Keywords

1 Introduction

1.1 Characteristics of Small Waterbodies

As an important part of the freshwater system, small waterbodies are the most numerous freshwater environments in the world and can be seen everywhere. Up to now, however, small waterbodies have no specific definition; they usually encompass ponds, small lakes, small rivers, streams, ditches, and springs [1, 2]. In general, small standing waters are defined according to the area of watershed. For example, small lakes vary in size from 1–5 ha to around 50–100 ha in area (but the Water Framework Directive (WFD) limit 50 ha) [1, 3], and the area of ponds ranges from 1 m2 to about 2–5 ha [1, 4, 5]. At one extreme, the tiny puddle which forms after rain is classified into pond [1]. Small running waters such as small streams and small rivers are relatively harder to define, and the term small stream is often used interchangeably with the term headwater. Furse et al. [6] define the length of stream within 2.5 km from the source as headwater streams, while small linear headwaters are those with catchments less than 10 km2 in the WFD terms [3]. In addition, other small waterbodies are distinguished mainly depending on their functions or hydrological characteristics. Ditches are man-made channels build primarily for agricultural or street drainage; normally the width of ditches is 1–3 m [7]. Spring is a strictly delimited place where the groundwater appears at the surface [8]. Furthermore, small waterbodies are naturally created or man-made and permanent or seasonal.

Small waterbodies are not only various but also numerous. The river network in Europe is more than five million kilometers long; more than 80% consist of small rivers, commonly known as headwaters, streams, creeks, or small rivers [9]. In addition, there are around 5–10 million small lakes and ponds in Europe and about 17 million ponds up to 1 ha in the United States, but the latter data omits the waters less than 1,000 m2 [10]. However, half to two-thirds of all waterbodies are in the range 25–400 m2 in some area (e.g., southeast Great Plains in the United States) [11]. Accordingly, the current global number of small waterbodies may be underestimated because of omitting those smaller ones [11]. For instance, there are approximately 32,000 and 120,000 ponds ranging from 100 m2 to 5 ha in area in Switzerland and Denmark, respectively [12, 13]. Furthermore, ditches constructed for agriculture are likewise widespread. In England, ditch length is about 600,000 km and in Netherlands 300,000 km [7, 14, 15]. Overall, inland small waterbodies are globally abundant freshwater habitats.

Compared to the large waterbodies, small waterbodies, especially ponds and small lakes, are areas of high biodiversity [1]. Despite small waterbodies only occupy 2% of Earth’s surface area, their species richness is nearly equal to marine environments [16]. Support for this idea comes from several studies which have surprisingly found that the ponds sustain a larger proportion of freshwater species than lakes or rivers [3, 17, 18]. In addition to the common freshwater species such as macrophytes and micro- and macroinvertebrates, there are specific species such as that of frogs and toads in small waterbodies [19, 20]. It has been suggested that the patchy and comparatively isolated nature of small waterbodies may be a reason of higher speciation rates [1]. Moreover, the dominant number, physicochemical heterogeneity, and low contamination rate of small waters may be another possible explanation of their high species richness [1, 18, 21].

With the increasing concern about small waterbodies, their ecosystem services are gradually recognized by researchers and policy-makers [1, 22]. Indeed, there are three main categories of ecosystem services: (1) provision, (2) regulation and maintenance, and (3) cultural support and entertainment [1]. For example, natural and man-made stormwater ponds provide a wide range of functions, including flood control, water supply, elimination of nutrients and pollutants, enjoyment and recreation, etc. [23]. Clearly, ditches constructed near the farmlands are mainly used for land drainage and irrigation [24]. The ecosystem services of small waterbodies have been highlighted within the literature [1, 2, 5].

1.2 Environmental Pollution in Inland Small Waterbodies

Because of their patches, small volumes, and catchments, small waterbodies are much more likely to fall entirely within unpolluted areas and to be less exposed to pollutants. Once they form in regions of frequent human activities such as development and intensive agriculture, however, they are easily contaminated by a variety of pollutants [1, 2]. Because small waterbodies have a considerably small water volume, there is less potential for dilution of contaminants. Therefore, they are exceptionally vulnerable to input of even small amounts of pollutants from their surroundings [25, 26].

