Present Situation of Eutrophication of Landscape Water Body and Its Prevention and Control Measures

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Tianjin Agricultural University, Tianjin 300384, China

Abstract At present, all kinds of pollutants are discharged into the landscape water body, and the landscape water body is mostly static or slow flow state. This leads to the deepening of water pollution, eutrophication, blackening of water body, and stench and flood disasters occur frequently. This makes the urban landscape water lose its ornamental value, affects the effective operation of ecological function, and will also endanger the health of residents. Reclaimed water is one of the common methods of modern landscape water, but reclaimed water contains a lot of nitrogen and phosphorus, which will make the eutrophication of water body increase, and easily lead to "algal blooms", water quality deterioration, and loss of landscape function. Therefore, understanding the current situation of landscape water eutrophication and effective prevention and control measures has become one of the water environment problems to be solved.

Key words Landscape, Water eutrophication, Water environment

1 Introduction

Water eutrophication means that due to the high proportion of nitrogen and phosphorus in rivers and lakes, the expansion growth of algae and other aquatic organisms in water, the decrease of transparency of water and the decrease of oxygen, dead organisms increase and then water resources are corroded. When rotting organisms begin to decompose, they consume a lot of oxygen in water, resulting in a rapid reduction of dissolved oxygen in water[1]. At the same time, it will produce a series of toxic gases, such as methane, hydrogen sulfide and mercaptan, which seriously pollute the water body and lead to the stench of the water body. The harm of water eutrophication is great, it affects the natural circulation and biological balance, and also leads to the reduction of people’s domestic water and agricultural water. The continuous expansion of human activities also leads to the deepening of water eutrophication.

Under natural conditions, the process of water body evolution is called natural water eutrophication, and human factors will make its process greatly accelerated[2]. At present, the eutrophication problem which received extensive attention refers to artificial water eutrophication and chemical pollution. According to the relevant data, most of the lakes in China have eutrophication. From 1978 to 1980, 3.2% of the lakes were in the state of poor nutrition, 5.0% in the state of eutrophication, and now there are as high as 80% of lakes in the state of eutrophication.

The emergence of eutrophication naming is related to the study of nutrient classification and lake evolution. Weber put forward the concept of poor nutrition and lake eutrophication. Current studies have found that the slow increase in nutrients in lakes should not be called eutrophication, or it can be called natural eutrophication. Only a sudden and rapid increase in nutrients can be called true eutrophication, which is the usual study[3]. The problem of water eutrophication belongs to the category of water pollution, so it actually refers to "man-made eutrophication". From 1900 to 1970, during the eutrophication of Lake Eric, a large quantity of nutrients discharged into Lake Eric was equivalent to the sum of nutrients over the past 10 000 years.

The phenomenon of eutrophication is mainly manifested in the large-scale reproduction of phytoplankton. Due to the emergence of phytoplankton of different colors, the water surface is often blue, red, brown, milky white and so on, this phenomenon is called "algal blooms". It’s called a red tide in the ocean. Therefore, lake eutrophication is mainly the dominant position and reproduction process of some phytoplankton in the water body.

2 Harm

In recent years, with the accelerating process of urbanization and the continuous expansion of urban area, many water systems that were not in the urban area have become an important natural landscape of the city with the expansion of urban area. The transportation function of water system is weakening or even losing, while the ecological value and purification function of water body in the process of urban construction is becoming more and more obvious. At the same time, in order to meet the growing environmental demand of urban residents, artificial landscape water body has become an indispensable part of beautifying the urban environment and purifying the urban environment. Especially in the construction of large parks and beautiful residential areas, it has many important functions, such as proper distribution of trees in scenery, and ecological purification. However, while pursuing a higher quality of life, water eutrophication is becoming more and more serious. People’s daily activities also aggravate the degree of eutrophication of water body.

Nitrogen and phosphorus are nutritional elements to promote plant reproduction; under the promotion of human activities, the two begin to enter a large number of water bodies. They mainly appear in lakes, rivers, reservoirs and slow-moving waters, promoting the proliferation of plants in the water, algae growth and death, resulting in serious water eutrophication.

