Distribution of hypoxia and pycnocline off the Changjiang Estuary, China

The distributions of hypoxia and the pycnocline off the Changjiang Estuary were investigated by making several field observations from June 2 to 11, from July 18 to 23, from August 20 to 30, from October 3 to 13, 2006, and from August 27 to September 3, 2009. The observations from July 18 to 23, 2006, mainly focused on analyzing the relationship between hypoxia and the extension of the river plume and vertical stratification. In July, the Changjiang diluted water (CDW) was influenced by the easterly typhoon winds, causing it to extend northward rather than northeastward. By using the maximum vertical density gradient as a stratification intensity index, we found that the area of low (<3mg/L) dissolved oxygen (DO) was similar to the area of the pycnocline (>2.0kg/m⁴), which indicated that the summer pycnocline can effectively block vertical DO exchange and maintain hypoxia near the bottom. The observed hypoxic area was 500km², which was much smaller than the hypoxic areas observed in previous studies, and occurred because of the enhanced mixing that resulted from Typhoon Bill. During the observation period of August 20-30, 2006, the maximum density gradient was weaker due to distinct low river discharge. No hypoxia was observed in the eastern and southeastern sea off the Changjiang Estuary where hypoxia often occurs. However, hypoxia occurred over a large area of 15,400km² in the northern observation domain where hypoxia rarely occurs. During June 2-11 and October 3-13, 2006, the maximum density gradient was weaker, and the area with low DO was smaller than in July 2006. This finding resulted from relatively low river discharge and weaker solar heating. Consequently, no hypoxia occurred in the bottom layer. The area of low DO was similar to that of the maximum vertical density gradient. From August 27 to September 3, 2009, high river discharge and strong solar heating produced a larger and more intense pycnocline. The hypoxic area reached 3735km² and was very similar to the area of the pycnocline, which was greater than 3.0kg/m⁴. The seasonal variations of the pycnocline were consistent with those of hypoxia, and the pycnocline played an important role in preserving hypoxic conditions. The seasonal influences of biogeochemical process on hypoxia in 2006 were discussed. The residual current speeds at the bottom were small and favorable for maintaining hypoxia during the summer.