Eutrophication of water body refers to the overall effect that lakes, rivers, reservoirs, bays and other slow-moving water bodies receive too much nutrients such as nitrogen and phosphorus, which leads to a large number of algae and aquatic organisms in the water body, which leads to the decline of water transparency and dissolved oxygen, the death of fish and other aquatic organisms and the deterioration of water quality. When the water body is eutrophic, a large number of fast-propagating algae surface. Due to the photosynthesis of algae, a large number of bubbles are emitted, and clusters of algae resemble flowers. In lakes, reservoirs and other water bodies with relatively slow water flow, this phenomenon is called "water bloom" or "splash". In the Gulf, this phenomenon is called "red tide". It is the result of the deterioration of water bodies.

(2) Conditions of water eutrophication

Eutrophication of water body is the result of interaction between biological reproduction ability of water body and environmental conditions of water body.

There are about 20 ~ 30 kinds of nutrient elements in water, among which C, N and P are essential elements for the growth of aquatic organisms. The atomic ratio of C∶N∶P in algae is 106∶ 16∶ 1. The carbon required for algae growth mainly comes from the atmosphere, and algae such as cyanobacteria also have the functions of fixing atmospheric nitrogen and increasing nitrogen in water, so N, P, especially P in water become the main limiting factors of water eutrophication. There are both inorganic and organic forms of nitrogen and phosphorus in water, among which inorganic nitrogen and phosphorus can be directly absorbed and utilized by plants. At present, it is generally believed that when the total P concentration in water is greater than 0.02mg/L and the total N is greater than 0.2mg/L, water can be regarded as eutrophication.

The abundance of nitrogen and phosphorus in water is the basis of water eutrophication, but whether it can form "bloom" or "red tide" depends on environmental conditions such as pH and water temperature of water and hydrological conditions such as area, shape, depth and fluidity of water. In shallow lakes and reservoirs with sufficient nutrients such as nitrogen and phosphorus, weak alkaline water quality, slow water flow and high transparency, it is easy to cause a large number of cyanobacteria to multiply and form "water blooms". When seawater containing dissolved organic matter enters the bay, and the light is strong, the water temperature is high and the seawater is relatively stagnant, it is easy to cause algae to multiply in large quantities, and then form a "red tide".

(C) ways to prevent water eutrophication

Eutrophication of water bodies in China is serious. According to the investigation and statistics of Jin Xiangcan (1992, 1999) on 25 lakes in China, the total nitrogen content of almost all lakes exceeded the eutrophication index, and the total phosphorus content of 92% lakes exceeded the standard, exceeding 10 times or exceeding 10 times. The main ways to prevent water eutrophication are: ① reducing the input of nutrients such as nitrogen and phosphorus into water. At present, the pollution sources of lakes and reservoirs in China are mainly point pollution (industrial wastewater and domestic sewage) and non-point pollution (soil erosion, farmland runoff, etc.). ). All kinds of wastewater should be treated to meet the discharge standards. At the same time, we should strengthen the scientific utilization and management of agricultural land, reduce soil erosion, and control the amount of N and P substances entering lakes, reservoirs, swamps and other water bodies from the source. (2) Injecting gas into water or adding chemical phosphorus-reducing flocculant to reduce dissolved phosphorus in water and improve the balance of oxygen in water. At the same time, clean up the sediments and sediments with high phosphorus content in the water body to avoid the inorganic or organic phosphorus that has been precipitated from being released into the water again under anaerobic conditions.