Why does the number of cyanobacteria in Taihu Lake increase rapidly with high temperature and low water level?

The main cause of cyanobacteria outbreak in Taihu Lake is eutrophication.

Eutrophication in Wuxi section of Taihu Lake is closely related to air temperature and water temperature, and there are also geographical particularities.

The water quality in Shangyou Lake has deteriorated. In recent years, the protection of lakes in the upper reaches of Taihu Lake is not enough, and the lake ecosystem is seriously degraded. Taihu Lake lost its first natural protection barrier, which directly led to an increase in the total amount of pollutants entering from the northwest of Taihu Lake, and the algae content in most waters of Taihu Lake was at a very high level.

The temperature of the lake is higher than that in normal years. The annual maximum water temperature in Taihu Lake appears in July and August, and the annual minimum water temperature appears from the end of 65438+February to the beginning of February. The annual maximum water temperature reaches 38℃ and the minimum water temperature is 0℃. The annual water temperature is between 29.5℃ and 38.0℃, with an average range of about 34℃. The average water temperature is 17. 1℃, which is higher than the land temperature. The winter of 2006 was the warmest winter since the meteorological record in Wuxi, and the average daily temperature in the whole winter was more than 2℃ higher than that in previous years, which caused cyanobacteria not to freeze to death in a large area and accumulated conditions for the outbreak in 2007. Following the "warm winter", the temperature has been rising continuously since the spring of 2007, which provides suitable conditions for the wild growth of cyanobacteria. From June 5438 to April 2007, the accumulated water temperature in Taihu Lake was higher than the average for many years, especially in April, which was the highest in the last 25 years. Since April 25, the water temperature in Taihu Lake has been above 20℃, and May is also the highest month in history. All these are beneficial to the growth of algae.

The water level in Taihu Lake is very low. From June 5438 to April 2007, due to continuous drought, the water level in Taihu Lake dropped sharply, and the water level in Taihu Lake was already very low, so it quickly triggered a flood, causing all the dirty things below Taihu Lake to float up. Taihu Lake has been at a low water level, with an average water level of 2.94 meters in four months, which is lower than the highest accumulated water temperature in the past 25 years in the same period of 2002 13.4 cm. The extremely low water level increases the concentration of pollutants, further leading to the deterioration of water quality.

Wind direction concentration effect of southeast wind. Meiliang Lake is a large lake bay extending from Taihu Lake to land. When the southeast wind blows, almost all the cyanobacteria in Taihu Lake gather here. An expert once said that 100 square kilometers of Meiliang Lake is like a relatively static water "pocket". As soon as the south wind blows, it becomes a gathering place for cyanobacteria in Taihu Lake. In fact, Meiliang Lake in Wuxi has concentrated cyanobacteria from Taihu Lake, Zhejiang and Shanghai, and the water intake of Wuxi City in Taihu Lake is here, so the outbreak of cyanobacteria has a great impact on the drinking water of Wuxi citizens.

There is little precipitation. The lack of rainfall weakens the purification ability of water itself and further worsens the water quality. In addition, the ecological dredging project in Taihu Lake caused the water body to be unable to flow, and Taihu Lake was almost like water, which aggravated the disaster of cyanobacteria.

However, the above-mentioned phenomena that lead to the deepening of water eutrophication are not naturally formed, and the large-scale outbreak of cyanobacteria is not the fault of cyanobacteria. The chief culprit of Taihu Lake pollution is human beings. Since the end of 1980s, the main pollutants in Taihu Lake, such as total nitrogen and total phosphorus, have seriously exceeded the standard, with obvious eutrophication, and the contents of mercury compounds and COD in some areas have also greatly exceeded the standard. This is mainly caused by agricultural non-point source pollution, industrial point source pollution and domestic sewage discharged into Taihu Lake.

The primary cause of eutrophication in Taihu Lake is rural non-point source pollution. Non-point source pollution refers to dissolved and solid pollutants flowing into receiving water bodies from non-specific places, including rivers, lakes, reservoirs and bays, and causing eutrophication or other forms of pollution. Non-point source pollution is more related to rural areas. Excessive and unreasonable use of pesticides and fertilizers in rural areas of China, livestock manure produced by small-scale livestock farming, untreated agricultural production waste, rural domestic garbage and wastewater are all direct factors that cause non-point source pollution.