The harm of water eutrophication is mainly manifested in the following aspects: (i) Destroying the ecological landscape. Urban rivers are the main place for contemporary people to have fun. In the case of eutrophic water, aquatic organisms such as fish are killed because of lack of oxygen. If not cleared in time, dead fish and other aquatic organisms will rot on the surface of the water body, making the water body stink, turbid, not conducive to human and mental health, thus reducing the social benefits of the water body[4]. (ii) Reducing species diversity and stability in aquatic ecosystems. It is difficult for sunlight to enter the water covered by algae, which inhibits the photosynthesis of plants and reduces the content of dissolved oxygen in the water. In addition, dead algae continue to sink to the bottom of the water, accelerating the consumption of oxygen at the bottom, resulting in the anaerobic state of water below the surface and the death of aerobic organisms. (iii) Endangering human life. Algae proliferate explosively in eutrophic water. Many algae, such asMicrocystisandAnabaena, secrete algal toxins, causing mass deaths, mutations in fish eggs, and animals such as birds, cattle and sheep. When these animals and some shellfish products in contaminated water are eaten, they can cause chronic poisoning in humans. Toxic by-products, such as trihalomethane formed during chlorination, also endanger human safety. (iv) Causing economic losses. When eutrophic water is used as a drinking water source, in order to ensure water quality, the complexity of the water plant operation process will increase the cost of treatment. At the same time, eutrophication water will corrode the pipe network in the process of transportation, increasing the difficulty of transportation[5].

After the emergence of eutrophication, a large number of propagators spread on the water surface, blocking the spreading of light to the bottom of the water, hindering the photosynthesis of lower aquatic plants and reducing the release of oxygen. When algae get bigger because of a lot of nutrition, they die on a large scale. When the body is broken down by microbes, it consumes a lot of oxygen. As a result of these two effects, the concentration of dissolved oxygen in water decreases, resulting in the death of animals, especially fish[6]. Some blue-green algae also release chemicals that are toxic to fish and humans, causing the death of a large number of aquatic organisms. If eutrophication is severe, anaerobic conditions will be formed at the bottom of the water, and the substance in the water will be reduced by the action of anaerobic bacteria. This will produce harmful gases such as H2S，NH3，CH4, the peculiar smell of algae itself, and lead to the complete deterioration of water quality, the destruction of water ecological structure, the rupture of biological chain and degradation of other water functions.

The green algae formed in eutrophication water body lead to the decrease of water transparency and the decrease of landscape value. Dianchi Lake is a famous plateau lake in China. It was originally the receiving organization of drinking water source and sewage in Kunming. In the 1990s, because the water body was polluted, Dianchi Lake could only meet the requirements of irrigation water quality, the growth thickness and coverage area of water hyacinth increased year by year, the phenomenon of blue-green algae appeared, losing the glory of the past.

If the reservoirs, rivers, lakes, urban industrial and domestic water on which people depend are polluted or eutrophicated, organic matter is increased in the water and algal toxins are produced. This allows water phytobacteria to survive in rivers and lakes. The aquaculture environment has endangered human health and caused a variety of diseases. Due to serious water pollution, six of Guangzhou’s eight water plants have been relocated.

3 Influencing factors

3.1NutrientsofnitrogenandphosphorusThe eutrophication of water body is usually considered to be the excess of total phosphorus (TP) and total nitrogen (TN), which leads to the growth of a large number of phytoplankton and the outbreak of algal blooms. When the content of nitrogen and phosphorus in water is low, eutrophication will not occur, on the contrary, the high content of nitrogen and phosphorus in water will not necessarily cause nutrition. In the water system, it contains a certain amount of nitrogen, phosphorus and other nutrients, but the content is low, the natural remediation ability of the water body can be controlled at a relatively stable level without causing pollution. The relationship between nitrogen and phosphorus nutrient content and phytoplankton in rivers is very complex. However, more studies show that the main factors affecting the number of phytoplankton and nutrients in water are basin area, water retention time, riparian vegetation coverage, water turbidity, human activities and other factors. With the rapid development of human society, a large amount of nitrogen, phosphorus and other nutrients produced in irrigation, fertilization, food processing, industrial sewage and farmland fertilization and drainage are discharged into the water body. A large number of pollutants enter into the water body, so that the content of water and nutrients is too high, promoting the explosive growth of phytoplankton. Algae and a variety of aquatic plants continue to consume dissolved oxygen in the water. Their corpses are broken down by microorganisms to produce toxic gases, such as hydrogen sulfide, leading to deterioration in water quality and the death of fish and various aquatic organisms. Algae and plankton release excess nitrogen, phosphorus and other substances into the water during decay, which are used by the next generation of algae[7]. Even if some measures are taken to control the input of exogenous nutrients, this vicious circle will be difficult to improve the polluted water body.

