The policy “Do not let Wujiang River become polluted river”: removing fish culture make nitrogen retention rate to reduce

Abstract:

Wujiang River is an upstream tributary of the Yangtze River, China, and flows in the Three Gorges Reservoir. The 11 cascade power stations were build in Wujiang River stream, and their hydropower development reserves are 18,745 million kW. The reservoirs are responsible for power generation, farming, and flood control. The 50km-long cages were distributed in different reservoirs with an annual output of 20,000 tons of fish. Feed dosage was 2%-3% of the weight of the fish. The nitrogen content of fish feed was 70%-80%, the remaining feed and fish excrement were imported into the water body of the breeding area, where the dissolved inorganic nitrogen accounts for 58%-78%. The abundance of nutrients also led to the increase of phytoplankton . In 2018 cages of fish farming were dismantled. We took samples in summer 2017 and 2019 in Hongfeng Lake (fish farming was abolished in 2008, first-level reservoir of Maotiao river, the tributary of Wujiang river), Wujiangdu Reservoir and Suofengying Reservoir to compare the effect of fish farm. When fish culture was allowed, compared with reservoir N₂O flux for China of (36 ±51) μmol/(m²·d), which is 3.4-fold higher than previous estimates in worldwide reservoirs of (19~53) μmol/(m²·d)^[1], step reservoirs of Wujiang river N₂O fluxes of (1.05~17.15)^[2]μmol/(m²·d) are about one fifth of those for China. Before and after removing fish farm, nitrogen retention rates were changed from (-20.2~36)% to (5~21)%. Due to the hysteresis compared to rivers, long-term monitoring are required. Because of effects of extreme heavy rainfall and in the absence of large controlled reservoirs of upstream, the weak El Nino and premature rainy season made 2020 China’s floods occur from May in small and medium-sized rivers. Wujiang river was affected by the heavy rain. The cascade reservoir results in the deposition of large particle size particles and the small particle size particles are discharged into the downstream rivers. It is remained to answer which one is larger to effect the N exchange in air-water interface of reservoir. Adding fish feed every day, or ten days of heavy rain?

[1]Li, S., et al. (2018). Water Res 147: 13-24.

[2]Xia L, et al.(2019). EST, 2019, 53(20): 11745-11754.

Figure(A) Vannote R L, et al. (2018). Canadian journal of fisheries and aquatic sciences, 1980, 37(1): 130-137.