In addition, in recent years, Taihu Lake in Wuxi and other places regards chemical industry as a pillar industry, which makes large and small chemical, electroplating and printing and dyeing enterprises in this area spring up like mushrooms after rain. Thousands of polluting enterprises line up along Taihu Lake, and sewage is discharged directly into Taihu Lake. The township industry in Taihu Lake basin is quite developed, accounting for less than 0.4% of the national land area and about 1/8 of the national gross national product. The upstream region failed to contain its "GDP complex", unilaterally pursued economic benefits and neglected the remediation of pollution sources around Taihu Lake.

As the efficient and rapid economic development model attracts a large number of migrants, the population density of Taihu Lake Basin reaches about 1 10,000 people per square kilometer, which is one of the regions with the highest population density in the world and the highest urbanization level in China. With the acceleration of urbanization and the increase of foreign population, the amount of urban domestic sewage has increased rapidly, and a large number of domestic sewage has been discharged into Taihu Lake without exception.

Originally, the lake was a large natural system, but for a long time, people developed and utilized it without restraint, which led to systematic problems in the lake. The dialectics between man and nature shows that the rude use of nature will inevitably be retaliated by nature. The relationship between man and nature has been seriously unbalanced, and nature has begun to retaliate against the overuse of human beings. Compared with rivers, lakes need to "recuperate" more. The ancient Taihu Lake has nurtured countless children around it. It's time for them to give up their greed and let her recuperate.

Reflecting on my hometown of Wuhan, the data of the fifth census (2000) showed that the average population density in Wuhan was 947 people per square kilometer, which was 132 more than 1990. According to this growth rate, the population density of Wuhan should have exceeded per square kilometer 1000 people, which is equivalent to the situation in Taihu Lake Basin. The urbanization rate of Wuhan is 82.86%, which is also quite high, and the proportion of floating population is also very large. However, unlike Taihu Lake Basin, the highest population density in Wuhan is Wuchang, with a population of 0/2020 per square kilometer. Wuchang is the education center of Wuhan, where college teachers and students account for a large part of the population. As far as cities are concerned, the fastest growing population is also college students. But this can't reduce the pollution level in Wuhan. Among the peers I grew up with in Wuhan, there are many people with upper respiratory diseases; Although a large number of factories have moved to the suburbs, this can only lead to the accelerated evolution of the pollution level in the suburbs to the urban areas, and there is little change in the urban areas. I have also seen the green Yangtze River water, but it is not the same as the green of Taihu Lake. Small fish and tadpoles at the bottom of the river are clearly visible. I have drunk bitter tap water, and sugar can't hide the pain, but it was caused by an emergency, and it will be fine in less than a day. Now, I often see water blooms in large and small lakes in Wuhan. Almost all lakes have water blooms, but I have never seen any signs of improvement. I often hear the news that the lake has turned over in a large area, but it happens almost every year. Playing with the small fish in the river, those happy childhood days have become permanent memories, and I have never seen those fish in the river since I grew up. Maybe my peers and I are the last generation with this memory! -What a sad fact! As the sons and daughters of the Yangtze River and the Han River, guarding the province of thousands of lakes and the county of hundreds of lakes, it is high time to be alert.

Some people suspect that the outbreak of cyanobacteria in Taihu Lake is related to the paddy fields around the lake, but it is not. Paddy field is the largest artificial wetland ecosystem on the earth and plays an important role in regulating regional climate. The research shows that under the same level of soil organic carbon, paddy soil can keep more nitrogen and phosphorus, which can not only significantly improve soil fertility, but also significantly reduce the pollution of nitrogen and phosphorus to the surrounding environment. Therefore, rice will not cause the outbreak of cyanobacteria, but will limit the wild growth of cyanobacteria, so it should play a significant role in carbon fixation, emission reduction and cooling and air purification. However, the traditional concept of planting rice around Taihu Lake needs to keep pace with the times. On the basis of scientific planning of rice field layout, the rice production goal of "high yield and higher yield" will be changed to "high quality and safe production", so as to effectively reduce pesticide input, innovate protective rice cultivation techniques, actively develop environmental protection protective rice cultivation techniques, and create a win-win situation of economic benefits and environmental protection.

