Knowledge of temporal and spatial variations of nutrient supply and POC export rates is of great importance for understanding the processes controlling the cycles of carbon and associated elements in the ocean. There are a few methods that allow for the in-situ assessment of POC flux and new production. However, discrepancy has often been observed by oceanographers in the results derived by the various methods. In this thesis research, in-situ nutrient supply and POC export rates in the upper water column were determined using the naturally occurring radionuclides Ra-228, Th-228 and Th-234 in the South China Sea ¨C the largest marginal sea in the world, which is an important but under study area.

Investigations in a time series station and an intercalibration station were conducted in the southern South China Sea during the winter cruise in 1997 and the spring cruise in 1999. Vertical distributions of Ra-228 in the upper water column favor the steady-state assumption. By developing a one-dimensional single source model and a one-dimensional two-source model, Ra-228 was used to estimate the eddy diffusion coefficient. The vertical diffusivities derived at the time series station and the intercalibration station were 8.6, 6.6 and 2.0 m^2 d^-1, respectively. Upward fluxes of nitrate into the euphotic zone were then calculated with coupled Ra-228/nitrate approach and further converted into the new production of 5.5, 5.0 and 4.4 mmol C m^-2 d^-1, respectively, using a Redfield ratio 6.6 for C:N.

POC export rates were determined based on the Th-234/U-238 disequilibria and measured POC concentrations. Results at the time series station showed that POC export rates in the euphotic zone had varied dramatically over a relatively short time span of a week or two, from 46.5 to 13.1 mmol C m^-2 d^-1. Meanwhile, the measurement at the intercalibration station showed that POC export flux was ~5.7 mmol C m^-2 d^-1.

POC export flux was also determined at the intercalibration station using Th-228/Ra-228 disequilibria. The Th-228-based POC flux was estimated to be ~1.7 mmol C m^-2 d^-1. DOC transport and/or accumulation, data uncertainties and different time scales to which the various methods were applied could be the potential factors that caused the discrepancy between these estimates at the intercalibration station. The vertical fluxes of nitrate and phosphate to the euphotic zone bear a N/P molar ratio of 35, which is significantly higher than the Redfield ratio and suggests a P-limitation in the southern South China Sea.