Lots of studies show that small waterbodies are threatened by a range of pollutants due to the acceleration of urbanization and agriculture [25, 27, 28]. A typical problem of inland small waterbodies, particularly static waters, is “eutrophication” [18, 29,30,31]. Forty years ago, many researchers have started to study the causes and mechanisms underlying the process of eutrophication. To date, nutrient inputs such as nitrogen and phosphorus to inland waters are considered as the main causes of eutrophication [18, 29,30,31]. Dodds et al. [32] analyze the total nitrogen (TN) and phosphorus (TP) concentrations of rivers, streams, and lakes in the US Environmental Protection Agency (EPA) nutrient ecoregions and find median TN and TP values in the range of 0.248–3.372 mg L−1 and 0.012–0.184 mg L−1, respectively, much higher than the reference median values. Likewise, the freshwater eutrophication is serious in developing countries. More than 80% of urban rivers are contaminated in China [33,34,35]. For instance, ammonium nitrogen concentrations of water collected from the black-odor rivers in Wenzhou are 1.17–18.51 mg L−1, and TP concentrations range from 0.42 to 3.0 mg L−1 [33]. These values have shown us a high nutrient concentration in freshwater systems.

In addition to nutrients, heavy metals, organic matter, pesticides, and plastics also cause inland small waterbodies to suffer from toxic pollutions [34, 36, 37]. These pollutants may enter and accumulate in small waterbodies via rainfall runoff, atmospheric deposition, mismanagement, etc. For example, stormwater ponds used extensively in stormwater management receive a variety of pollutants from rainfall runoff [27]. Weinstein et al. [37] demonstrate that polycyclic aromatic hydrocarbons (PAHs) are pervasive in the sediments of 19 stormwater ponds located in coastal South Carolina, and the high PAH levels in 5 stormwater ponds suggest that there are moderate to high risks to organisms and humans. Moreover, due to extensive application of pesticides, streams and ditches near farmlands are commonly exposed to pesticides via spray drift, edge-of-field runoff, or drainage [36]. Nearly half of European waters are at risk from pesticides [38], and 26% of 2,369 sampling sites of small streams in Germany are found to have considerable exceedances of regulatory acceptable concentrations (RAC) [36].

With increasing plastic production and usage year by year, plastic trash becomes “huge” and “ubiquitous”. Plastic pollution and risks have been gradually realized by the general public and reported by media [39]. Plastic litter entering the aquatic environment degrades to millions of smaller pieces [40,41,42], namely, MPs, which negatively impact waterbody ecosystems. Although MP pollution has been well documented in marine and large freshwater system, studies in small waterbodies are limited [28] (Fig. 1). So far, only a few papers have reported MP pollution in small waterbodies. Indeed, MPs can enter small waterbodies through sewage effluent and road runoff as well. Therefore, in this chapter, we will specifically introduce characteristics, risks, and management of MP pollution in inland small waterbodies.

Fig. 1
figure 1

Plastic litter in a pond located in Hangzhou, China

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2 Characteristics of Microplastics in Small Waterbodies

2.1 Sources of Microplastics in Small Waterbodies

Nowadays, activities involving plastics encompass packaging, textile, transportation, agriculture, electronics, and buildings and constructions, nearly covering all fields [43]. Human activities are considered as one of the major factors of MP pollution in freshwater environments [44, 45]. Particularly, plastic production, usage, and discard increase with the growing of population density, such as in Shanghai and Paris, both megacities [46, 47]. For example, Yonkos et al. [48] investigate MP contaminations in four estuarine rivers in Chesapeake Bay, USA, and find that the concentration of MPs is positively correlated with population density and proportion of urban/suburban development within the watersheds.

Illegal trash disposal and poor management may result in the increase of plastic trash from household and agricultural plastics in terrestrial environments, especially in developing countries. These plastic trashes enter small waterbodies directly or via wind transport, surface runoff, or agricultural fertilizers and then degrade into MPs, indicating a potential source of pollution for small waterbodies. For instance, lakes in tourism areas and ponds near residential areas or in parks are the recipient waters of plastic trash and MPs [28]. Another potential input of MPs to the pollution of small waterbodies may be the discharge of sewage comprising fibers from laundry wastewater [49] and microbeads from personal care products [50]. The effluent overflows into small waterbodies during storm events due to the limited treatment capacity of wastewater treatment plants (WWTPs) [51]. Some rivers in old cities are even the direct receiving waters of effluent from residential areas and factories. People still wash their garments directly in small waterbodies in some underdeveloped areas. In recent years, atmospheric fallout has been realized as a possible source of MPs, especially microfibers [46].

2.2 Occurrence of Microplastics

Up to now, to our knowledge, there are only a few papers about MP pollution in small waterbodies (Table 1). The average MP abundance in water and sediment samples varies greatly. This difference results from some key factors such as sampling sites and methods, human activities, and features of small waterbody [60].