In addition, there is a potential factor that has long been forgotten-the ecosystem food chain. Taihu Lake is rich in 48 species of fish, among which whitebait and whitebait are famous economic fish. In the 1960s, the traffic between Taihu Lake and the Yangtze River was blocked by building dikes and dams around the lake, which reduced the number of silver carp that could have swam into Taihu Lake. Silver carp is a kind of fish that eats algae and is a natural nemesis of cyanobacteria. Silver carp can't enter Taihu Lake, a link in the food chain is missing, and cyanobacteria lack ecological natural enemies. It's only a matter of time before they grow wild. It can be seen that man-made water conservancy projects laid the curse of today many years ago. Everything in nature has its own laws, among which there are many subtle connections that we humans can't see clearly. This is the greatness of nature and the smallness of us human beings. If man is omnipotent, what does God need to do? The existence of Babel is a warning to all living things-nature created you, so you will never be stronger than nature.

"Water bloom" is a natural ecological phenomenon in fresh water. Most water blooms are only caused by algae, such as cyanobacteria, green algae and diatoms. Some water blooms are also caused by zooplankton flagellates. When a water bloom occurs, fresh water is usually blue or green.

In the natural ecosystem, nutrients such as nitrogen and phosphorus are the limiting factors of plant growth, that is, the short board of barrel. If the content of nitrogen and phosphorus is greatly increased, that is, after eutrophication, the growth of plants will be lifted and then multiply in large quantities. Protophytes (mainly phytoplankton) at the bottom of the food chain reproduce the fastest, and they quickly occupy a lot of resources, thus limiting the growth of other plants and becoming the dominant population. In fresh water, the most common organism causing water bloom is cyanobacteria. Because algae use light energy as energy, photosynthesis will produce oxygen, so when algae grow in large quantities, the dissolved oxygen on the water surface is saturated or even supersaturated; But at the same time, cyanobacteria will also form a thin film on the water surface, blocking the entry of sunlight and air. Moreover, when a large number of algae die, due to the process of decay and degradation, a large amount of dissolved oxygen will be consumed in the water, resulting in a serious shortage of dissolved oxygen in the water, which is in an anaerobic state and a serious odor in the water. In addition, in order to inhibit the growth of other organisms, algae will release a toxin-lake indigo, which is toxic to fish, destroys the diversity of organisms in the water, leads to the increase of biological corpses and residues in the water, greatly increases the organic load, and produces a lot of nitrogen and phosphorus, further deepening the anaerobic decomposition of microorganisms, thus forming a vicious circle. Therefore, after freshwater eutrophication, water blooms will appear frequently, the area will spread year by year, and the duration will be extended year by year.

The biggest harm caused by water bloom is that drinking water sources are threatened. Microcystins affect human health through the food chain. MCRST, a secondary metabolite of cyanobacteria bloom, can damage the liver, promote canceration and directly threaten human health and survival. In addition, the filtration device of the waterworks will be full of algae, and the algae floating on the water will affect the landscape, and the water will have an unpleasant smell and so on. Therefore, every time a water bloom occurs, it will bring great losses or disasters to human beings and nature.

The most important factor leading to water bloom is eutrophication of water body. The main reason of water eutrophication is that a large number of chemical fertilizers are applied along the water area, and a large number of domestic sewage and industrial wastewater are discharged into rivers and lakes, which leads to the increase of nitrogen, phosphorus and potassium contents in rivers and lakes. In addition, water temperature, pH value, light intensity and other factors will affect the degree of water bloom. 20 ~ 30℃ is the suitable temperature range for water bloom. The sudden rise of water temperature above 2℃ in a week is a precursor of water bloom. Under suitable conditions such as nutrients, the pH value of around 8 can promote the occurrence of water bloom. If a water bloom occurs, even if the initial pH value is not conducive to the growth of algae, the adaptability of algae can adjust the pH value of water body to an alkaline range suitable for growth through a series of physiological and biochemical reactions. The stronger the light intensity, the more beneficial it is to the photosynthesis of algae, thus promoting the reproduction of algae and greatly improving the reproduction speed of algae. When the water bloom occurs, the water area is characterized by drought and little rain, sultry weather, high water temperature, weak wind and slow tide.

