Ecosystem Functions in Urban Stormwater Management Ponds: A Scoping Review

Abstract

Stormwater management ponds (SWMPs) are an important tool for sustainable urban stormwater management, controlling the quantity and quality of stormwater runoff in cities. Beyond their engineering purpose, SWMPs may hold ecological value that is often overlooked. This is especially the case for the array of geochemical, physical, and biological processes (i.e., ecosystem functions) in SWMPs. Here, we performed a scoping review of ecosystem function in SWMPs to summarize current knowledge and identify research needs. We searched peer-reviewed papers using the Web of Science database. Papers that did not report specifically on SWMPs, did not discuss ecosystem function, or were solely based on ecotoxicological tests were excluded from further assessment. For the remaining papers, information on year of publication, scope, and key findings was extracted. We found that a total of 55 papers on ecosystem function in SWMPs have been published since 1996. Our review identified important areas for advancing knowledge about nutrient dynamics, contaminants processing, sedimentation, temperature, habitat provisioning, and biodiversity in SWMPs. Overall, we identified a need to further understand how factors related to pond design and landscape and management practices influence ecosystem function. There is also a need to understand the effect of climate change on ecosystem function and to examine the interactions between ecosystem function and humans. Such information will not only provide opportunities for researchers to better understand ecological value, but also facilitate more effective sustainable management of SWMPs.

1. Introduction

Urban areas are continuously expanding in many parts of the world with a projected 65% of all land area on the planet becoming urban by 2030 [1]. Such areas are broadly covered with impervious surfaces that prevent rainwater from infiltrating soil, increasing stormwater runoff. Excessive stormwater runoff can rapidly overwhelm drainage systems leading to flooding and sewage overflow in areas with combined storm and sanitary sewers [2]. This can have profound effects on the environment and society by spreading contaminants and pathogens, leading to physical health ailments and psychological stress [3]. Predictions suggest climate change will increase flood risk, which can further threaten the environment and society in the near future [4]. Therefore, city planners and practitioners have focused on managing urban stormwaters to mitigate potential environmental and societal impacts [5,6].

There are multiple approaches to managing urban stormwater. Early techniques focused on collecting and disposing of stormwater away from urban areas as quickly as possible, often relying on natural waterways (streams) to convey stormwater [7]. More recent approaches, often referred to as best management practices, acknowledge the ecological and social importance of stormwater for sustainable management [8]. Such an approach includes separating combined sewers and retaining stormwater for slow release into the environment [7]. To achieve this, a variety of systems have been developed, including detention basins, swales, permeable pavements, and stormwater management ponds (SWMPs) [9].

The SWMPs are designed to collect and retain stormwater runoff, working as a catchment basin. Urban stormwater is loaded with nutrients (mainly nitrogen and phosphorous), contaminants (e.g., road salt, heavy metals, pesticides, herbicides), and sediments (i.e., dissolved solids) washed from urban areas, including landfills, lawns, and roads [10,11]. Stormwater enters the SWMP through an inlet that leads to a retention pool that stores stormwater, allowing water treatment through settling, biochemical reactions, and biological uptake of pollutants and sediments [12]. From the retention pool, stormwater is slowly released through an outlet back into the environment, often into nearby streams. This engineered design differentiates SWMPs from regular ponds, lakes, and reservoirs.

The SWMPs are one of the most widely used structures for urban stormwater management worldwide [13]. Despite their widespread use, the ecological aspects of SWMPs are poorly understood [14]. Specifically, their ecosystem functioning (i.e., the array of geochemical, physical, and biological processes [15]) is not well understood [14,16]. Such information is fundamental to effectively managing SWMPs for their ecological value beyond engineering goals, such as the provisioning of habitat for species and green space for people [17].

