Causes of continuous and short-term hypoxia in rivers entering the sea: a case of Minjiang River in Fujian Province

In the last 10 years, the Minjiang River, which is the longest river in the Fujian Province in Southeast China, has been facing a downward trend of dissolved oxygen (DO) and a frequent occurrence of hypoxia. In this study, the development of the continuous and short-term presence of low DO was investigated by using the water age concept and average DO consumption concept based on a three-dimensional Environmental Fluid Dynamics Code in the Minjiang River. The results revealed that the spatial distribution of DO was affected by temperature, runoff, pollution emission, tidal advection, and hypoxic water discharge from the reservoir bottom. The continuous low DO in the water of the North Channel occurred frequently when the enough pollutants were aerobically decomposed faster than the rate of oxygen reaeration during the high temperature and low river discharge period. In addition, the water age and reaeration time decreased with a rapid increase in the water flow from the Shuikou dam when the reservoir capacity was released via drainage. The results of this study provide scientific insights on the mechanism involved in the occurrence of hypoxia and suggest countermeasures for addressing hypoxic problems in estuaries.