On May 6th, 2007, cyanobacteria broke out in Taihu Lake. This "frequent visitor" of Wuxi citizens, who is harassed every year, came earlier this time and came with greater force. Wuxi urgently opened the pumping station of Changshu Water Control Project to carry out emergency water transfer in the Yangtze River. 16, the water quality of Meiliang Lake turned black. On the 22nd, Li Xiaowan Waterworks stopped supplying water. On 25th, the water quality of Gong Hu Waterworks still met the water supply requirements. On the 28th, the water quality of Gong Hu Waterworks deteriorated seriously, the water source stinked and the water quality turned black. Dissolved oxygen decreased to 0mg/L, and ammonia nitrogen index increased to 5mg/L per liter. Residents' tap water smells bad. Since the 29th, the water quality of tap water has become sticky.

Since then, Wuxi citizens can't drink tap water, even wash their faces and take a bath. Citizens can only buy pure water to solve the drinking water problem. In the old city, citizens began to use well water for first aid. Wash your face, take a bath and brush your teeth in your daily life. You must endure the pungent smell and keep your nose away from your hands. Some citizens left Wuxi to avoid the bad smell.

In order to ensure that residents can drink pure and delicious tap water, the water supply company spends 6 cents per ton of water to remove algae and deodorizer, and needs to treat 1 10,000 tons of water every day. This alone costs more than 60,000 yuan per day. Moreover, it is difficult to completely remove the odor with chemicals, and adding water purification agent, activated carbon and strong oxidant has no effect.

The landscape, fishery and shipping industry in Taihu Lake basin will be hit hard and the economic losses will be huge.

emergency measures

Facing the sudden outbreak of cyanobacteria in Taihu Lake, Wuxi Municipal Government has taken a series of emergency measures: First, it has increased the supply of "diverting water from the Yangtze River to Taihu Lake" to dilute the eutrophic water quality of Taihu Lake. As of June 3, * * * has diverted 430 million cubic meters of water from the Yangtze River, of which 246 million cubic meters have been introduced into Taihu Lake; Second, urgently invite domestic cyanobacteria control experts to discuss relevant countermeasures to improve the water quality of Taihu Lake; Third, pay close attention to the biochemical changes of tap water quality in order to respond positively. In addition, the gate connecting Taihu Lake and inland river in Wuxi city was opened, and a number of closed deep wells were opened to supplement domestic water for the citizens in case of emergency. The municipal government organized manpower to salvage cyanobacteria, strengthened monitoring, purified water as soon as possible, and stabilized water prices.

In order to alleviate the blue-green algae disaster in Taihu Lake, in view of the low rainfall, the relevant departments carried out rocket artificial precipitation enhancement in Taihu Lake area on the afternoon of May 3 1 and the early morning of June 1, and achieved obvious precipitation enhancement effects. In view of the public's water shortage, the government has opened a number of closed deep wells to supplement the public's domestic water in an emergency.

On May 3 1, Professor Tsinghua University Zhang Xiaojian and his assistant came to Wuxi. Experts found that it takes two and a half hours for the 7-kilometer-long water pipeline from the water intake of Taihu Lake to the water treatment plant. If a large amount of potassium permanganate oxidant is put into the water intake, most of the odorous substances can be oxidized in more than two hours. Putting powdered activated carbon into the water plant can remove the remaining odor substances and decompose the remaining oxidants. After repeated experiments for 17 hours, at 7: 40 a.m. on 1 day, the odor in the effluent of the water plant was successfully eliminated. This key breakthrough has brought hope for solving the drinking water crisis in Wuxi. On the evening of June 1, Wuxi began to restore normal water supply, but it was not until the old water in the water pipe was used up that water could be used. On the morning of June 3, the health departments of Jiangsu Province and Wuxi City monitored and compared the water quality in factories and residents' homes. The water quality of tap water is stable and qualified, and the sensory index, general chemical index, toxicology index and bacteriology index of all water plants have reached the national GB 5749-85 water quality standard. In addition to drinking and cooking, citizens still rely on pure water, and other domestic water is normal.

