Establishment and application of the health evaluation system for rain-sourced rivers and lakes in coastal cities

Rivers and lakes constitute vital components of aquatic ecosystems, playing pivotal roles in flood control, drainage, water supply, irrigation, and the sustenance of aquatic habitats (Chen et al., 2021). However, in recent years, the natural ecosystem model of rivers and lakes has been disrupted due to climate change (Wang et al., 2023), urbanization (Ziyan et al., 2023), and increased anthropogenic disturbances (Ulrika & Tawfiqur, 2023), resulting in a range of ecological, environmental, and social issues. These issues include river disruptions (Han & Endreny, 2014), eutrophication of lakes and reservoirs (Pap et al., 2023), biodiversity decline (Yifan et al., 2022), black smelly water bodies (Wang et al., 2020), and heavy metal pollution (Rady et al., 2023). In response, conducting a health assessment of rivers and lakes, accurately diagnosing the main factors causing the deterioration of river and lake ecosystems, and carrying out water and ecological management are crucial ways to recover the ecological environment of rivers and lakes (Su et al., 2020).

Initially, the evaluation of river and lake health relied on a limited set of physical indicators (e.g., flow velocity) or chemical indicators (e.g., nitrogen concentration) to assess their health status (Norris & Thoms, 1999). However, as human understanding of river and lake health has evolved, the assessment process has grown more intricate. The current evaluation of river and lake health considers the integrity of their ecosystems' structure and function, their self-healing ability under external disturbance, and their social service function in meeting human demands (Zhengxian et al., 2021).

In response to this refined perspective on river and lake health, two main evaluation methodologies have emerged: the biological monitoring method and the comprehensive index method (Zhao et al., 2019). The former relies on the use of representative aquatic organisms, such as fish, macroinvertebrates, zooplankton, and aquatic plants, to reflect the environmental problems. Qualitative and quantitative monitoring data, such as changes in the species composition, biomass, and density of indicator species, are employed in this approach. Examples of typical biological monitoring methods include the River Invertebrate Prediction and Classification Scheme (RIVPACS) (Wright et al., 1984), the Australian River Assessment Scheme (AUSRIVAS) (John, 2004), and the EU Water Framework (WFD) (Kallis & Butler, 2001). The comprehensive index, on the other hand, provides a more comprehensive approach to assessing the health status of a water body by considering multiple physical characteristics, chemical indicators, and the distribution of aquatic organisms. Although this approach is laborious and time-consuming, it provides a more holistic understanding of the specific problems of a river or lake. Examples of typical comprehensive index methods include the Swedish Riparian and Riverien Environment Specifications (Petersen, 1992) and the Australian Stream Condition Index (ISC) (Ladson et al., 1999).

Numerous studies have focused on evaluating the health of estuaries, urban rivers, and lakes using the biological monitoring method, such as the Fish-based Index of Biotic Integrity (F-IBI), and the comprehensive index method, such as the pressure–state–response (PSR) framework (Yifan et al., 2021; Zhang et al., 2021; Wang et al., 2023). In coastal cities, most of the rivers and lakes form separate embankments that are isolated from the sea to counteract the high salinity of seawater. These rain-sourced water bodies have limited water recharge, mostly from tail water of domestic sewage and sewage treatment plants, resulting in the characteristic micro-capacity and heavy load (Hill et al., 2020), which exacerbates pollution loads, degrades river water quality, reduces self-cleaning capacity, and impairs ecological environments. Evaluating the health of rain-sourced water bodies in coastal cities remains a pioneering endeavor. This study focuses on the typical coastal city of Lingang City Shanghai, selecting typical rain-sourced rivers and lakes in the region, developing an applicable regional evaluation system for river and lake health, and conducting a health evaluation to accurately diagnose the water ecological and environmental problems of rivers and lakes in coastal cities.

In this study, the Huangrigang River (HRGR) and DSL were chosen as the primary evaluation objects. The HRGR and DSL were chosen as study sites based on the following considerations: Firstly, the HRGR serves as a temporary water intake channel for the region, playing a pivotal role in connecting Lingang and another city. Secondly, the DSL serves as the core of the regional water system and is a multifunctional artificial freshwater lake that integrates various purposes such as climate regulation, leisure activities, and aesthetic enhancement.

