Water bloom is a natural ecological phenomenon in which algae propagate in fresh water, and it is a feature of water eutrophication. It is mainly due to the fact that cyanobacteria (strictly called cyanobacteria), green algae, diatoms and other algae become the dominant populations in the water body after the wastewater containing a lot of nitrogen and phosphorus in life and industrial and agricultural production enters the water body, which makes the water body appear blue or green after a large number of reproduction. Some water blooms are also caused by zooplankton flagellates. The phenomenon of "water bloom" has been recorded in the ancient history of China. In addition, this phenomenon (usually red) in seawater is red tide.

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Water body coverage: After the eutrophication of fresh water, "water bloom" appears frequently, the area spreads year by year, and the duration is prolonged year by year. There are "water blooms" in Taihu Lake, Dianchi Lake, Chaohu Lake and Hongze Lake, and even in flowing rivers, such as Hankou section in the lower reaches of Hanjiang River, the largest tributary of the Yangtze River. The biggest harm caused by freshwater blooms is that drinking water sources are threatened. Microcystins affect human health through the food chain. The secondary metabolite of cyanobacteria blooms, MCRST, can damage the liver and promote cancer, which directly threatens human health and survival. In addition, the filtration device in the waterworks is full of algae bloom, which affects the landscape and has an unpleasant smell. Therefore, every time a water bloom occurs, it will bring great losses or disasters to human beings and nature. When algae grow in large quantities, these algae often float a layer of blue-green or red-yellow spray or film on the surface of Indigo Lake, which is downwind, and grass, green, silver carp and bighead carp cannot be digested, which affects the growth of fish. Although algae grow fast, they also die quickly because the nutrients in the water are used up. After a large number of algae die, in the process of decomposition, it also consumes a lot of dissolved oxygen in the water, which will rise to the surface of the water to form a layer of green mucus, which will make the water seriously stink. The main reason for the water bloom phenomenon is that a large number of chemical fertilizers are applied along the waters, and a large number of domestic sewage and industrial wastewater are discharged into rivers and lakes, resulting in an increase in the contents of nitrogen, phosphorus and potassium in rivers and lakes. Eutrophication of water body is one of the important factors leading to water bloom. Eutrophication of lakes and other water bodies is still the main water environment problem facing China at present and for a long time to come. It is of great ecological and environmental significance to study the formation mechanism of cyanobacteria bloom in lakes and take corresponding measures to reduce its impact. In order to explore the formation mechanism of cyanobacteria bloom in eutrophic lakes, the research status of the causes of cyanobacteria bloom in large shallow lakes in China and the general understanding of the formation mechanism of cyanobacteria bloom were summarized. The main environmental factors such as chemistry, physics and biology were analyzed, and the possible reasons why cyanobacteria, especially Microcystis, became the dominant species of cyanobacteria bloom were discussed. It is considered that the formation of cyanobacteria bloom needs a comprehensive understanding, and the increase of nutrient concentration may be only one of the factors that people can control. When exploring the cause of water bloom, we should not only study and observe the environmental characteristics of blue-green algae bloom in summer, but also pay attention to the physiological and ecological characteristics of blue-green algae in winter, the ecological inducing factors and their thresholds of spring recovery, and how blue-green algae form groups in the growth process after recovery. And gradually become the dominant species in the lake aquatic ecosystem and even in the process of forming water bloom. It is necessary to have a sufficient understanding of the survival countermeasures of cyanobacteria in winter, the conditions for the formation of cyanobacteria groups, the trigger conditions for the recovery of cyanobacteria in spring and its ecological threshold, and the physiological and biochemical characteristics of cyanobacteria winning in competition with other cyanobacteria groups. The "bloom" of cyanobacteria is an apparent phenomenon, and its premise is that algae have a certain biomass. This is a gradual process. According to the basic theory of ecology and the field observation of the formation process of cyanobacteria bloom, a four-stage theoretical hypothesis of the cause of cyanobacteria bloom is put forward. That is, in a large shallow lake in the middle and lower reaches of the Yangtze River with four distinct seasons and serious interference, the growth and bloom formation of cyanobacteria can be divided into four stages: dormancy, recovery, biomass increase (growth), floating and aggregation, and the physiological characteristics and dominant environmental factors of cyanobacteria in each stage are different. In winter, the recovery process of Spring Festival is mainly controlled by the temperature and dissolved oxygen on the surface of lake sediments. The substances and energy required for photosynthesis and cell division determine the growth of cyanobacteria blooms in spring and summer. Once there are suitable meteorological and hydrological conditions, a large number of cyanobacteria blooms that have accumulated in the water body will float to the surface of the water body for accumulation. Form visible flowers. To study the formation mechanism of cyanobacteria bloom, it is necessary to find the trigger factors or specific factors that lead to the formation of cyanobacteria bloom in each main physiological stage, and to study the physiological characteristics of cyanobacteria in different stages in depth. Only in this way can we gradually understand the formation mechanism of cyanobacteria bloom, predict every process of its occurrence, and seek more targeted control measures. In addition to eutrophication, there are also water temperature, ocean current, PH value of water body, light intensity and so on. Will affect algae and other blooms.