Table 1 Microplastic pollution in small waterbodies worldwide
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Among these researches, MP abundance is the lowest (0.014 ± 0.009 items L−1) in water collected from fish ponds and rivers of the European Carpathian Basin [54]. The different sampling methods might explain the low concentrations of MPs. In this study, the authors use a mobile sampling system which only retains MPs between 0.1 and 2 mm in size, resulting in the loss of larger and smaller MPs [54]. Similarly, Dikareva and Simon [58] collect plastics using a phytoplankton net (63 μm mesh) and find that MP abundance is in the range of 0.02–0.3 items L−1 in streams in Auckland, New Zealand. However, a bulk sampling approach is used in most of other studies. Generally, water is filtered through a mesh filter in small size (e.g., 10 or 20 μm), and MPs larger than this size are supposed to be collected [28, 52, 57]. Hu et al. [28] investigate MP occurrence in the waters of six types of small waterbodies in East China and find MP concentration in the puddles up to 15.7 ± 4.6 items L−1. Usually, a puddle is a downfold that has a considerably small total water volume. The rainfall runoff with MPs enters the puddles, leading to a higher MP pollution. Of course, the MP abundances are also high as well in other types of small waterbodies in the same survey area [28]. The authors suggest that the high accumulation of MPs is related to the higher population density and more anthropogenic activity [28]. For example, Shanghai is currently the most populated city in China. Shanghai has a population of 24.2 million, a primary plastic product of 3807.3 thousand tones, and a chemical fiber product of 430.0 thousand tones [61]. Furthermore, most of the sampling sites of East China locate near residential areas and textile processing plants. Surprisingly, the highest MP concentration (270 items L−1) is detected in a stormwater pond in Viborg, Denmark [57]. Possible explanations for the high MP abundance are that the stormwater retention time of the stormwater pond is about 10 days and its drainage area includes production industries, retailers, building supply stores, parking lots, as well as roads.

Similarly, the trend of MP abundance in sediment is consistent with that in water (Table 1). The lowest abundance of MPs in sediment is detected in European fish ponds and rivers (0.8 ± 0.4 items kg−1) [54], and the highest MP density is determined in a stormwater pond in Viborg, Denmark (9.5 × 105 items kg−1) [57]. These indicate that the functions of small waterbodies probably influence their MP abundances. Stormwater retention ponds are versatile in terms of MP input pathways, and it is likely that they are important sinks for MPs. In addition, MP abundances in sediment samples collected from other types of small waterbodies are approximately hundreds per kilogram (Table 1). To sum up, MPs are abundant in small waterbodies, and such waterbodies play a role in receiving diffuse MP pollution from urban and highway areas.

2.3 Shape and Size of Microplastics

The literature in Table 1 demonstrates that MPs in small waterbodies are commonly categorized into four types according to their shapes: fiber (elongated), fragments (small irregular pieces), film (thin flat), and granule/pellet (spherical and ovoid pieces). Fibers are dominant in water. For instance, Luo et al. [52] and Hu et al. [28] find fibers to be the most abundant, accounting for 88% of the total MPs. Compared with MPs in water, the shape distribution of MPs in sediment significantly varies. Pellets are predominant in samples from urban river in Shanghai [47], fibers are prevalent in riceland [53] and Edgbaston Pool [59], while fragments are more abundant in small waterbodies in East China, United States, and New Zealand [28, 55, 58]. The morphological characteristics of MPs can be used to indicate their potential sources. For example, the pellets in urban river possibly originate from personal care products, and the potential sources of fragments in stormwater ponds are stormwater runoff containing road plastic debris.

Small size (<5 mm) is another key parameter of MPs involving their bioavailability [62]. Various sampling methods and size classifications are used in fieldworks, which make it difficult to compare data from different studies. Therefore, in this review, our concern is the main size of MPs in small waterbodies. Results show that 0.1–1 mm MPs are most abundant in small waterbodies (Table 1). On one hand, <0.1 mm MPs are hard to observe under microscopes. On the other hand, MP abundance generally increases with the decreasing of their sizes [28, 63]. Thus, these may be the reason that 0.1–1 mm MPs are dominant in these studies.

