1. Introduction
Urban rivers have been closely related to the development of human society and can provide various services such as water supply, flood control, shipping, landscaping and recreation, and at the same time, they are under pressure from urban construction and development [
1,
2,
3]. The ecosystem health of urban rivers has been more prominent in recent years and is receiving increasingly widespread attention. Due to insufficient water quantity, water contamination and channelization of rivers, the species and number of aquatic organisms in rivers have decreased, the community structure has changed, biodiversity has been lost and the water ecosystem has been severely degraded [
4,
5,
6]. The deterioration of urban river health has threatened the immediate interests of the public, and there is an emerging demand from the public for the improvement of river ecological environment quality. As the most critical resource and environmental carrier, urban rivers are related to the development of human society, and it is an urgent issue at present to reconcile the contradiction between economic development and urban river ecosystem to achieve human-water harmony and coexistence [
7].
As one of the important means to improve river health conditions and enhance the ecological service functions of rivers, water ecological restoration has received increasingly extensive attention both domestically and internationally [
8]. Studies and practices on ecological restoration of rivers are more frequent in the United States, Japan and a number of countries in Europe [
9,
10,
11], and the related studies mainly concentrate on the restoration strategies for river ecosystems and the restoration of water quality, water quantity, riparian zones and other elements in river ecosystems [
12]. Recently, many cities have implemented the use of reclaimed water to replenish rivers, which has played a vital part in alleviating the contradiction between urban water supply and demand as well as improving the quality of the water environment. However, eutrophication is more likely to occur in landscape water bodies replenished by reclaimed water than in natural rivers [
13]. To further purify the water quality of rivers, improve the habitat conditions of rivers and enhance the landscape effect, submerged plant community construction is gradually applied to urban river water ecological restoration [
14]. Submerged plants refer to large aquatic plants whose plant bodies are all located below the water layer for survival in fixation, belonging to herbaceous plants, and most of the leaves of these plants are ribbonlike or filamentous; examples include
vallisneria,
hornwort,
Myriophyllum verticillatum and
Hydrilla verticillata. Submerged plants are not only significant primary producers of the water ecosystem, but also important regulators of the quality of the water environment; they occupy critical interfaces in the water ecosystem and have an important effect on the material and energy cycles in the water ecosystem. When submerged plant communities are constructed, the submerged plants can not only play a purifying role in water quality, but also provide appropriate habitats for fish, benthic animals and others and embellish the surrounding environment [
15,
16,
17]. There are many examples of restoration for eutrophic water bodies with the application of submerged plants [
18,
19], and the application of submerged plants in urban river restoration has been reported to achieve more desirable outcomes, forming a “submerged forest” landscape in the river through reasonable planting of submerged plants and better artificial maintenance in later stages.
Despite the fact that submerged plant construction improves the ecological condition of the river in both laboratory experiments and examples of urban rivers with strong human intervention, there remains a certain degree of uncertainty about the effect of restoration with submerged plant communities in field practice. The growth process of submerged plants is affected by a combination of various factors [
20,
21], and when the dominant factors affecting submerged plants are not known and human intervention is lacking, it is sometimes difficult to establish a large area of survival or a stable population or community after restoration [
22]. For example, according to experimental data, organic-rich sediment can hinder the absorption of nutrients by submerged plants and may also exacerbate the effects of other stress factors [
23]. Meanwhile, the restoration effect often cannot be demonstrated for at least one year after the construction of submerged plants. In response to the drawbacks of water ecological restoration, follow-up monitoring and effect evaluation after river water ecological restoration have been carried out abroad [
24,
25,
26,
27,
28]. For example, an effectiveness evaluation of Project River Recovery (PRR) was conducted in New Zealand to restore the habitat of rivers and wetlands in the Waitaki Basin. Monitoring and analysis of habitat improvements over the past decade of the PRR program yielded strengths and areas in need of improvement, making it an effective way to ensure restoration success. Therefore, it is considered that monitoring and evaluation after restoration is an effective method to ensure the successful completion of restoration [
29,
30]. A series of evaluations have also been conducted in China for the evaluation of river water ecological restoration effects [
31,
32,
33].