The primary objective of this paper is to perform a scoping review of the literature on ecosystem function in SWMPs. Scoping reviews are a type of research synthesis aimed at determining the coverage of literature on a given topic, providing the volume of studies available and their focus [18]. This approach is especially useful in developing fields, such as the study of SWMPs as ecosystems, to provide an overview of the field and identify gaps [18]. Based on this review, we identify information and research needs required to advance towards a better understanding of the ecological value of SWMPs.

2. Methods

The term “ecosystem function” has been broadly used in different contexts, and there is no consensus on its meaning [19]. Therefore, in this review, we searched for specific terms that denote the array of geochemical, physical, and biological processes related to ecosystem function in urban SWMPs. We co-produced search terms based on discussions with government agencies, researchers, managers, and students during a workshop on ecosystem functions and services in SWMPs [20] (Figure 1). We performed separate searches for each term, coupled with an identifier, in the following format: “search term” AND “stormwater pond”. Searches were performed by topic (i.e., matches on the title, abstract, and keywords) of peer-reviewed papers using the Web of Science database. The search was performed between February–April 2024 on the full database, including all years. There is no consensus on the term to refer to SWMPs in the literature. We used “stormwater pond” as an identifier because that is the most commonly used term to refer to SWMPs. Alternative searches using the synonym identifiers “stormwater management pond” and “wet pond” were also performed (Figure 1). A unique list of papers was produced for each search term with the combined matches from the three identifiers. The abstract for each paper was assessed for eligibility; papers that did not report specifically on SWMPs, did not discuss ecosystem function, or were solely based on ecotoxicological testing, were removed. We categorized the remaining papers into geochemical, physical, and biological processes that describe ecosystem function in SWMPs. We counted the number of papers per category, and we summarized key aspects of ecosystem function (i.e., data charting). Such data charting was accomplished by accessing information contained in the results, discussion, and conclusions of each paper and was performed independently by the lead author, following PRISMA guidelines for reporting on reviews [21]. We identified information available and exposed venues for advancing knowledge in SWMPs.

3. Results

Following our screening process, we kept a total of 55 papers that explicitly described ecosystem function in SWMPs. Most papers focus on geochemical processes (contamination and nutrients, 23 papers), followed by physical processes (sedimentation and temperature, 21 papers), and biological processes (biodiversity and habitat, 11 papers). Among these, the most studied ecosystem functions are related to contamination and sedimentation (17 and 13 papers, respectively), while little is known about functions related to habitat (4 papers) (

Table 1. Search terms used in this review were derived from a co-production workshop. Each term was grouped into ecosystem function processes (geochemical, physical, or biological). The total number of studies returned in each search (hits) and the total number of papers considered following screening (retrieved) are shown.

Search Term	Process	Hits	Retrieved
Contamination	Geochemical	27	17
Nutrient	Geochemical	48	6
Sediment	Physical	63	13
Temperature	Physical	24	8
Biodiversity	Biological	13	7
Habitat	Biological	12	4

). We found no clear trend of publication over time, with the earliest paper being published in 1996 (Supplementary Figure S1). The full list of search hits and papers retained after screening is available in Supplementary Table S1. We provide a summary of our key findings here and a more detailed interpretation in the Discussion section.

We extracted key information from each paper (i.e., data charting) (Supplementary Table S2) (Figure 1) and found venues for advancing the knowledge. Specifically, we found that studies on contaminants in SWMPs tend to focus on substance fate, and little is known about microorganisms and biological toxins (Figure 2). Also, there is little evidence on the environmental factors regulating the concentration of contaminants in SWMPs. Studies on nutrients in SWMPs tend to focus on nitrogen and phosphorous, but the sources, pathways, processing, and overall nutrient cycling remain unclear (Figure 2). This likely contributes to the poor understanding about nutrient removal efficiency in SWMPs that we detected. Studies on sedimentation in SWMPs suggest that, although these structures can retain large amounts of sediment from stormwaters, there is still a need for understanding long-term nutrient dynamics and the use of devices and designs for increasing nutrient removal capacity (Figure 2). Studies on temperature in SWMPs tend to focus on thermal stratification, but the factors modulating stratification are poorly known. It is possible that design parameters have an important role in stratification, but empirical evidence is needed (Figure 2). Studies on biodiversity indicate that multiple species are present in SWMPs, from algae to birds to mammals. However, the factors defining which species dominate SWMPs are yet to be understood. It is likely that management strategies, pond design, and landscape characteristics determine biodiversity, but empirical evidence is needed (Figure 2). Studies on habitat in SWMPs show the potential for habitat provisioning for multiple species, facilitating dispersal and gene flow. However, the extent to which SWMPs can contribute to habitat connectivity remains under debate. The quality of habitat in SWMPs is also uncertain, and they could represent low-quality, ecological trap habitat for some species (Figure 2).