As far as lake waters are concerned, the conclusion that the species composition and quantitative characteristics of phytoplankton are closely related to the content of nitrogen and phosphorus in the water has been widely recognized, but the relationship between the content of nitrogen and phosphorus in the river and phytoplankton is very complex. Friedrichetal. studied the lower Rhine River and concluded that the development of aquaculture would increase the content of nitrogen and phosphorus in the water, thus affecting the composition and quantity of phytoplankton. The positive correlation between phosphorus content and phytoplankton in rivers in southeastern Ontario and rivers in western Quebec has also been confirmed. Based on the eight-year monitoring and data analysis of the Rhine River in France, Descyetal. considered that reducing the phosphorus content in the water had a significant effect on inhibiting the reproduction of green algae. It is generally considered that the eutrophication index of water is that the content of nitrogen in water is higher than 0.2-0.3 mg/L, the content of phosphorus is higher than 0.01 mg/L, and the biochemical oxygen demand is higher than 10 mg/L. The total amount of bacteria in fresh water reaches 104 mg/L, and the concentration of chlorophyll α in algal growth was higher than 10 mg/L. In the world, the concentration of lake eutrophication should be controlled as follows: the concentration of total phosphorus is not higher than 0.02 mg/L, and the concentration of total nitrogen is not higher than 0.2 mg/L.

The release of nitrogen and phosphorus nutrients from sediments has a great impact on the degree of eutrophication of water body, and is one of the important factors affecting aquatic ecology. The relationship between phytoplankton and nitrogen and phosphorus nutrients in rivers varies in different types of water bodies. Therefore, the release, precipitation, distribution and circulation of nitrogen and phosphorus nutrients in water and the biological absorption of endogenous nitrogen and phosphorus in water are combined with the monitoring of phytoplankton biomass to study and analyze the ecological situation of the water body, aimed at reducing the eutrophication of landscape water and controlling algal blooms pollution, improving the quality of landscape water, improving the landscape’s ornamental, ecological and economic value[8].

3.2AlgalbloomsThe essence of lake eutrophication is that nutrient input and output break the interspecific balance in lake ecosystem, resulting in the rapid growth of single species, such as blue-green alga, further damaging the lake ecosystem. Material circulation and energy flow eventually lead to the gradual demise of the whole ecosystem.

China is one of the countries with the most serious outbreak and the most widespread distribution of blue-green algae in the world, and it is also one of the countries where most blue-green algae species breed. The outbreak of cyanobacterial blooms not only changes the characteristics of low dissolved oxygen, high chemical oxygen demand of environmental factors, but also produces a large number of derivatives, including microcystin, nitrogen and dimensionless nitrogen. Hydrogen sulfide, odorous substances, sulfides, monomethyl isobutanol and so on seriously affect the structure and function of water quality and aquatic ecosystems, and even endanger the health of aquatic organisms and human beings[9].

The most direct manifestation of eutrophication is the outbreak of cyanobacterial blooms. Cyanobacterial blooms mostly occur in summer, the phytoplankton blue-green alga in the water reaches the eutrophication level of nitrogen, phosphorus and other nutrients in the water, and the water temperature reaches more than 20℃. The pH value of the water body is high, the light intensity and lighting time are suitable, the blue-green alga floats on the water surface, the blue-green alga population propagates rapidly. Algal blooms are formed on the surface of the water. When a large number of blue-green algae die, they not only change environmental factors, water, low dissolved oxygen and high chemical oxygen demand, but also decompose and produce a large number of derivatives, including microcystins, nitrogen and nitrate. Nitrogen, hydrogen sulfide, odorous substances, sulfide, monomethyl isopropanol, snails and so on seriously affect the water quality and the structure and function of aquatic ecosystems, and even endanger the health of aquatic organisms and human beings. In addition, due to the different functions of water bodies and different impacts on aquatic ecosystems, it will show different degrees of harm. The excessive proliferation of blue-green alga and the decomposition of dead algae continue to deteriorate the light and air exchange conditions of aquatic ecosystems, inhibit the growth and reproduction of other algae and photosynthesis of submerged plants, resulting in a serious shortage of dissolved oxygen, single algae, disappearance of submerged vegetation, death of fish and a sharp decline in aquatic biodiversity. Secondly, blue-green algae decompose and release a large number of derivatives, directly harming aquatic organisms, dead blue-green algae release a large number of organic substances, which can stimulate heterotrophic bacteria. Reproduction causes aquatic organisms to be infected with bacterial diseases. Third, in the process of cyanobacterial blooms and algae death, the physical and chemical indicators of the water body often exceed the tolerance of aquatic organisms, resulting in death. Photosynthesis in blue-green alga, for example, increases the pH of the water body to about 10 during the day, far exceeding the tolerance of some aquatic organisms and leading to death.

