Dissolved oxygen (DO) is not only a fundamental parameter of coastal water quality, but also an indication of organics decomposed in water and their degree of eutrophication. There has been a concern about the deterioration of dissolved oxygen conditions in the Minjiang River Estuary, the longest river in Fujian Province, Southeast China. In this study, the syntheses effects on DO was analyzed by using a four year time series of DO concentration and ancillary parameters (river discharge, water level, and temperature) from the Fuzhou Research Academy of Environmental Sciences, at three automated stations along the Minjiang River Estuary. Hypoxia occurred exclusively in the fluvial sections of the estuary during the high temperature and low river discharge period and was remarkably more serious in the river reach near the large urban area of Fuzhou. Enhancement of respiration by temperature and discharge of domestic sewage and industrial wastewater, versus regeneration of waters and dilution of pollutant concentration with increased river discharge, which regarded as the dominant antagonist processes that controlled the appearance of seasonal hypoxia. During the high temperature and the drought period, minimal mainstream flow above 700 m3·s−1, reduction of pollutants and forbidding sediment dredging in the South Channel should be guaranteed for strong supports on water quality management and drinking water source protection.

The Minjiang River, a typical tidal channel in Southeast China, plays an important role in the supply of drinking water, flood control and drought relief, farming and navigation, as well as shipping and other functions. Dissolved oxygen (DO), as a basic living condition for aquatic biota, has been deteriorating in the Minjiang River in recent years. In order to understand how the spatial distribution of DO responds to river discharge, nutrient loading and water temperature, a three-dimensional Environmental Fluid Dynamics Code model was used to simulate water age and the distribution of DO in the Minjiang River. The model presented in this paper was used for water resource and water quality simulations under various physical, chemical, and biological scenarios. Sensitivity simulation results indicated that the three factors had a significant impact on the spatial distribution variation of DO in the Minjiang River. Increased river discharge or split ratio of the North Channel resulted in decreased water age and increased DO. Increased nutrient loading and water temperature caused lower DO. In order to protect coastal environments in the Minjiang River, river discharge should be increased and pollutants of local cities should be reduced during the high temperature and drought period.

In 2008, a simple punitive eco-compensation method was implemented in the Taihu pilot region, China. However, due to the use of a flawed formula and weak compensation criterion the payments were considered unsuitable. To improve the scheme, the following issues were considered: determination of compensation criterion; compensation when water quality is acceptable; consideration of reciprocating flow; control of the errors in pollutant fluxes due to the non-synchronization of river flow and water quality data. Two alternative ways to calculate eco-compensation payments were assessed for a case study in 2013: a payment based on the “Water quality exceedance rate (WQER) method” was found to be 172 million CNY (24.9 million USD). This method avoided errors caused by the pollutant flux and considered the situations of reciprocating flow and acceptable water quality; and the “Pollutant treatment cost (PTC) method” was considered suitable for immediate implementation, although the payment was higher at 245 million CNY (35.4 million USD). The determination of compensation criterion using this method had a scientific basis, but it required perfect and reliable monitoring data. If these conditions are met, the method was considered suitable for future implementation.