Exploration of Ecological Function and Restoration of Small and Micro Wetlands

Due to their unique ecological functions, small and micro wetlands have received more extensive attention and recognition from the international community, and have become an important part of China’s ecological civilization construction. In this paper, the concept, characteristics and classification of small and micro wetlands are described, and the ecological functions and services of small and micro wetlands are discussed. On this basis, the principles of ecological restoration of small and micro wetlands were analyzed, and suggestions for the protection and management of small and micro wetlands were put forward, in order to provide reference for the protection and restoration of small and micro wetlands in China.


Concept, classification and characteristics of small and micro wetlands
1. 1 The concept of small and micro wetlands At present, due to the great differences in the topography, geomorphology and hydrological conditions of SMW, the definition of "SMW" in various countries around the world is still relatively vague, which is not conducive to the protection, restoration and management of "SMW ".Some European countries refer to SMW as "ponds" and "small lakes" and set their starting areas ranging from 1.2 hm² to 2 hm² , and the current definition of SMW in China also has local differences.Based on the description of the Code for the Conservation and Management of Small and Micro Wetlands, it is believed that SMW refer to natural or artificial wetlands with water throughout the year or part of the time and with an area of less than 8 hm 2 , which have the characteristics of patch-like, small area and uneven distribution [3][4][5][6] .

Classification and characteristics of small and micro wetlands
The classification of SMW is the basis of research, and the classification from different perspectives and research methods can provide a clearer understanding of them.However, due to the complex structure of wetland ecosystems and large regional differences, different scholars have great differences in the classification, and have not formed a relatively complete classification system.In this paper, SMW were classified based on the formation factors, hydrological periods, and geographical location.

Classification according to formative factors
According to the research of Zhao H et al. [7] , SMW are divided into natural and artificial ones.
Table 1.Classification of small and micro wetlands according to their formation factors.

Category Basis and characteristics of the division Types
Natural SMW Small and micro wetlands formed by natural evolution are small in area, rich in biodiversity, large in gradient change and sensitive to environmental changes.

Artificial SMW
It is a composite wetland landscape with artificial construction or control, weakened natural attributes, prominent human attributes, and significant social service functions such as landscape and leisure.
Low-lying land of farmland, breeding farms, rainwater wetlands, wetland sewage treatment plants, small urban landscape water bodies, etc.

Classification according to hydrological cycle
Boulton et al. [8] divided SMW into transient and permanent ones.
Table 2. Classification of small and micro wetlands according to their formation factors.

Category Basis and characteristics of the division
Transient SMW Alternate wet and dry.According to whether it has a certain regularity, it can be divided into seasonal small and micro wetlands and intermittent small and micro wetlands.

Permanent SMW
Regular flooding, water level fluctuations, about a year of dryness in ten years, most of the plants living in the wetland belong to wet plants, do not have drought tolerance attributes.

Categorized by geography
Xiang L et al. [9] divided SMW into rural and urban ones, considering the different locations of them.Cui L et al. [3] summarized the domestic research and abroad, and divided SMW into which associated with large wetlands and those that are relatively independent in geographical location according to their location relationship with large wetlands.
Table 3. Classification of small and micro wetlands according to geographical location.

SMW in the countryside
It is located in the countryside and has an area of less than 8hm² .

SMW in the city
It is located outside the countryside and covers an area of less than 8 hm² .
Urban rain gardens, urban wetland sewage treatment plants, urban small landscape water bodies, etc.

SMW associated with large wetlands
It is located in the same large geographical area as the large wetland and belongs to the subsidiary part of the large wetland.
Small swamp wetlands in river basins, river waterfront wetlands, natural seepage areas, etc.

Relatively independent SMW
A type of small and micro wetland that lacks obvious surface water connection with upstream and downstream waters.
Grassland and grassland basin wetlands in the central part of the continent, semi-desert and desert basins and plain wetlands, coastal intertidal wetlands, etc.

Ecological functions and services of small and micro wetlands
Wetlands are composed of soil, water, animals and plants, which are inseparable from each other, forming a relatively stable SMW ecosystem, providing hydrological, biogeochemical and socio-economic benefits.Although the impact of a single wetland may be limited to some extent, SMW are characterized by large numbers and wide distribution, and the cumulative effect of many wetlands is significant [10][11] .

