1 Temporal and Spatial Distribution of Dissolved Oxygen in the Pearl River Estuary and Adjacent Coastal Waters

2 The Pearl River is one of the large rivers in the world and it discharges to the northern part of the South China Sea. There has been a concern about the deterioration of dissolved oxygen conditions in the Pearl River estuary and adjacent coastal waters. In this study, historical data on dissolved oxygen (DO) from 1980, recent data from a summer cruise in 1999, and a 10-year time series in DO for 1990-2000 were used to examine spatial and temporal distribution of DO in the Pearl River estuary and adjacent coastal waters near Hong Kong. In the adjacent coastal waters, low oxygen waters <4 mg l-1 occurred in large areas during the summer of 1981, but DO rarely dropped to <3 mg l-1. In the Pearl River estuary, DO was 3.5-4 mg l-1 in the eastern part, but was>4 mg l-1 in the western part in August 1984. In July 1999, DO was < 4 mg l-1 in a near bottom 2 m layer in a large area of the estuary and was <2.5 mg l-1 in the eastern section, just inside the entrance of the estuary. In the coastal waters adjacent to Hong Kong, DO was> 4 mg l-1. The 10-year time series showe