2.4 Polymer Types of Microplastics

Plastics are made from a wide range of polymers. The polymer types affect MP density, longevity, and performance and indicate their probable origins [43]. In the studies (Table 1), MPs in water and sediment samples are randomly selected to identify their polymer types using Fourier Transform Infrared spectroscopy (FT-IR). Overall, polypropylene (PP), polyester (PES), and polyethylene (PE) are the most common polymers in small waterbodies (Table 1). A larger global demand for these three types of polymer makes them more widespread in the environment [64]. Specifically, PES and PP are the dominant polymers found in water, while PP and PE are the most abundant polymers in sediment, which is consistent with the results from studies of large freshwater environments [65, 66].

3 Occurrence of Microplastics in Freshwater Biota from Small Waterbodies

Organisms in the aquatic environment are believed to be impacted by MP ingestion [62]. Based on the above data, MP contamination is pervasive in small waterbodies of high biodiversity. Therefore, exploring MP pollution in freshwater biota in small waterbodies is necessary. Here we summarize the related researches in Table 2. Six papers investigate the uptake of MPs by fish. Results show that MPs have a relatively low detection rate and concentration, except for a high concentration of MPs (3.3 ± 0.5 items individual−1) in eels [53], and a fuzzy data by Olesen et al. [57] indicate that the average MP abundance in three-spined sticklebacks and young newts is 65 items individual−1 (340 items g−1). In the study by Lv et al. [53], MPs found in crayfish (2.5 ± 0.6 items individual−1) and loach (1.8 ± 0.5 items individual−1) are likewise high. Additionally, MP contaminations are widely found in different species of tadpoles, especially Rana limnochari and Microhyla ornata, with great abundance expressed in terms of weight [28].

Table 2 Microplastic pollution in organisms from small waterbodies
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The difference of MPs in organisms may result from MP contamination of habitats and organismal feeding strategies [28, 70, 71]. For example, Setälä et al. [71] expose a range of animals (bivalves, free-swimming crustaceans and benthic, deposit-feeding animals) to microbeads with different concentrations (5, 50, and 250 beads mL−1). Results show that the amount of microbeads in animals is concentration dependent and free-swimming crustaceans ingest more microbeads compared with benthic animals. In addition, Mizraji et al. [70] observe that herbivorous or carnivorous fish have a lower MP concentration than omnivorous ones, which have a wider range of food sources. On the other hand, the number of MPs ingested by organisms is related to the MP bioavailability as well [72], just as some invertebrate species with a wide feeding size range have been demonstrated to selectively forage on specific sizes when expose to multiple size particles.

Fibers <1 mm are the most abundant MP type in most of organisms in small waterbodies (Table 2). Furthermore, PES and rayon are the prevalent polymers, with exception of the study by Lv et al. [53]. The authors identify that PE dominate in organisms; this result is similar to that found in sediment samples. The authors deduce that the ingestion of MPs in eels, crayfish, and loach might be related to their habits. These three animals are considered as typical benthic organisms, which commonly forage and live in the bottom. Conversely, Hu et al. [28] find that the shape and polymer distributions of MPs ingested by tadpoles are most similar to that found in water, mainly resulting from tadpoles ingesting MPs when they swim through the water column.

Small waterbodies provide habitats for a variety of organisms and are areas of high biodiversity. Once they are polluted, their resident organisms will be affected by the pollutants. Here, we demonstrate that MPs are ingested by animals in small waterbodies such as amphibians which could transport MPs from aquatic to terrestrial food webs. However, the study area and species are limited. Hence, in the future, more surveys will need to be carried out in order to widely investigate MP contamination in the creatures of small waterbodies.

4 Effects of Microplastics on Freshwater Biota

Field studies have proved that MPs are widely detected in aquatic ecosystems, which increases the attention to adverse impacts on freshwater ecosystems. Scherer et al. [72] have summarized laboratory studies about the interactions of MPs with freshwater biota. They discuss biotic and abiotic factors affecting MP ingestion. Biotic factors focus on the feeding type of invertebrates and vertebrates, particularly invertebrates (e.g., flagellate, rotifers, cladocerans, blackworm, bivalves, etc.). Abiotic factors include microparticle size, shape, and taste, which affect the bioavailability of MPs. Next, the effects of MPs on freshwater organisms have been summed up by Scherer et al. [72]. A variety of physical impacts induced by MPs to algae, Daphnia magna, bivalves, gastropods, and fish include blockages, reduced dietary intake, and internal injuries [62]. In addition, MPs can act as carriers of chemicals and microorganisms, aggravating the adverse effects of MPs to the organisms [73,74,75].