Two field surveys were conducted at the HRGR and DSL in November 2021 during the dry period and in July 2022 during the rich period. The collected data from each site were subsequently averaged twice. To monitor the health of typical rivers and lakes in Lingang City, a total of seven points were established for investigation purposes, including three points (H1–H3) in the HRGR and four points (D1–D4) in DSL (Figure 1). The investigation focused on the physical characteristics of the rivers and lakes, including lake area, river and lake shoreline conditions, and water level, as well as chemical characteristics such as total nitrogen, total phosphorus, and dissolved oxygen. Additionally, aquatic organisms such as phytoplankton and fish were examined, along with social services such as public satisfaction.

Water samples were collected from the water bodies using stainless steel buckets. At each sample point, three collections were taken, and the samples were mixed before random sampling. The mixed samples were then fixed with acid and stored frozen in PVC bottles until analysis for biogenic substances in the laboratory.

Fish samples were collected using an electrofishing backpack and subsequently identified, weighed, and counted in the field. Phytoplankton samples were collected in 1,000 mL of water, fixed in the field with 1% Lugo's reagent, and then concentrated to 30 mL after 48 h of sedimentation for phytoplankton community analysis (Chen et al., 2020).

Total nitrogen, total phosphorus, and other water quantity and quality indicators were measured in accordance with the monitoring analysis method of water and wastewater (Ministry of Water Resources of the People Republic of China, 2020). The water temperature, turbidity, and other indicators were measured using a multiparameter water quality analyzer (Hydrolab HL4, HACH Inc., Tampa, FL, USA). Phytoplankton was quantified using a light microscope (Olympus BX41) at 400× magnification (Hongjun & Yinxin, 2006).

Based on standards such as ‘Technical Guidelines for River and Lake Health Assessment of Shanghai’ (Shanghai Municipal Water Administration, 2023) and ‘River and Lake Health Evaluation Guide (trial)’ (Ministry of Water Resources of the People's Republic of China, 2020), a typical river and lake health evaluation index system for Lin-gang was established using the Australian ISC method (Ladson et al., 1999) and the Analytic Hierarchy Process (AHP) (Zsombor & Szabolcs, 2022).

The selected evaluation indexes were required to be highly sensitive and responsive to changes in river and lake ecosystems, representative, and able to fully reflect the health condition of the river and lake. Additionally, they needed to be independent among the indexes and show a low correlation among the indexes.

The health status of representative rivers and lakes in Lin-gang can be classified into five tiers based on their health assessment score: natural, healthy, sub-healthy, unhealthy, and morbid (Table 1).

Table 1

Qualitative description and representation of each health level.

Health level .	Score .	Expressions .
Natural	90 ≤ H ≤ 100
Healthy	75 ≤ H < 90
Sub-healthy	60 ≤ H < 75
Unhealthy	40 ≤ H < 60
Morbid	H < 40

Health level .	Score .	Expressions .
Natural	90 ≤ H ≤ 100
Healthy	75 ≤ H < 90
Sub-healthy	60 ≤ H < 75
Unhealthy	40 ≤ H < 60
Morbid	H < 40

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The methodology employed in this study involved the application of the quadratic method to ascertain the scoring criteria for each index. The resulting scoring criteria were then evaluated by calculating various statistical measures, including the mean, standard deviation, minimum value, maximum value, and five quartiles ranging from 5 to 95% of the dataset for each index at all data points. The grading criteria of the quadratic method were compared based on these measures. Each index was scored equally, and the resulting scores were used to classify the river health levels into five categories: natural, healthy, sub-healthy, unhealthy, and morbid, which are arranged in descending order.

The health evaluation index system for the HRGR and DSL comprises three layers: the target layer, the category layer, and the indicator layer (Table 2). In the HRGR, there are nine evaluation indicators, encompassing two indicators for physical habitats, four indicators for water quantity and quality, two indicators for aquatic organisms, and one indicator for social services. Similarly, DSL's evaluation index system consists of nine indicators, including three indicators for physical habitats, four indicators for water quantity and quality, one indicator for aquatic organisms, and one indicator for social services. Additionally, the evaluation index system for rivers and lakes comprises seven public indicators and four specific indicators. The water fluidity and fish diversity index are specific to rivers, while the percentage of lake area shrinkage and trophic level index are specific to lakes.