processing method

It has been a long time since the history of pollution control in Taihu Lake, but the water quality in Taihu Lake has gone from bad to worse. One of the reasons is underestimating the possible anthropogenic pollutant emissions. In the 1990s, the goal of "clearing Taihu Lake in 2000" was put forward. At that time, it was estimated that only 540 million cubic meters of industrial wastewater entered rivers and lakes every year, and the annual discharge of domestic sewage was only about 320 million cubic meters. In 2000, the amount of pollutant discharge increased five times, and the monitoring data showed that the point source pollutant discharge in Taihu Lake Basin was 5.33 billion cubic meters. The second reason is that the water resources protection and water pollution control in Taihu Lake Basin are characterized by the basin, involving water conservancy, environmental protection, construction, agriculture, fishery, transportation and other departments, and must be comprehensively controlled by the basin. Taihu Lake involves two provinces and one city, and the cities around the lake must join hands to manage it together, otherwise it will be impossible to manage Taihu Lake well. The third reason is that the current situation of cyanobacteria outbreak is not formed in one day, and it takes a long ecological cycle to control it. If a new water plant is to be built, the cycle of site selection, water intake scheme, technical design and construction is also quite long.

From the ecological point of view, as early as 65438+ 10 before the outbreak of cyanobacteria, some experts proposed to raise fish in Taihu Lake to prevent cyanobacteria. It is estimated that silver carp or silver carp can "kill" 40 to 50 kilograms of cyanobacteria per 1 kg of body weight. A 50-gram silver carp fry will be caught ashore when it grows to about 1.5 kg in Taihu Lake. From this calculation, a silver carp fry can eat about 60 kg of cyanobacteria from releasing the fry to fishing ashore.

The key laboratory of ion beam bioengineering, Hefei Institute of Material Science, Chinese Academy of Sciences has developed an organic sewage treatment agent. When it is evenly added into a cup of sewage, nitrogen, phosphorus and algae can be flocculated and precipitated in 3 ~ 5 minutes, and then pollutants can be completely removed by magnetic separation technology. The treated water body reaches the national secondary discharge standard. According to experts, in the past, some water treatment products could only concentrate and precipitate pollutants and could not be removed. After a long time, pollution often breaks out again. The organic sewage treatment agent they developed is made of several non-toxic and harmless industrial wastes, such as fly ash and chitosan, and magnetic materials such as magnetic powder, which are modified by physical and chemical methods to make powder or aqua master batch. Experiments show that this kind of water treatment agent coagulates with eutrophic lake water, which first produces flocs, and then extends to magnets to adsorb and remove nitrogen, phosphorus and algae. This new achievement can completely eradicate nitrogen, phosphorus and cyanobacteria by adding magnetic separation technology, which makes up for the shortcomings of some water treatment products in the past.

In view of the outbreak of cyanobacteria, experts proposed spraying diluted clay slurry into the lake, and using the cohesive adsorption of clay to algae cells, the cyanobacteria sank to the bottom of the water to control their growth and spread. At the same time, adding activated carbon to raw water can also adsorb and precipitate cyanobacteria.

Deng Chunhui, an associate professor in the Department of Chemistry of Fudan University, conducted a research entitled "Removal of Microcystins by Adding Copper Ions to Magnetic Nanomaterials". Studies have proved that copper ions can combine with microcystins through coordination. By attaching copper ions to magnetic nano-materials, magnetic nano-materials attached with microcystins can be easily extracted by magnets, thus achieving the purpose of separating microcystins from water. In the laboratory stage, the separation rate of this method has reached more than 90%.

In order to prevent and alleviate the problem of water eutrophication, three students, including Hu Huan from the Department of Environmental Science and Engineering of Fudan University, invented the "Pollution Control Method of Terrestrial Plants in Hydroponics", which provided a brand-new solution. In this method, two common garden plants, bergamot and Guanyin lotus, are planted in the seriously eutrophic water body to absorb the redundant nutrients in the water body to the maximum extent.

Algae is an organism that can grow as long as there is sunlight and water, and can be converted into fuel oil. Scientists began to launch a plan to convert fuel oil with algae. Vegetable oil extracted from algae can be converted into biodiesel, which can be used as fuel for almost all diesel engines. Therefore, some people think of using cyanobacteria to make biofuel.