4. Discussion

Despite the widespread use of SWMPs as best management practices for stormwaters in many cities worldwide, our review suggests little is known about their ecosystem functioning. We identified a need for further research on geochemical, physical, and biological processes to better understand ecosystem functioning and advance the ecological management of SWMPs.

4.1. Geochemical Processes

Assessing the type, concentration, and distribution of chemicals is important to understand the capacity of SWMPs to capture, store, and process chemicals that can determine ecological responses in eutrophication and environmental toxicity. Two geochemical processes of interest in SWMPs are related to the fate of contaminants and nutrients (Table 1).

4.1.1. Contaminants

Stormwater entering SWMPs is loaded with a wide range of substances (e.g., heavy metals, road salt, microplastics), microorganisms (e.g., E. coli), and biological material (e.g., cyanotoxins) that are known or suspected to cause detrimental ecological and/or human-health effects. Such substances are collectively referred to as contaminants [22]. Searched studies indicate that contaminants processing and retention in SWMPs vary widely.

For substances such as heavy metals, the particulate fraction is retained in SWMPs mainly by sedimentation processes, while the retention of the dissolved fraction requires additional mechanisms of removal (e.g., filters) [23]. Retention capacity for heavy metals can vary with the chemical considered, pond-related factors (e.g., age, volume, residency times, land use), and water-chemistry (e.g., pH, alkalinity) [23,24]. For substances such as road salt (chloride), sedimentation does not play a major role in retention; instead, chloride is stored in diluted form [25]. The retention capacity for chloride is still under debate, with storage and release varying with season and pond design [26]. In some cases, SWMPs can be considered a source of long-term chloride contamination to surface and ground waters [27]. However, the context in which chloride and other substances leak from SWMPs is still unclear (Figure 2).

Studies on SWMP contaminants focus on substance fate, and little is known about microorganisms (E. coli, intestinal enterococci) and biological material (cyanobacteria toxins) (Supplementary Table S2) (Figure 2). Notably, microorganisms and their biological material seem to be a prolific field of study in natural aquatic ecosystems [28,29]. In SWMPs, the retention of microorganisms seems uncertain with studies, indicating that effects are negligible or dependent on flow rate (water level), with effective retention during dry conditions under reduced outflow [30,31]. Likewise, the processing of biological material such as cyanotoxins is unclear because their levels are not always correlated with the density of cyanobacteria [32,33] (Figure 2).

In general, the concentration of contaminants in SWMPs can widely vary, sometimes surpassing health and safety standards [34,35,36]. Multiple factors can influence the concentration of some contaminants in SWMPs (Figure 2). For example, the concentration of synthetic chemicals in SWMPs can be related to nearby road type, while coliform and microplastic contamination is related to commercial land use and magnitude of rainfall [37,38,39]. Also, the concentration of some herbicides (atrazine and 2,4-D) is related to seasonality, being particularly low during the summer months [36]. Despite such studies, how environmental factors modulate the concentration of contaminants in SWMPs is still unclear (Figure 2). This is fundamental for better understanding the role of SWMPs as a source/sink of harmful substances, microorganisms, and biological material.

