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Hypoxia has become a prevalent phenomenon in the plain river network region. To reveal the causes of hypoxia in these regions, the Sihu Canal in the Hanjiang River Basin, one of China's most important freshwater aquaculture areas, was selected as a case study. The spatiotemporal variations in water quality, including dissolved oxygen（DO）and nutrients, were analyzed for the period 2010~2023, and the spatial distribution of nutrients in water and sediments were investigated. The impact of parameters such as water temperature, ammonia nitrogen, and flow on DO levels in the water was evaluated using a Random Forest model. The results indicated significant seasonal fluctuation in DO levels, which exhibited a 'V'-shaped pattern throughout the year. DO concentrations were relatively low during flood seasons, while during non-flood seasons the requirements for Class III surface water quality were generally satisfied. In 2021, severe hypoxia（DO<2mg/L）was observed, with the annual hypoxic days amounting to 79,116, and 96 at the Yunlianghu, Xinhecun, and Xintan sections respectively. Evident hypoxic zones were identified in the mid- and upstream sections of the Sihu Canal, where DO concentrations ranged from 2.61 to 3.22mg/L. From 2010 to 2023, the water quality of the Sihu Canal consistently ranged from Class IV to Class V, with occasional further deterioration recorded. The main parameters exceeding the standards were identified as DO, permanganate index, ammonia nitrogen, and total phosphorus. The total nitrogen and phosphorus contents in the sediments ranged from 857.70 to 2846.87mg/kg, and 545.99 to 2475.59mg/kg, respectively, indicating that the sediments were subjected to mild to moderate pollution, with tributaries being more polluted than the main canal. High accuracy in predicting DO levels was demonstrated by the Random Forest model, which yielded an R2 of 0.995 and an RMSE of 0.2085. Water temperature had a relative importance exceeding 35% in influencing DO levels, followed by pH, ammonia nitrogen, conductivity, turbidity, and flow. To mitigate the hypoxic conditions during flood seasons, it was recommended that the systematic management of the basin be strengthened, the water quality of shrimp-rice and aquaculture drainage systems be improved, and the operation and scheduling of pump stations be optimized.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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