The nitrogen content of cyanobacteria is as high as 10%, and the phosphorus content is nearly 1%. 1 m3 cyanobacteria can produce 4 kg of nitrogen, which is equivalent to 8 kg of urea. Combined with the income from biogas production, the total income from comprehensive utilization of cyanobacteria in 1 m3 can reach 50 yuan. They have successfully made cyanobacteria into high-quality biological fertilizer, which can replace chemical fertilizer well. At present, this achievement is about to be piloted in Taihu Lake. Once the project is finally successful, it can solve the fertilizer supply problem of 400 thousand mu of land around Taihu Lake. The water body is eutrophic, but the surrounding farmland lacks nutrition. This treatment itself reflects the unscientific and unreasonable situation of agricultural production in China.

Subsequent treatment

Real-time monitoring of cyanobacteria. Jiangsu Bureau of Surveying and Mapping and Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences jointly developed the "Dynamic Monitoring and Early Warning System of Cyanobacteria Bloom in Taihu Lake". Through satellite remote sensing space, geographic information technology and lake sampling, the relationship model between remote sensing spectrum and cyanobacteria biomass was established. After the system is completed, it will dynamically monitor the occurrence, development and spatial distribution changes of cyanobacteria bloom, judge the growth trend of cyanobacteria through uninterrupted satellite image data output, make forecast and early warning, provide timely water quality information for the construction of new countryside in the lake area, and provide benign management and technical services for agriculture, fishery production and people's domestic water use.

Harness pollution sources. Zhou Tie Town, Wuxi, adjacent to Taihu Lake, covers an area of less than 73 square kilometers, but there are more than 400 industrial enterprises, including more than 50 highly polluting enterprises/kloc-0. After years of rectification, no effect has been achieved. After the outbreak of cyanobacteria in 2007, Wuxi will close 772 enterprises with high consumption, high pollution, high risk and low efficiency before the end of 2008. In Yixing, Jiangsu, all new projects of nitrogen and phosphorus emission were stopped, and nearly ten such old enterprises closed down and switched to production according to the new environmental protection threshold, although they met the previous emission standards. Jiangsu will close 2 150 small chemical enterprises before the end of 2008.

condition

At the beginning of April this year (2008), cyanobacteria broke out on a large scale in Taihu Lake, one month earlier than in 2007. The lake was like green paint and could write. Taihu Lake is like a dye vat. Everything that falls and is fished out is dyed green. It is undeniable that the water bloom phenomenon has not improved, but has become more serious. However, due to all the preparations, the domestic water consumption of residents has not been affected.

The algae-water separation project is expected to run on a large scale. Wuxi Taihu algae separation station demonstration project is located at the northern end of Meiliang Bay in Taihu Lake, which is one of the key demonstration projects for the harmless treatment and comprehensive utilization of cyanobacteria in Wuxi City. The project introduces algae separation and integration technology and adopts mechanical methods to control cyanobacteria. Mature technology, low consumption, high efficiency, safety and practicality, and the daily treatment capacity can reach 5000 tons of algae-rich water. After the implementation of the project, three major problems, namely, efficient and rapid separation of algae, dehydration of algae slurry and preparation of organic fertilizer from algae residue, can be solved, and a large area of water bloom formed by the outbreak of cyanobacteria can be quickly and effectively removed, with an algae removal rate of over 95%, and a large amount of nitrogen and phosphorus can be taken away, thus reducing the nutritional load of water body and improving water quality. The algae removal water treated by the algae separation station in Taihu Lake can be directly reused, which can improve the transparency and landscape of the water body and create conditions for the ecological restoration of the lake.

impression

A lot of information has been collected and some related knowledge has been consulted, but no control measures have been found for eutrophication itself. Can only wait for the occurrence of water bloom, and then in the process of water use, treatment and purification. It means pollution first and then treatment, and this treatment is not a fundamental treatment. Although it may take a long time to fundamentally manage, there seems to be no solution now, which makes people feel discouraged and sad!

Eutrophication of water body involves two environmental problems at the same time, namely environmental pollution and ecological destruction. In the above research, these two aspects are mentioned, but obviously, the social reflection on the outbreak of cyanobacteria in Taihu Lake only sees environmental pollution, but pays insufficient attention to ecological destruction, and does not make a specific study and analysis on the current situation of the ecological environment. But I think it is faster and more effective to solve the problem of cyanobacteria in Taihu Lake by biological or ecological methods.

In addition, the situation in Wuhan and Taihu Lake Basin is also very similar. As a native of Wuhan, I am deeply worried that the story of Taihu Lake will be repeated in Wuhan.