Water storage and flood control
SMW play an important role in water storage and flood control, water conservation, and groundwater level regulation, which are mainly reflected in: firstly, the soil of SMW has special physical and hydrological properties, strong water storage capacity, and can absorb a large amount of rainwater; Secondly, the plants of SMW can reduce the peak and slow down the flow of a certain amount of floodwater, regulate the stormwater, and protect the surrounding land and buildings from flooding.Finally, SMW can regulate uneven precipitation in time and space, and through their throughput regulation, they can avoid floods and droughts, and are known as the "natural sponges" for water storage and flood control [12][13][14][15] .

Water purification
SMW are usually used as buffer areas to alleviate agricultural non-point source pollution.Their substrates, plants and microorganisms can remove pollutants and improve water quality through adsorption, precipitation, and biotransformation.It is mainly reflected in: first, the soil, plants and artificial substrates in SMW can adsorb suspended solids and organic matter in water.These substances are adsorbed and gradually settle to the ground, reducing the amount of suspended solids in the water.Second, microorganisms in SMW can also decompose organic matter in water.Microorganisms use their own metabolic activities such as redox, hydrolysis, and esterification to convert a large amount of organic matter and fat contained in sewage into inorganic matter, carbon dioxide, and water, thereby improving water quality.Finally, SMW are also able to remove nutrients from water through plant uptake and transformation and convert them into harmless forms [16][17][18][19][20] .rich in species diversity, which are carbon dioxide "sinks" and climate "stabilizers", can effectively alleviate global warming and urban heat island effect.It is mainly reflected in: 1. Plants in SMW can absorb a large amount of carbon dioxide through photosynthesis and convert it into organic matter, which is stored in the soil, thereby reducing the carbon dioxide content in the atmosphere and slowing down the rate of global warming.2. SMW can release water to the atmosphere through evaporation and return to SMW after a new round of precipitation, and the wetlands and the environment exchange materials in the rainwater cycle, which can reduce the temperature and regulate the climate of the surrounding environment while exchanging energy, and effectively alleviate the heat island effect.In addition, SMW can also improve air quality by increasing the concentration of negative ions in the air, which indirectly improves people's health [21][22][23][24] .

Principles of restoration of small and micro wetlands
Table 4. Principles of restoration of small and micro wetlands.

Remediation Principles Protection Recommendations
Ecological priority, close to nature restoration The landscape restoration design should conform to the undulating terrain, maintain the tortuous form of rivers, roads and farmland boundaries, and use natural or ecological measures to create a wetland landscape without destroying the original wetland habitat as much as possible.

Adapting measures to local conditions to increase biodiversity
Adhere to the principle of protecting biodiversity, use local materials, so that the structure and function of the ecosystem are in a state of dynamic equilibrium of adaptation and coordination, and avoid biological invasion and competition between populations leading to plant decline or even death.

Scientific layout to improve self-healing ability
Improve the synergistic relationship between substrate, plant and microorganism, change the spatial arrangement of patches, corridors and substrates to improve the landscape space, and increase the self-repair and self-sustaining ability of small and micro wetland ecosystems.

Recommendations for the protection of small and micro wetlands
SMW are of great significance to people's daily life and natural geographical environment, but people have little understanding of the role and importance of SMW.Based on this, it is necessary to strengthen the popularization of science, enhance the awareness of protection, improve the laws and regulations on the protection of SMW, advocate the rational development and utilization of SMW on the basis of protection, and actively find a balance between the protection and development of SMW.At the same time, government departments should play a leading role, strengthen supervision and management, and individuals should also establish a good awareness of protecting the ecological environment and reduce pollutants from the source.

Summary and outlook
Although the area of SMW is small, they are numerous and widely distributed, which are of great significance in biodiversity conservation, improvement of surrounding living environment and climate regulation, and their ecological functions and services are very significant.We should firmly establish and practice the concept of "lucid waters and lush mountains are invaluable assets", adhere to the principle of "ecological priority and green development", and make rational use of wetlands while protecting and restoring wetlands, so as to realize the harmonious coexistence of man and nature.