Constraint of nitrogen behavior on dissolved oxygen deficit in the tidal river
WEN jing, HUANG Bang-jie, HUANG Zhi-wei, FANG Huai-yang, WANG Long-le, ZHANG Ying-min, ZENG Fan-tang, DU Hong-wei
Guangdong Key Laboratory of Water and Air Pollution Control, National Key Laboratory of Water Environment Simulation and Pollution Control, South China Institute of Environmental Science, Ministry of Ecology and Environment, Guangzhou 510535, China
Abstract：The Dongjiang River basin was selected as the study area to monitor the spatio-temporal variations in dissolved oxygen (DO), nitrogen concentration and multi-isotope in the basin, so as to provide a better understanding for the quantitative contribution of nitrogen turnover processes to DO depletion. The DO concentrations ranged from 2.96mg/L to 11.47mg/L in the Dongjiang River. In the spring and summer, the downstream of the river became the low DO hot spot, where the highest value in the apparent oxygen utilization (AOU) was up to 4.8mg/L, and a high correlation between water temperature and Cl- concentration and AOU was observed. However, the accumulation of different nitrogen species and δ15N fractionation of nitrification mainly occurred in the downstream river network area in the winter and spring. Based on the end-members mixing model and isotope Rayleigh fractionation model, it was found that nitrogen mainly reflected the conservative mixing driven by hydrological process in the summer whereas nitrogen prone to occur nitrification in the downstream in the winter and spring. The degree of nitrification reached 14.4% and 32.3% in the winter and spring, respectively, which also corresponded to about 7.5% and 5.9% of AOU in maximum, respectively. Therefore, there were certain limitations in the contribution of nitrogen behavior to the low DO in tidal river. Especially, the oxygen deficit in the summer might be dominated by other factors. Our results will provide a new insight into the limitations of current environmental management pattern in judging whether DO will satisfy the water quality standard through the concentration of ammonia nitrogen, and will also provide a support for exploring and establishing the DO assessment systems focused on distinguishing different regions and watersheds.
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