As an international metropolis, Beijing is also a water-scarce city where human–water conflicts have always been prominent and the water ecological condition of rivers is not positive. In recent years, Beijing has carried out a series of river water ecological restoration projects, and the water quality, hydrology and riparian landscape of some rivers have been significantly improved. The construction of submerged plant communities as an important means of water ecological restoration has been gradually applied to improve the ecological conditions of urban rivers. To verify the ecological restoration effect and properly adjust and optimize the restoration measures, it is necessary to carry out water ecological monitoring and evaluation of the restoration effect for urban rivers that are restored by submerged plant communities.
Using site survey and field monitoring methods, in this study, we took the urban rivers replenished by reclaimed water in Beijing—Liangshui River and Dalong River as the research objects; carried out the monitoring of water ecological environment quality of the rivers; selected the appropriate water quality, aquatic biological indicators and habitat indicators; constructed the water eco-environment quality index; and took the Yongding River, which is replenished by natural water, as a comparison to evaluate the restoration effect of submerged plant communities. Meanwhile, in this study, we tentatively determined the major environmental factors affecting the submerged plant communities by calculating the correlation coefficients, providing a certain basis for further guiding the construction and maintenance of submerged plant communities.
2. Materials and Methods
2.1. Study Area and Data Sources
The main rivers replenished with reclaimed water in the urban area of Beijing were selected for the site survey in this study. According to the distribution, habitat characteristics, substrate features and sampling conditions of submerged plants in the rivers, we selected two rivers replenished by reclaimed water, namely the Liangshui River and the Dalong River, and selected the Shanxia segment of the Yongding River as a comparison for the restoration effect evaluation of submerged plant communities. The Liangshui River and Dalong River are located in the North Canal Basin (within Beijing), with a watershed area of 4348 km2, and many rivers in the basin are important components of the urban water system in Beijing, serving the functions of flood control and drainage, ecological landscaping and recreation of the urban water system. The climate of the North Canal Basin is a temperate continental monsoon climate, characterized by cold and dry winters, hot and rainy summers, and dry and windy springs, with large temperature variations in winter and summer. The wind in winter is mostly from the northwest, with an average wind speed of 3.0–3.5 m/s and a maximum wind speed of 22.0 m/s. The average annual evaporation on land is 400–500 mm, and the average annual evaporation on water is 1120 mm. Both the Liangshui River and the Dalong River are typical urban rivers sourced from the Xiaohongmen Reclaimed Water Plant. A series of near-naturalized water ecological restorations have been carried out in the rivers in recent years. According to the site survey, a large variety of submerged plants are distributed in the rivers, which is suitable for the study of the restoration effect of submerged plant communities.
The data of this study were mainly derived from the field monitoring of water quality and sediment, field sampling of aquatic organisms and field surveying of habitat conditions. According to the “Technical Guide for Monitoring and Evaluation of River Water Ecological Environment Quality” issued by the Ministry of Ecology and Environment and literature research, we determined the water and sediment quality parameters. All the indicators were monitored once. The water quality monitoring items included pH, dissolved oxygen (DO), permanganate index (CODMn), five-day biochemical oxygen demand (BOD5), ammonia nitrogen (NH3-N), total phosphorus (TP), total nitrogen (TN), arsenic (As), mercury (Hg), cadmium (Cd), chromium (hexavalent) (Cr), lead (Pb), cyanide, volatile phenols, petroleum, electrical conductivity, turbidity, nitrate nitrogen (NO3-N) and nitrite nitrogen (NO2-N). Water quality indicators were monitored in accordance with the “Technical Specification for Surface Water and Sewage Monitoring” (HJ/T91-2002). Sediment monitoring items mainly included total nitrogen (TN), total phosphorus (TP), mercury (Hg), arsenic (As), lead (Pb), cadmium (Cd) and organic carbon (OC). The monitoring methods were performed with reference to the relevant monitoring norms and standards for soil. The apparatus used for the collection of water was a 1 L plexiglass water sampler. A Peterson mud sampler with a sampling area of 1/16 m2 was used for sediment collection. The aquatic organisms were mainly investigated in terms of the species and quantity of macrobenthic invertebrates and submerged plants. The samples of benthic animals were collected with a D-type hand-operated net. The submerged plant community was surveyed using the quadrat method. Each sample plot was set up with 3–5 quadrats with an area of 1 m2. The importance value of submerged plants was expressed as the mean value of relative height, relative coverage and relative frequency. The coverage of submerged plants was characterized by the ratio of the area of submerged plants in the waters within 6 m depth to the water area.