Notably, the weight assigned to the water quantity and quality and social service layer in the category layer is 0.3, which is higher than that of the physical habitats and aquatic organisms (Table 3). In HRGR's physical habitat category, the weight distribution is based on the riparian naturalness, the riparian degree of illegal exploitation and utilization, and an equal distribution of the weights for each indicator in the aquatic organisms category. Conversely, in DSL's physical habitat category, the largest weight is assigned to the percentage of lake area shrinkage. Furthermore, in the water quantity and quality category, the degree of ecological flow guarantee rate and the trophic level index carry greater weight than the degree of water quality and self-purification capacity.

According to the quadratic method to obtain the assignment criteria of each indicator of the HRGR and DSL, each indicator assignment criterion is shown in Table 4.

The spatial distribution of the health status of the HRGR and DSL is illustrated in Figure 2(b). In the HRGR, the health condition improves gradually from upstream to downstream, where H1 is the sub-healthy state, and H2 and H3 are healthy states. Notably, 33.3%of the points are in a sub-healthy state, while the remaining 66.7% are in a healthy state. Conversely, DSL displays a more even distribution of health conditions, with 50% of the points classified as healthy and the other 50% as sub-healthy. Specifically, D4 and D1 are the healthy state, while D2 and D3 are sub-healthy states. Additionally, the southwest region of the lake exhibits a relatively better health condition than the northeast region.

The variation in the health condition of the HRGR between upstream and downstream is attributed to anthropogenic activities in the surrounding area. The H1 section, situated in the main university district of Lin-gang Special Area, is densely populated, and the river's health is negatively impacted by the direct discharge of domestic wastewater. The H2 section, a primary fishing and agricultural area, is also contaminated by waste garbage and farming wastewater, leading to poor health conditions. Recently, ecological restoration projects, such as underwater forest construction and shore softening, have been implemented by the Lin-gang government to improve the area's health condition. As a result, H2 is the healthy state, and H3, located in an integrated park area of coastal leisure, cultural tourism, and ecological protection, has a circular purification wetland that has significantly improved its health condition.

DSL is an inner urban rainfall type lake, where ecological replenishment mostly occurs through regional rainfall and a small portion via tidal water from the HRGR through the northern and eastern river network, while the lake's water is exchanged through the western HRGR and the southern river network draining Hangzhou Bay. The northeastern part of the lake is affected by agricultural activities, resulting in the entry of water with higher nutrients through the northeastern river network, leading to water pollution. Additionally, due to regional development and construction, some rivers in the northeast are severely disconnected, resulting in reduced water inflow into the lake. The weaker water flow in the northeast compared to the southwest causes nutrient deposition in the former, leading to a lower health condition of the lake.

The evaluation results of the water quantity and quality category indicate that 42.9% of the assessment points are classified as sub-healthy, while an equivalent of 42.9% are deemed unhealthy. Moreover, 14.2% of the points fall under the morbid, with no points being evaluated as healthy (Figure 3(b) and 3(e)). The regions displaying poor chemical indicators are mainly located in the upper reaches of the HRGR and the northeastern section of DSL. These regions comprise built-up areas, such as the primary university town in Lin-gang, where alterations in land use patterns have altered the substrate's properties. Consequently, the impervious area has increased, and pollutants present in the land area enter rivers and lakes through rainfall-runoff, resulting in a pooling area for pollutants.

The aquatic organisms category evaluation results reveal that 28.6% of the points are sub-healthy, 57.2% are unhealthy, and 14.2% are morbid (Figure 3(c) and 3(e)). Lin-gang is a plain city river network characterized by a relatively flat terrain, with a ground elevation generally ranging from 3.5 to 4.5 m. The area exhibits an undulating topography, with the rivers and lakes largely in a non-flowing state, making it susceptible to water body eutrophication. The scores of phytoplankton density in the HRGR and DSL are 24.3 and 21.2 (Table 5), thereby increasing the risk of water bloom.

