Seasonal variation of DO and formation mechanism of bottom water hypoxia of Changjiang River Estuary

Seasonal variations of Dissolved Oxygen(DO) concentration in the water samples from Changjiang River Estuary and adjacent area were analyzed,which were obtained during 2006 and 2007 cruises.In winter,DO concentration was higher in the outer shelf than that in the coast area and homogeneous in the water column.In spring,water column stratification,algae bloom and the settling organic matter consumed the preformed low oxygen provided necessary conditions for the development of bottom water hypoxia.In summer,there was a "low oxygen zone" in the northern of the front with the minimum value was 2.0 mg/L.In autumn,low concentration patches were observed in the east area of 122.5°E,these maybe the residue of summer hypoxia.Because of the mixed influence of anthropogenic activity and natural variation,the biogeochemistry dynamics and ecology response were complicated.Changjiang plume provided inorganic nutrients for phytoplankton growing,Taiwan Warm Current intrusion offered a low preformed oxygen value,water column stratification prohibited gas exchange between bottom and upper water.On the other hand,the distribution patterns of Particulate Organic Carbon(POC) and Total Organic Carbon(TOC) illustrated that terrigenous organic matter maybe no significant contribution to the formation and development of bottom water hypoxia.