| Dissolved oxygen plays an important role in the biogeochemical cycling of seawater. Dissolved oxygen concentration of seawater is affected by both biological and physical processes, whereas dissolved Ar in seawater is affected strictly by physical processes. Oxygen (O2) and argon (Ar) have similar solubility properties. Measurements of the O2/Ar ratio can therefore be used to assess the influence of biological processes on the O2 concentration. Deviations of the O2/Ar ratio from equilibrium concentrations can reflect biological O2 supersaturation. The marine oxygen cycle is tightly linked to the carbon cycle via photosynthesis and respiration. In the absence of physical effects, dissolved O2 constrains the balance of these two biological processes, i.e., net community production (NCP). NCP represents the largest organic quality from surface waters to the deep ocean, however, the NCP in surface water have great spatial and temporal variations. Taking into account of the climate sensitivity and its role in the global carbon cycle, continuous and accurate determination of the NCP is necessary for understanding the global carbon cycle and diagnosis of climate-carbon feedback.In this thesis, we carried out high-frequency, continuous measurements of dissolved gases (O2, Ar and CO2) by membrane-inlet mass spectrometry (MIMS) in the marginal seas of China to obtain the spatial and temporal distributions of O2/Ar, estimate the net community production of the marginal seas of China and discuss the main impacting factors. This research may help us further understand the biological pump effect on ocean carbon sinks. The main research results are as follows:1 The continuous underway measurements of O2/Ar by membrane-inlet mass spectrometry methods have been developed based on the references. The measured precision of O2/Ar, N2/Ar and CO2 is 1.5%,1.6% and 1.9%, respectively, which is comparable to that reported in the literatures.2 Dissolved gases (O2, Ar and CO2) in surface waters are measured using MIMS during a survey on the Yellow Sea and East China Sea in July 2013. O2/Ar ratio showed great variation (5.1-15.6), with a slight decrease offshore. The △(O2/Ar) in the surface waters ranged from -55.6% to 36.0%. NCP ranged from-68 to 1860 mmol C m-2 d-1. Specifically, the mean NCP of Yellow Sea, East China Sea and Yangtze river estuaries in July 2013 are 111±92,165±162 and 259±159 mmol C m-2 d-1, respectively.3 Dissolved gases (O2, Ar and CO2) in surface waters are measured using MIMS during surveys on the Yellow Sea and East China Sea in August and October 2013. O2/Ar ratio ranged from 5.0 to 22.2 in August 2013, with a slight decrease offshore. High surface 02/Ar appeared at the Yangtze River estuaries and the coast water of Zhejiang and Fujian. The A(O2/Ar) in the surface waters ranged from 57.2%to 91.1%, with the average of 5.0±16.9%, indicating the study area is autotrophic. The NCP were estimated to range from -175 to 1902 mmol C m-2d-1,mean NCP of the Yellow Sea, East China Sea (before the typhoon) and Yangtze river estuaries in August 2013 are 92 ±264,13±36 and 166±533 mmol C m-2d-1 respectively. After a typhoon, NCP of the ECS decreased to -24±73 mmol C m-2d-1. In October 2013, the A(O2/Ar) in the surface waters ranged from -31.4% to 22.4%, with the average of-6.1±5.7%, A(O2/Ar) in most of the study area are blow than 0. The NCP ranged from-901 to 430 mmol C m-2d-1, with the average of -154±148 mmol C m-2 d-1. Factors that affecting the distribution of O2/Ar ratios and NCP in Yellow Sea and East China Sea include hydrological structure, sea surface temperature, biological activity and nutrient. In summer, the distribution of O2/Ar showed a similar trends with Chla, and showed a mirror profile of CO2, indicating that the major factors regulating O2/Ar and NCP in this region is biological processes. In October 2013, due to tough weather conditions, air-sea exchange and vertical mixing are very strong, physical processes became the major factors affecting the distribution of O2/Ar and NCP. In such tough weather conditions,O2/Ar ratio normalization is insufficient to eliminate the impact of physical processes, using the O2/Ar ratio method to estimate NCP may susceptible to a significant underestimate.4 Dissolved gases (O2, Ar and CO2) in surface waters are measured using MIMS during a surveys on the South China Sea in June-July 2014. A(O2/Ar) in the surface waters of the SCS ranged from -6.2% to 7.5%, with the average of 2.0±1.7%, indicating the study area is autotrophic. NCP ranged from -265 to 250 mmol C m-2 d-1, with the average of 52±55 mmol C m-2 d-1, and have a similar trends with O2/Ar. Nutrients is probably the major important factors regulating the distribution of O2/Ar and NCP in this region. Physical factors such as the mesoscale eddy and typhoon can enhance the vertical mixing of seawater and bring more nutrients to the euphoric zone, which is beneficial to the growth of phytoplankton, and may have an important impact on the distribution of net community productivity. |
PDF Full Text Request