4 Prevention and control measures

Governments should enact strict laws and regulations to reduce the contribution of human activities to nitrogen and phosphorus in water. We can use physical clearing, filtration, chemical algaecide, biological manipulation, aquatic biological treatment, microbial treatment and other methods to achieve the effective control of water eutrophication.

4.1PreventivemeasuresIt is necessary to reduce nutrients. We can use low-phosphorus or phosphorus-free detergents, soap or soap-based detergents in soft water areas, and use the detergents made from alternatives in hard water areas.

It is necessary to increase the use of green manure, develop biological nitrogen fixation technology, eliminate the direct loss of nitrogen, reduce the use of chemical fertilizers, properly treat phosphorus-containing residues, and increase the steps of removing nutrients from sewage discharged from sewage treatment plants. Current water purification plants can remove 90 percent of purified organic pollutants, while nutrients can only be removed by 30 percent. There is a need to develop a technology for wastewater with low cost, low investment cost, which can remove nitrogen and phosphorus, and to take measures to remove the phosphorus in washing powder in urban catchment areas[10].

It is necessary to plant trees to slow down soil and water loss. We should change the traditional agricultural management model, put the development of pollution-free, green and organic agriculture around the lake in the first place, and reduce the non-point source pollution caused by livestock runoff. It is necessary to speed up the construction of urban environmental protection facilities, enhance the treatment and comprehensive utilization of municipal solid waste, and the innocuous disposal of hazardous waste. It is necessary to promote the centralized and safe disposal of rubbish and non-hazardous waste on the basis of preferential reduction of solid waste.

In order to reduce the influence of municipal solid waste and hazardous waste on water body, we must pay attention to domestic water pollution and save water, and solve the problem of domestic sewage by optimizing the resources. For water shortage areas, water resources protection is of great significance, it can reduce the demand for water resources and reduce the discharge of sewage. First of all, it is necessary to pay attention to the relationship between water conservation and the economic benefits of users and the implementation of rated water consumption, adopt modern new water-saving technology, provide excellent equipment, to improve residents’ domestic water use, and strengthen the drinking water and domestic water supply system. Secondly, it is necessary to increase the intensity of treatment, implement water price reform, and increase the proper use of water resources to a certain extent.

4.2UsingbiologicalchainsforrepairFor the water body where eutrophication has appeared, the pollution sources should be cut off quickly, and the photosynthesis of phytoplankton and the vortex movement caused by water should be relied on to help the water body gradually return to normal. Fish can consume algae and large aquatic organisms to alleviate the symptoms of eutrophication. We can introduce carp and squid for reasonable matching to control water eutrophication, and can also develop dense box culture. If the substrate is eutrophicated, snails and river otters can be put into the water to reduce organic matter and nutrients in the sediments, or copper sulfate can be put into the water to inhibit the growth of algae.

4.3EcologicalrestorationandlandscapeplanningThrough the investigation of eutrophication of some landscape water bodies, it is found that the reasonable combination of ecological restoration and landscape planning can control the characteristics of water system more effectively.

Ecological restoration technology is to use ecosystem principles, and a variety of biological methods to repair or restore the damaged ecosystems and ecological structures, improve ecological functions, and ultimately achieve the sustainable cycle of ecosystems. Based on the principle of ecosystem, landscape water ecological restoration technology adds the artificially cultivated plants and microorganisms, with the function of anti-pollution and powerful purification, so it has established a balanced ecosystem, restored the ecological function, and improved its purification ability[11]. Finally, the water is purified. In waterscape planning, it is necessary to understand the mechanism and pollution characteristics of eutrophication and algal blooms, use the original land for landscape water and riparian planning, and reasonably allocate the land with fewer leaves so that the roots can effectively solidify the soil and purify the nutrients of the water body. It is necessary to reasonably allocate the aquatic plants which can purify nitrogen, phosphorus and other nutrients, so that the waterscape design is not only beautiful but also able to give full play to the ecological value of the landscape water body.