Amphibians are typical animals living in small waterbodies. At present, a field work completed by Hu et al. [28] has confirmed that tadpoles can ingest MPs from their living environment. However, limited information regarding MP ingestion and its effects on amphibians is available in the laboratory. Currently, thereby, we mainly described the ingestion and effects of MPs on amphibians.

As representatives of amphibians, Xenopus laevis/tropicalis are always used as model organisms in the world. The tadpoles of Xenopus laevis/tropicalis are filter feeders which are supposed to be especially prone to MP ingestion because of their extensive feeding activities [76, 77]. Multiple studies have shown that MPs ingested by Xenopus tadpoles accumulated in the gills, alimentary canal, stomach, and gut, and nano-plastics are also detected in the blood, cytoplasm, nucleus, and periphery of digestive gut cells [77,78,79,80]. Moreover, micro- and nano-plastics are potential threats for the growth and development of Xenopus larvae. The embryos of Xenopus laevis exposed to 50 nm polystyrene nanoparticles display numerous malformations including disorders in pigmentation distribution; anomalies of the head, eyes, intestine, and tail; edema in ventral anterior zone; and a stunted body [79]. Additionally, embryo mortality rate exhibits a dose-dependent relationship with MP exposure [79]. However, De Felice et al. [78] expose Xenopus tadpoles to polystyrene MPs (2.75 ± 0.09 μm of diameter) at different concentrations (1 × 105–8.7 × 105 particles mL−1) and find that neither body growth nor swimming activity of tadpoles are affected. These differences among the researches may be induced by organismal development stage, exposure time, MP concentration, etc. Thus, further studies will need to be performed in order to fully explore MP toxic effects and risks on amphibians.

5 Management of Small Waterbodies

To sum up, small waterbodies are a critical but vulnerable part of freshwater ecosystems. Their functions are of equal importance to the larger waters, but they are more susceptible to many human activities (e.g., development and intensive agriculture, pollutant discharge) and climate change [1]. Despite this, nevertheless, small waterbodies are still the least studied part of the freshwater environment and are largely excluded from freshwater management planning, even rarely recognized by people in developing countries. In recent years, more and more countries and organizations recognize the importance of small waterbodies and pay attention to their pollution and management [1,2,3, 9]. For example, in 2013, a workshop organized by the European Environmental Bureau (EEB) took place in Brussels to discuss possible ways to better protect and manage small waterbodies. In 2015, a special session on “Small waterbodies – knowledge base, importance, threats, and future research priorities” was carried out at the 9th Symposium for European Freshwater Sciences (SEFS) in Geneva. This session emphasized the importance of small waterbodies and aimed to refocus research attention on these resources.

Nowadays, there is a broad consensus that small waterbodies are reservoirs of biodiversity and significantly contribute to catchment diversity and that they should be integrated into the existing legislative framework to get better protection and management [1, 2]. While there are lots of gaps in our knowledge of small waterbodies, compared to larger waterbodies. Therefore, first and foremost, more scholars should be appealed to conduct relevant researches about small waterbodies, and to reduce the gaps in knowledge. In addition, sufficient research and activity funds should be provided for environmental action. Because small waterbodies are globally abundant, this may signify a significant financial and administrative burden if they are included in the legislative framework process more extensively.

Since it is very hard to remove MPs from the environment, plastic source control and management may be an excellent way to reduce MP pollution in small waterbodies [81]. Firstly, we should take measures to reduce the use of plastics such as the ban of microbeads in personal care products, increasing the production and use of cotton clothing, replacing non-biodegradable plastics with biodegradable ones, etc. [50, 82]. Secondly, comprehensive implementation of garbage classification conduces to waste management and the mitigation of MP diffuse pollution from rainfall runoff and agricultural fertilizers [83, 84]. Finally, we should enhance the ability of wastewater treatment plants to remove MPs and avoid the release of untreated effluent directly into the receiving waters [51, 85, 86]. MP contamination in small waterbodies should be reduced through the above measures applied.

In conclusion, small waterbodies are the most numerous freshwater environments worldwide and are under all the threats affecting larger waters, but they are largely excluded from water management planning. This section introduces small waterbodies and reports their MP distribution, characteristics, and toxic effects. It suggests that MPs are prevalent in water, sediment, and resident animals, especially in the form of <1 mm fibers, probably due to human activities nearby. In addition, we discuss the potential adverse effects caused by MP exposures to resident animals, particularly amphibians. Corresponding high abundances and potential adverse effects of MPs strongly suggest the need for increasing attention and researches, reducing inputs of plastic waste, and supporting protection and effective management of small waterbodies.