The evaluation results of the social service layer indicate that 28.6% of the points are in natural, 42.8% points are healthy, and 28.6% of the points are sub-healthy (Figure 3(d) and 3(e)). In line with the ‘water city coexistence’ model, DSL and its drainage channel HRGR have established water culture promotion projects such as nautical museums, marine parks, walkways around the lake and river, and ecological wetlands. These initiatives have facilitated the integration of scenic planning with river and lake management, creating a significant public activity platform that enhances the social function of the river and the lake service. As a result, tourists and citizens can now enjoy water-based leisure and entertainment activities.

Improving the hydrodynamics of rivers and lakes plays a pivotal role in enhancing the oxygen saturation levels of water bodies, expediting the chemical and biological purification processes, bolstering the self-cleansing capacity of water bodies, and restoring aquatic habitats (Li et al., 2013b). Specifically, enhancing the mobility of water bodies in urban river networks can effectively reduce nutrient concentration, thus mitigating the risk of malodorous water bodies (Pierfranco & Carmelina, 2021). In the case of lake hydrodynamic enhancement, it can substantially reduce the concentration of chl-a and total phosphorus (TP) in lake water bodies (Li et al., 2013a; Boqiang et al., 2021). However, the flow velocity of water bodies in the HRGR is generally within the range of 0.00–0.01 m/s, while the flow velocity of water bodies in DSL is less than 0.03 m/s, indicating a state of weak flow. An increase in the residence time of river water bodies can reduce water flow mixing and dilution, leading to the accumulation of suspended particles and a propensity for eutrophication (Li et al., 2014). Additionally, the abundance of nutrients received by algae at the bottom of the water enhances biomass growth, ultimately resulting in the formation of cyanobacterial blooms (Wu et al., 2015).

Freshwater ecosystems in coastal cities, particularly those formed by rainfall, are facing ecological challenges due to their limited capacity and heavy pollution load. It is important to evaluate the health of these water bodies and carry out targeted restoration measures to maintain their ecological function. The article selected the HRGR and DSL as typical rivers and lakes in the Lin-gang Special Area and established a health evaluation index system using ISC and AHP methods. The evaluation results showed that the HRGR was sub-healthy, while DSL was healthy. The main problems facing these water bodies were related to low flow velocity, weak mobility, and high phytoplankton density.

Regarding the typical rivers and lakes in the Lin-gang Special Area, the primary task in the current water environment conditions is to control exogenous pollution, which includes the treatment of domestic sewage, as well as point and surface source pollution control. Subsequently, river and lake hydrodynamic control measures can be implemented by adopting multi-source diversion of clear water replenishment and constructing gates and pump projects to increase the fluidity of river water, thereby reducing the hydraulic residence time of the lake. Finally, long-term control of the river and lake water environment can be achieved through river and lake ecological restoration and modernized intelligent control measures.

This paper proposes a health assessment method for coastal rain-sourced lakes and rivers and applies it, providing theoretical support for achieving sustainable utilization of regional water resources. However, the research outcomes are only applied to a typical lake and river in Lingang, Shanghai. In future research, it is necessary to expand the application of the health assessment method for coastal rain-fed lakes and rivers. Starting from a watershed scale, the reliability of detection and evaluation results and the applicability of the assessment method need to be examined. Furthermore, the current evaluation indicator data heavily rely on manual sampling, which is influenced by various natural factors like weather and artificial factors such as sampling personnel's skills and proficiency. Future research should combine emerging monitoring technologies like satellite remote sensing and environmental DNA with health assessment, aiming to enhance the reliability of assessment results.

S.W. conceptualized the study, performed data curation, did formal analysis, investigated the methodology, wrote the original draft, and reviewed and edited the manuscript. Y.L. conceptualized the study, did funding acquisition, and supervised the work. X.P. was involved in data curation and formal analysis, and reviewed and edited the manuscript.

Research leading to this paper has been partially supported by Research on the Educational Reform of Higher Education in Jiangsu Province in 2021 (2021JSJG474) and the Qin Lan Project of Jiangsu Province.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.