4.1.2. Nutrients

Studies on the nutrient dynamics in SWMPs tend to focus on different forms of nitrogen (N) (e.g., nitrate, nitrite, ammonia) and phosphorous (P) (e.g., dissolved organic and inorganic, particulate organic and inorganic) (Supplementary Table S2). In natural aquatic ecosystems (i.e., not SWMPs), such nutrients are often seen as a priority because they are related to eutrophication that leads to algal blooms and fish die-offs [40]. The specific pathways of N and P in SWMPs (i.e., input, chemical transformations, removal) were summarized by [41]. Major sources of N to SWMPs can be traced to atmospheric deposition, runoff from fertilized lands, and sewage leakages or combined sewer overflow, while processing and removal are mainly performed by bacteria through denitrification [41]. Major sources of P to SWMP are animal wastes, leaf litter, and soil particles transported by stormwater, while processing and removal occur mainly through plant uptake and absorption into sediments [41].

The effectiveness of SWMPs in removing nutrients from stormwater runoff is debated. While some studies show SWMPs can efficiently remove N and P from stormwater, others show SWMPs can be a source of nutrients to downstream ecosystems [24,42,43]. Such contrasting evidence can be related to the effects of design, pond age, and seasonality on the efficiency of nutrient processing [41,44,45] (Figure 2). For example, removal of N can decline with increasing depth, volume, and relative area of the SWMP [46], while hydrodynamics (e.g., storage capacity) can be fundamental for nutrient retention [47]. Likewise, seasonality can lead to vertical stratification of P with accumulation on the pond bottom and nutrients reducing concentrations towards the surface as a result of limited particle resuspension [48]. However, evidence of the factors regulating nutrient dynamics is still very limited compared to studies in natural aquatic ecosystems [49,50]. This can impair efficient planning and use of SWMPs [46]. This is especially the case for less-studied nutrients, such as carbon [51]. Evidence shows that newly constructed SWMPs can be sources of carbon to the atmosphere, despite high carbon storage in sediments [52]. This suggests that SWMPs could be included in global carbon budgets, but evidence on the factors determining carbon sequestration/emission in SWMPs is still needed [51,52].

Better understanding nutrient cycling is fundamental to clarifying the role of SWMP in net nutrient processing and eutrophication. Specifically, information on the sources and pathways of nutrients, the factors influencing nutrient processing (e.g., design, environmental), and the mechanisms for nutrient cycling have the potential to help advance towards SWMPs that are more efficient in processing nutrients [53] (Figure 2). This information can contribute to improving current modeling predicting nutrient removal in SWMPs [54,55]. In the future, mesocosm experiments and long-term monitoring of existing SWMPs can help identify factors modulating nutrient dynamics in these systems [24,56].

4.2. Physical Processes

Physical processes can be critical in modulating chemical reactions and biological interactions in SWMPs [57]. Two important physical processes in SWMPs are sedimentation and temperature (Table 1).

4.2.1. Sedimentation

Sedimentation (i.e., the removal of suspended solids) is an important process in SWMPs. Sedimentation is one of the few SWMP functions regulated by governments with clear targeted goals. For example, in Ontario, Canada, 60% to 80% of suspended solids must be removed by SWMPs to safeguard water quality in downstream ecosystems [58]. At the same time, sedimentation is an important process for pond managers because it reduces stormwater storage capacity, determining the frequency of SWMPs maintenance operations (i.e., dredging) [59]. Sedimentation is also an important ecological factor modulating primary productivity through light limitation and the behavior of the biota by visibility restraint [60,61].

Existing efforts have been placed on understanding the rates of sediment settling and regulating factors such as the physical characteristics of the particles, vegetation, stratification, turbulence, and residence time [62] (Supplementary Table S2). Although SWMPs seem effective in removing a large amount of suspended solids from stormwater [63], there is still a need to consider long-term suspended sediment dynamics and the use of devices and design for efficient sediment removal [62,64,65] (Figure 2). Recently, numerical models and machine-learning approaches have been proposed as a tool to understand and predict sediment dynamics in stormwater ponds, and floating treatment wetlands and technologies for real-time control of the outflow show promise in improving sediment retention in SWMPs [55,64,66,67,68].

