The ecosystem of the Three Gorges Reservoir has remained relatively stable for a long time. Which of the following aspects is necessary?

General situation of fluctuation in three Gorges reservoir area

The fluctuating zone of the Three Gorges Reservoir consists of two parts: the fluctuating zone in the reservoir area and the buffer zone of the fluctuating zone. Among them, the water level fluctuation zone is the water level fluctuation zone formed when the Three Gorges Reservoir operates according to the water level scheme of 175- 175 m after impoundment, with an area of about 300 km 2. However, the buffer zone of the water-level fluctuation zone is a non-submerged area that affects and protects the ecological function of the water-level fluctuation zone, that is, a belt-shaped area with a width of about 30m at the altitude of 175. These two parts together constitute the fluctuating zone of the Three Gorges Reservoir, and their response to the operation of the Yangtze River directly affects the water quality of the Yangtze River.

The fluctuating zone of the Three Gorges Reservoir is mainly the bank slope, alluvial flat dam, terrace and floodplain of the main tributaries of the Yangtze River, and some areas are canyons. Most of them are land, which is an ecosystem of nature, economy and society [l].

After the impoundment of the Three Gorges Reservoir, the water level fluctuates periodically in the fluctuating zone in the form of alternating winter water and summer land, which is contrary to the natural law of flood and drought. The original terrestrial ecosystem disappears with the fluctuation of water level, and animals and plants that can adapt to the new environment will gradually survive and multiply, forming a new ecosystem, but the process will be very long.

The area and fluctuation range of water level of the Three Gorges Reservoir are the highest among the built reservoirs in China. Many immigrant towns are adjacent to the water-level fluctuation zone, and some urban groups are surrounded by the water-level fluctuation zone, which has become an important part of immigrant survival and regional economic development.

In the fluctuating zone, tributaries and steep canyons are the most widely distributed, endangering the stability of reservoir banks and the safety of towns. At the same time, the water-level fluctuation zone is also a region where human activities are frequent and intense, and the ecological and environmental problems that may be caused will be the most concentrated and prominent.

2 Possible ecological and environmental problems in the water-level fluctuation zone

After the Three Gorges Reservoir 175m impoundment, the water level fluctuation zone has been completely formed, and the original terrestrial ecological environment has undergone fundamental changes, which may cause a series of ecological environment problems, directly affecting and endangering the ecological environment safety of the Three Gorges Reservoir, the overall ecological landscape and tourism environment of the Three Gorges Reservoir Area, the Three Gorges International Golden Tourism Zone, the production and living environment of the broad masses of people in the reservoir area, especially the healthy habitats of aquatic and terrestrial organisms, and the sustainable utilization of aquatic and terrestrial landscape resources in the water level fluctuation zone. Specific performance has the following aspects:

1) biodiversity is reduced and ecosystems are degraded.

Because the water level fluctuation in the water level fluctuation zone violates the laws of natural drought and flood, on land, the climate is hot and humid, there are many heavy rains and frequent droughts, so most of the original terrestrial plants are difficult to adapt to the habitat and die out, migrate or mutate. In the natural state, the reconstruction and regeneration of the water-level fluctuation zone ecosystem will be a long process. Therefore, in a certain period of time, the biodiversity of organisms decreases, the types of ecosystems decrease, and the structure and functions tend to be simplified.

2) Soil erosion and geological disasters will increase in a short time.

The violent fluctuation of water level will make the steep slope soil lose in the first few years of reservoir impoundment, especially the abandoned mines (quarries), farmland, dams and canals without reinforcement measures, which may collapse and landslide under the action of water flow or surge. Geological disasters induced by reservoirs are also inevitable.

3) Urban sewage, industrial wastewater and non-point source pollution affect the safety of reservoir water quality.

In the range of water level fluctuation zone, there are four main factors that affect the safety of reservoir water quality: first, the water quality of the reservoir is degraded due to incomplete cleaning before impoundment, and the residual pollutants and harmful substances in the soil are dissolved during impoundment; Second, after the operation of the reservoir, the discharge and treatment of urban sewage and industrial wastewater are not timely and thorough, which will pollute the reservoir water. In the tributary backwater area, the water body may be eutrophic due to rich nutrients, and in severe cases, "water bloom" will appear; The third is rural non-point source pollution. Due to the agricultural economic level, agricultural structure problems and overloaded land use in the reservoir area, domestic garbage, residual fertilizers, pesticides and crop residues enter the reservoir water body, causing water pollution. The fourth is the secondary pollution of water body formed by aquaculture.

4) It may form a hotbed of various epidemics.

The Three Gorges reservoir area is a high incidence area of various infectious diseases, natural focus diseases and endemic diseases. Because the impoundment operation of the Three Gorges Reservoir will change the ecology and environment of the water-level fluctuation zone and the reservoir bank zone, the pollutants accumulated in the water-level fluctuation zone may become the derivative sources of bacteria, viruses and mosquitoes and flies. If the pollution is aggravated, it may create conditions for the spread of infectious sources of natural diseases, insect-borne diseases and water-borne diseases and the generation of vectors. It is easy to induce the occurrence of primary disease.

5) The ecological landscape has changed and the tourism resources are facing reorganization.

After the impoundment of the Three Gorges Reservoir, the vegetation in the water level fluctuation area is becoming sparse, showing a phenomenon similar to "desertification". The Three Gorges landscape corridor, which was originally amazing to the world, is affected, and the water level fluctuation area with structural defects is in obvious contrast with land and reservoirs. The redistribution and characteristic changes of tourist attractions also bring new challenges to the development and utilization of tourism resources.

3. Ecological function and function of water level fluctuation area

Reservoir water-level fluctuation zone is an important wetland ecosystem between aquatic ecosystem and terrestrial ecosystem, which is influenced by land-water system. As a typical inland freshwater river wetland, the fluctuating zone of the Three Gorges Reservoir will include seasonal floods, meadows, swamps and open water bodies [3]. It should reflect the positive ecological function and function in the following aspects [4]:

1) As the last barrier of reservoir water quality safety, the water level fluctuation zone can retain a large amount of sediment and non-point source pollutants caused by land-shore soil erosion, and reduce reservoir siltation and pollution;

2) The wetland ecosystem with vegetation as the main body can decompose and absorb nutrients in the water in the reservoir area, thus reducing the eutrophication level in the reservoir area;

3) The diversity of the water level fluctuation zone and the universality of its distribution create diverse habitats for organisms, and the huge food chain supports rich biodiversity, forming "biological parks" and "biological corridors";