In October 2020, this study set a total of 12 sampling sites for field monitoring of the river water ecological environment for the Liangshui River (5 sampling sites), the Dalong River (4 sampling sites) and the comparison river Yongding River (Shanxia segment, 3 sampling sites), and the distribution of sampling sites is shown in
Figure 1.
2.2. Comprehensive Evaluation of River Water Ecological Environment Quality
This study included a comprehensive assessment of the water ecological environment quality of the river in which the submerged plant community was located, referring to the comprehensive index method in the “Technical Guide for Monitoring and Evaluation of River Water Ecological Environment Quality” issued by the Ministry of Ecology and Environment, combined with the condition of the study area, and the restoration effect of the submerged plant community was analyzed by comparison.
By weighting and summing the water chemistry index, aquatic organism index and habitat index, the water eco-environment quality index of the river (WEQIriver) was constructed. The water quality evaluation was carried out with reference to GB 3838 [
34] for single-factor evaluation (water temperature and pH were not used as evaluation indicators). The classification of the water quality category level referred to the water quality evaluation method of the rivers in the “Surface Water Environmental Quality Evaluation Measures” (General Office of the Ministry of Environmental Protection of the People’s Republic of China [2011] No. 22) and finally assigned points according to the water quality category level. River habitat evaluation was performed in accordance with the wadeable river habitat survey data sheet to score each of the 10 parameters. Each parameter was scored in the range of 0 to 20 and divided into five evaluation levels. The total score of each monitoring section was calculated by adding up the scores of the 10 parameters.
For the selection of aquatic organism indices, we referred to the recommended indices in the Guide and took the macrobenthos, which had strong regional characteristics and generally had less ability to avoid external disturbance, as the indicator species, combined with the availability of actual measurement data, and selected the biological monitoring working party (BMWP) scoring system and the Shannon–Wiener diversity index from the perspectives of macrobenthos tolerance to pollutants and species diversity to evaluate the water eco-environmental quality, and the worst evaluation results were adopted to represent the evaluation results for aquatic organisms.
(1) BMWP scoring system
The BMWP scoring system used the macrobenthos as indicator organisms and was based on the evaluation principle that different macrobenthic species have different sensitivity/tolerance to organic pollution (e.g., eutrophication), and scores were assigned according to the tolerance level of each taxon. The water quality condition was evaluated according to the range of the score distribution. Larger BMWP scores indicated better water quality [
14].
The BMWP scoring system was based on families, and the sum of the scoring values of each family in each sample was the BMWP score; families with only one or two individuals in the sample were not included in the scoring.
(2) Shannon–Wiener diversity index
The Shannon–Wiener diversity index reflected the complexity of the biological community structure. Its evaluation principle was established based on the following principles: normally, the greater the diversity index, the more complicated the community structure, the more stable the community, the better the eco-environmental condition; when the water body is polluted, certain species will die out, the diversity index decreases, and the community structure tends to be simple, indicating a decline in water quality.
The
WEQIriver is calculated according to the following formula:
where
WEQIriver—water eco-environment quality index of the
river;
According to the score of
WEQIriver, the water eco-environmental quality condition was divided into five grades, which are excellent, good, average, poor and very poor, and the specific index score and quality condition grading are detailed in
Table 1.
2.3. Statistical Analysis
Correlation analysis was applied to analyze the relationship between environmental factors and submerged plant coverage to identify the major environmental factors affecting the submerged plant coverage condition. The Pearson correlation coefficient was computed. IBM SPSS Statistics 22 software was used for the execution of correlation analysis.