4.2.2. Temperature

Thermal dynamics is another key ecological process in SWMPs. Thermal dynamics studies often focus on the role of SWMPs in changing the temperature of the outflow stormwater and in-pond stratification (Supplementary Table S2).

The SWMPs tend to absorb solar radiation, increasing the temperature of stormwater that flows out of the ponds. The temperature of SWMPs outflows can be up to 1.2 °C higher than inflows, posing a potential risk to receiving downstream ecosystems [69]. Several factors can modulate outflow temperature, such as catchment characteristics (e.g., drainage area), climate (e.g., rainfall depth and initial water temperature), and design (e.g., pond depth, length-to-width ratio, surface area, outlet depth (top-draw vs. bottom draw)) [70] (Figure 2). It is possible that controlling design parameters, such as increasing pond depth and positioning of the outlet at the bottom of the pond instead of mid-depth, is effective in providing a cooling effect, but empirical evidence is still lacking [71].

The ability of SWMPs to absorb solar radiation, in addition to input from overlaying air and lateral heat transfer, can lead to thermal stratification [48]. Despite being small in area and shallow in depth, SWMPs are frequently thermally stratified for extended periods of time, markedly over the summer [48,72,73]. Strong stratification, with temperature differences of 1–4 °C between top and bottom layers, has been reported over the summer [48,73], while water-column temperature can become relatively constant in the fall [74]. Thermal stratification seems to be determined by pond design, such as water depth and surface area, but the effects of other pond characteristics (e.g., riparian vegetation) are still unclear [48]. Most recently, machine-learning methods have been proposed to help predict thermal profiles in SWMPs [71].

In lentic ecosystems, such as SWMPs, thermal stratification influences both biological and chemical factors [75,76]. Evidence suggests that thermal stratification in SWMPs affects nutrient dynamics, particularly leading to higher phosphorous concentrations at the bottom, likely as a result of release from the sediments under anaerobic conditions [48]. The relationships between thermal stratification and dissolved oxygen, seston nitrogen, and total suspended solids are variable, which suggests that factors other than temperature can influence the chemistry of the water column in SWMPs [73]. The effect of thermal stratification on biological processes in SWMPs is unknown, but evidence from lakes suggests a potential relationship with algal growth/Chl-a production and secondary production could exist [77,78]. Further exploration of the relationship between thermal stratification and biological/chemical factors is fundamental to advance our understanding towards the determinants of ecosystem functioning in SWMPs (Figure 2).

4.3. Biological Processes

The SWMPs often integrate elements from terrestrial and aquatic systems (similar to wetlands) that can provide habitat for both aquatic and semi-aquatic/wetland species. This can be vital for biodiversity because such habitats are scarce in cities [79].

4.3.1. Biodiversity

The SWMPs can be spontaneously colonized or stocked with diverse species, from algae and zooplankton to birds and mammals, holding as much biodiversity as natural wetlands [17,80] (Supplementary Table S2).

Salt-tolerant plant species, such as the Hard-rush (Juncus inflexus) and Broadleaf Cattail (Typha latifolia), can dominate SWMPs [81]. Likewise, pollution-tolerant macroinvertebrates, such as odonates, Coenagrionidae, Libellulidae, and some gastropods, are commonly found in SWMPs [80,82]. Amphibians also use this habitat, primarily frogs and toads such as Green and Pickerel frogs (Lithobates clamitans, L. palustris) and American Toad (Bufo americanus) [83]. Other taxa, such as reptiles, fishes, birds, and mammals, are less studied, but their presence in SWMPs has been reported [17]. In general, SWMPs can harbor a large number of non-native species, but native species, including species with high conservation value, have also been found [81,82,84]. Whether native or non-native species dominate SWMPs likely depends on context, and management strategies are expected to have a fundamental role in shaping that biodiversity. For example, the use of algicide/herbicide to manage algal blooms in SWMPs can homogenize zooplankton communities [85] (Figure 2). However, the extent to which different management strategies and other factors, such as pond design and landscape heterogeneity, determine SWMPs biodiversity is yet to be fully understood. Such information is critical for incorporating SWMPs into a network that facilitates promoting urban aquatic biodiversity at large landscape scales [86].

