonally. In winter, the highly oversaturated pCO2 was caused by the combination of vertical mixing and the intrusion of YSWC. In spring, the undersaturated pCO2 was induced by the onset of spring bloom. In summer, the strong biological CO2 uptake in the CDW area dominated the effect of high temperature, leading to the undersaturated pCO2 signals, while in the region beyond the reach of Changjiang water, pCO2 was oversaturated due to high temperature. In fall, low biological production, high temperature and the break of stratification caused high pCO2. Overall, in the SYS the effect of biology on seasonal pCO2 variations may be larger than temperature.The pCO2 controlling processes in the NYS were different from those in the SYS. In summer, pCO2 in most area of the NYS was controlled by temperature, while high pCO2 in the Lubei coastal area was probably associated with the upwelling of cold bottom water and the resuspension of Huanghe mud; and in the Liaonan coastal area high pCO2 was related to river input and aquiculture. In spring and fall, in most area of the NYS biological activity was responsible for surface pCO2 distribution. In winter, surface pCO2 was mainly controlled by temperature, although affected by biological production in the Liaonan coastal area. In addition, vertical mixing (including the winter vertical mixing and the resuspension of Huanghe mud in the Lubei coastal area) and the YSWC also contribute to surface pCO2. Overall, in most part of the NYS, temperature plays a more important role in seasonal pCO2 variations than biology.The entire YS was a net source of atmospheric CO2, releasingy～8.02 Tg C yr-1 to the atmosphere (1 Tg=1012 g). The SYS was a net sink in spring, and was net sources in other seasons. Overall, on an annual basis the SYS was a net CO2 source of 1.99±0.39 mol m-2yr-1. The topography and associated current system in the SYS do not favor the export of carbon off the shelf, and therefore the CSP does not operate in the SYS. The air-sea CO2 flux in the general East China Sea (including the SYS) where CSP hypothesis was proposed was updated based on our study in the SYS, reducing to be-0.86 mol C m-2yr-1(Tsunogai et al (1999)'s result was-2.9 mol C m-2 yr-1). The NYS was a net CO2 source in each season, although undersaturated PCO2 area occurred in fall, winter and spring. The NYS on an annual basis was a net CO2 source of 0.74±0.12 mol C m-2 yr-1Buoy CO2 data continuously collected in the SAB from 18 July 2006 to 4 November 2007 with a time interval of 3h indicate that surface pCO2 was oversaturated in warm months (April-October), while undersaturated in cold months (Octoberb-March). The contribution from various controlling processes (temperature, biology, mixing and air-sea exchange) to pCO2 temporal vatiation in different periods was quantified using a 1-D diagnostic model. Model results show that temperature and biology are the most important processes, although the dominant process and the relative importance of each process vary with time. This site was a net CO2 sink of 0.34 mol C m-2 yr-1 during the entire study period. In addition, it was found that the uncertainty of air-sea CO2 flux may reach up to 17-20% due to under-sampling (even twice one month). Thus, it is crucial to enhance sampling frequency in the dynamic coastal ocean for reducing the uncertainty of air-sea CO2 flux.