中文摘要：

对1999年和2009年夏季珠江口的温、盐、溶解氧、叶绿素和径流量等数据进行分析。结果表明，由于1999年航次期间珠江较大的径流量，1999年冲淡水的扩展范围要大于2009年。1999年夏季珠江口水体存在较强的盐度层结，而2009年夏季受较小的珠江径流量和热带风暴“浪卡”的共同作用，水体盐度层结较弱，0—5m层水体垂向混合良好。两年夏季珠江口表层溶解氧（D0）的分布较为相似，而底层溶解氧的分布则有显著差异，主要表现在河口西岸浅滩区和河口口门外。1999年河口西岸浅滩区DO小于3mg．L-1，河口口门外DO在4—5mg．L-1之间，2009年河口西岸浅滩DO大于5mg．L-1，河口口门外DO小于3mg．L-1。1999年夏季伶仃洋海域内较强的盐度层结是其底层出现低氧环境的主要原因，2009年夏季强化的垂向混合使得表底层溶解氧得到了及时交换，阻碍了西岸浅滩区底层低氧环境的出现，而盐度层化和“浪卡”带来的浮游生物的耗氧作用可能是口门外底层出现低氧环境的原因。

英文摘要：

By analyzing the temperature, salinity, dissolved oxygen （DO）, chlorophyll, and river discharge data observed in the summers of 1999 and 2009 in the Pearl River Estuary （PRE）, we find that the spreading range of the freshwater plume in 1999 was larger than that in 2009 due to larger runoff from the Pearl River. The salinity profiles showed a strong stratification inside the PRE in 1999, but a weak stratification in 2009, owing to the co-effect of the smaller river discharge and stronger winds associated with the tropical storm ＂Nangka＂, which makes the water column well mixed in the top 5 m. The DO distribution patterns for the two years were similar near the surface, but were quite different at the bottom, especially in the western shallow area and outside the mouth of the PRE. In 1999, the DO concentration was less than 3 mg.L-1 in the western shallow area and between 4-5 mg.L-1 outside the mouth of the PRE; but in 2009 it was greater than 5 mg.L-1 in the western shallow area and less than 3mg.L-1 outside the mouth. Strong stratification in the PRE was the main reason for the low concentration of bottom DO in the western shallow area in 1999. The intensified vertical mixing induced by ＂Nangka＂ facilitated the exchange of DO between the surface and bottom waters, which prevented the formation of low concentration DO in this region in 2009; however, the salinity stratification and consumption of oxygen by the phytoplankton blooms triggered by ＂Nangka＂ might be the reasons for the low DO content in the bottom water outside the mouth of PRE.