Another important consideration is the role of SWMPs in facilitating species invasions (Figure 2). A large number of non-native species are found in SWMPs [87,88]. Several factors can contribute to the occurrence of non-native species in SWMPs. Newly constructed SWMPs may lack the ecological resilience of an established native ecosystem and provide ecological opportunities, making them susceptible to non-native species colonization [89,90]. Also, cities are hubs for the trade and transport of non-native species with commercial value, such as aquarium/ornamental trade and food production [91,92]. This provides diverse pathways for the introduction of non-native species, creating high propagule pressure that facilitates invasion [91,93]. It is likely that pet release is a common pathway for species introductions in SWMPs [87,94]. Empirical studies exploring invasion pathways into SWMPs are needed to allow for modeling invasion risk and designing mitigation actions [95]. The extent to which the characteristics (i.e., traits) of non-native species change to facilitate adaptation to SWMPs is also an important factor [96]. Non-native aquatic species can take advantage of urbanization by increasing reproduction, body size, and condition, which leads to enhanced invasive potential [97]. The extent to which adaptation in SWMPs enhances invasiveness of non-native species is unknown (Figure 2). Studies that contrast the traits of populations inhabiting SWMPs and urban/non-urban source ecosystems have the potential to expose the mechanisms through which invasive species adapt and succeed in such systems [89].

4.3.2. Habitat Provisioning

Reviewed papers indicate that SWMPs harbor ecological structure and processes related to habitat provisioning (Supplementary Table S2). The provisioning of habitat by SWMPs can facilitate the movement of organisms and their genes. Urban areas are characterized by high habitat heterogeneity [98]. This is especially the case for aquatic/semi-aquatic habitats that, in urban areas, are scarce, often isolated by barriers such as roads or buried in culverts [99]. This leads to habitat fragmentation with biota having few opportunities to disperse among habitats. In such scenarios, SWMPs could increase connectivity among habitat patches, facilitating the movement of organisms. However, the extent to which SWMPs contribute to aquatic/semi-aquatic habitat connectivity remains poorly known. Current evidence is conflicting, suggesting SWMPs can be of little or major importance for connectivity depending on their placement on the landscape and surrounding habitat [100,101]. Studies approaching functional landscape connectivity by considering not only distance between waterbodies but also surrounding land use are needed and have the potential for better mechanistic understanding of the role of SWMPs in connectivity [102] (Figure 2).

Despite its importance, the quality of the habitat provided by SWMPs can vary greatly. Habitat quality is dependent on the environmental and biological characteristics of each pond, the surrounding landscape, and management (Figure 2). For example, increased pond size, water depth, shoreline complexity, sediment depth, and macrophyte community can provide more complex (i.e., better) habitat [17,103]. Conversely, water pollution, the presence of invasive species, and poor vegetation management can reduce habitat quality [17,103]. Better understanding the context (environmental, landscape, management) in which SWMPs provide high- or low-quality habitat seems fundamental for advancing towards improved pond design and management to optimize ecological function.

Poor habitat quality in SWMPs can force the biota to rapidly adapt or become extirpated. In such cases, species that persist may express changes to their characteristics (i.e., traits) that ensure survival. For example, Gray Treefrog, Hyla versicolor, exposed to contaminated SWMPs sediments develop faster and grow larger at metamorphosis than treefrogs exposed to clean sediments [104]. Changes to traits can be pronounced to the point that an urban phenotype can be identified when comparing populations of the same species occurring in urban vs. non-urban environments [105]. However, the occurrence and magnitude of trait changes as a function of habitat quality in SWMPs remain poorly understood [106]. Assessing how species traits may change in response to varying habitat quality can help us advance towards understanding the mechanisms through which species persist and adapt in SWMPs [107].

