The focal area of this study is the Pearl River Estuary(PRE). In this study

the surface partial pressure of carbon dioxide pCO

2

temperature

salinity and dissolved oxygen (DO) were determined by underway measurement system during four field surveys

and their seasonal and spatial variations were shown. Significant seasonal variations of （pCO

2

）was observed in the upstream region of PRE

 pCO

2

in summer was the lowest at the same salinity

medium in spring and fall

the highest in winter

which indicated that the weatheringderived dissolved inorganic carbon was diluted in wet season and sea water was more close to upstream region in dry season. High pCO

2

and low DO occurred in the upstream region during four seasons

andpCO

2

and DO showed a strong negative linear correlation

and aerobic respiration was the most important factor in modulating high pCO

2

. Especially

the maximum of pCO

2

was 717.2 Pa and the minimum of DO was 41.4 μmol/L when salinity was 0.19‰ in spring. Pearl River runoff discharged an amount of nutrients into nearshore waters and promoted primary production in broad region in spring and summer. Phytoplankton photosynthesis produced oxygen into sea water and removed carbon dioxide from sea water. The lowest pCO

2

16.5 Pa and the highest DO

418.8 μmol/L existed in the western of Hong Kong where salinity ranged from 15‰ to 25‰ in summer.