Habitat quality can also cause SWMPs to become ecological traps [82,83]. SWMPs with low-quality habitat can represent ecological traps, which are areas where reduced reproductive success and/or increased mortality rates in species occur to the point that a stable population cannot be supported without immigration [108]. The extent to which SWMPs represent ecological traps is unclear [17]. For example, SWMPs can be both suitable habitats or ecological traps for amphibians, depending on the level of water pollution, hydroperiod, surrounding landscape, and the species considered [83]. Research on population dynamics, with special attention to the reproductive success of individuals, can help clarify the role of SWMPs as ecological traps (Figure 2). Such information can help to build a decision framework for identifying, preventing, or mitigating SWMPs as ecological traps [109].

5. Conclusions and Future Directions

Our review considered only primary, peer-reviewed, literature overlooking gray literature (non-peer-reviewed reports) that are often produced by governmental agencies and community groups. This may have restricted our overview of the field. Also, multiple alternative terms have been used to refer to SWMPs (e.g., roadside pond). We have limited our search to the most used terms (i.e., stormwater pond, stormwater management pond, and wet pond). This may have influenced our delimitation of knowledge in the field. Despite those limitations, our review exposes knowledge needs in the understanding of ecosystem functions in SWMPs. The greatest number of existing studies focused on geochemical processes, especially nutrient dynamics and contaminant processing. Less is known about physical (e.g., sedimentation and temperature dynamics) and biological (e.g., habitat provisioning and biodiversity) processes. Reviewed papers indicate that different ecosystem functions in SWMPs can be influenced by factors such as pond design (e.g., pond depth, length-to-width ratio, surface area, outlet depth, age), landscape (e.g., types of land use), and management practices (e.g., removal of vegetation). Therefore, we encourage future studies to expand beyond the description of patterns to explore the context dependency of ecosystem processes in SWMPs (Figure 2). This is also fundamental for placing the research into a broader context of global environmental changes.

Climate-change predictions forecast profound modifications to hydrology, with increased frequency and intensity of rainfall and droughts [110]. As a result, the volume of stormwater runoff and pollutant load discharged into SWMPs will likely increase in the near future [111]. The extent to which climate change will affect ecosystem function in SWMPs is unclear, with evidence showing varying effects. For example, SWMPs are expected to continue to remove contaminants, such as ammonia and phosphate, under climate change scenarios, while the removal of suspended solids is uncertain, with studies showing both sustained or loss of removal capabilities into the future [112,113]. A better knowledge of ecosystem processes can greatly improve our capacity to predict the efficiency of SWMPs in controlling stormwater quantity/quality in the near future. This will be fundamental for evaluating the effectiveness of SWMPs in the conservation of urban aquatic ecosystems.

It is also important that future research explores the role of SWMPs as socio-ecological systems. Beyond their primary purpose, SWMPs can provide a range of other benefits to urban residents (e.g., ecosystem services) [17]. Potential ecosystem services that SWMPs could provide include aesthetics, recreation (e.g., walking, nature watching), education (e.g., urban field laboratories), higher real estate values, and carbon sequestration [114,115]. Despite these potential services, very little is known about the range of ecosystem services provided by stormwater-control infrastructures, including SWMPs [116]. We recommend that assessments of ecosystem services in SWMPs take advantage of current approaches and place co-production of knowledge and environmental justice at the center of the research [117]. Further understanding ecosystem functions is the first step to identifying the links between social and ecological systems. Such links will be fundamental for advancing towards urban infrastructure that promotes the sustainable cities of the future, conserving aquatic biodiversity, and promoting human health and well-being.