Microplastic sampling techniques in freshwaters and sediments: a review

Abstract

Pollution by microplastics is of increasing concern due to their ubiquitous presence in most biological and environmental media, their potential toxicity and their ability to carry other contaminants. Knowledge on microplastics in freshwaters is still in its infancy. Here we reviewed 150 investigations to identify the common methods and tools for sampling microplastics, waters and sediments in freshwater ecosystems. Manta trawls are the main sampling tool for microplastic separation from surface water, whereas shovel, trowel, spade, scoop and spatula are the most frequently used devices in microplastic studies of sediments. Van Veen grab is common for deep sediment sampling. There is a need to develop optimal methods for reducing identification time and effort and to detect smaller-sized plastic particles.

Introduction

Ecosystems and their different organisms have been widely impacted by anthropogenic activities such as the discharge of pollutants (Hamidian et al. 2019; Mirzajani et al. 2016, 2015; Padash Barmchi et al. 2015; Rezaei Kalvani et al. 2019), including emerging contaminants (Jafari Ozumchelouei et al. 2020) such as microplastics. Endurance, flexibility, lightweight, being low cost and being waterproof allows for plastic use in different applications, leading to their accumulation in the environment (Pellini et al. 2018; Razeghi et al. 2020). Plastic materials are used in a wide variety of markets and industries, including packaging, building and construction, electrical, agriculture, consumer and household appliances such as toothpaste and facial scrubbers, etc. (Wu et al. 2018). According to data from Plastics Europe, world production of plastics reached 335 million tons in 2016 (Plastics Europe 2018). It is estimated that by 2050, this may increase to 33 billion tons (Horton et al. 2017). By then, 12,000 million metric tons (Mt) of plastic waste will have been accumulated in landfills and natural environments (Geyer et al. 2017). Recently, a sharp increase is induced in plastic waste production such as masks, gloves, and plastic shopping bags by the coronavirus disease (COVID-19) pandemic (Gorrasi et al. 2020). Once plastics are discharged into aquatic environments, they can persist for up to 50 years. Complete plastic mineralization may take hundreds or thousands of years (Holland et al. 2016).

Microplastics are generally defined as plastic particles smaller than a specified upper size limit (< 5 mm). However, sometimes smaller size limits have also been proposed. Currently, there is no specific lower size cutoff for this definition (Connors et al. 2017). Since the 1970s when the first reports of micro-sized particles were published, marine plastic pollution has been of concern (Carpenter and Smith 1972; Carpenter et al. 1972; Colton et al. 1974). Plastic debris in the ocean was recognized by the United Nations Environment Program (UNEP) as an emerging global environmental issue (Kershaw et al. 2011). However, “microplastics” were first described by Thompson and colleagues in 2004. They reported the occurrence and presence of plastics around 50 µm in size on shorelines and in water column (Thompson et al. 2004). Microplastics are commonly defined as plastic particles with sizes below 5 mm (Hidalgo-Ruz et al. 2012).

Depending on the way in which microplastics are produced, they can be classified into two classes as primary or secondary. Primary microplastics are small plastic particles released directly into the environment by domestic and industrial effluents, spills and sewage discharge or indirectly by runoff. Secondary microplastics are formed as a result of fragmentation of larger plastic particles already present in the environment. Fragmentation takes place due to UV radiation (photo-oxidation), mechanical transformation (e.g., via waves abrasion) and biological degradation by microorganisms (de Sá et al. 2018; Thompson et al. 2009).

There are hundreds of commercially available plastic materials. Polypropylene and low- and high-density polyethylene are the three most common used plastic polymers in packaging. Polyvinyl chloride, polyurethanes, polyethylene terephthalate, polystyrene and polyester are also widely used due to their various applications (Plastics Europe 2018).

In terms of shape, microplastics fall into five main groups. Fragments are three-dimensional and hard jagged-edged particles. Pellets have hard rounded shape. Fibers are fibrous or thin uniform plastic strands, and films are thin, two-dimensional plastic pieces. Foam is a mass of tiny bubbles (i.e., styrofoam-type material) (Anderson et al. 2017; Rezania et al. 2018). Using different measuring instruments, different results may be obtained in terms of shape, size and type of microplastics. The major sampling techniques are shown in Fig. 1.

Fig. 1

Common techniques to sample microplastics

Despite extensive research on microplastics in marine environments, less effort has been made to monitor microplastics in freshwaters. Freshwaters include water in ice sheets, ice caps, glaciers, icebergs, bogs, ponds, lakes, rivers, streams, marshlands, wetlands and groundwater. Freshwaters are generally characterized as having low concentrations (less than 1000 mg L⁻¹) of dissolved salts and other total dissolved solids (American Meteorological Society 2012). Although the ocean floor is considered to be the ultimate fate of marine microplastics, inland water bodies might also be a terminal or transient sink for microplastics (Z. F. Wang et al. 2018b). Freshwater bodies can have comparable plastic concentrations to marine waters.

Microplastics can cause several harmful physical effects on humans and living organisms through such mechanisms as entanglement and ingestion. They can cause blockage of the gastrointestinal tract or inflammatory responses and consequently a range of adverse effects. Some effects include lower energy reserves, reduced reproduction/growth, oxidative damage, metabolism disruption, cellular lesions, starvation and even death (Ding et al. 2018; Ogonowski et al. 2016). Exposure of microplastics to a cohort of human adults (hand-face skin, head hair and saliva) has been reported (Abbasi and Turner 2021).

The trophic transfer of microplastics in the aquatic food web has been demonstrated by researchers (Farrell and Nelson 2013; Setälä et al. 2014). Microplastics’ large surface area to volume ratio provides a high association potential for environmental contaminants. Microplastics have an affinity for certain hazardous hydrophobic organic chemicals, non-essential trace elements and persistent organic pollutants. Some examples include polychlorinated biphenyls, dichlorodiphenyltrichloroethane, additives, plasticizers and heavy metals (Brennecke et al. 2016; Hartmann et al. 2017; Holland et al. 2016; Koelmans et al. 2016; Naqash et al. 2020).

Wastewater treatment plants receive large amounts of microplastics among other pollutants. However, efforts of treatment (Mojoudi et al. 2018, 2019) including biological methods (Alavian et al. 2018; Hamidian et al. 2016; Mansouri et al. 2013; Mirzajani et al. 2017) remove most of these emerging pollutants.

Different microplastics treatment methods include sorption and filtration, biological removal and ingestion, and chemical treatments. Sorption of microplastics on green algae is based on charged microplastics. Membrane technology regarding durability, influent flux, size and concentration of the microplastics in water and wastewater have shown good efficiency. Coagulation and agglomeration processes, using Fe-based and Al-based salts, are also reported. Electrocoagulation technique and photocatalytic degradation using TiO₂ and ZnO semiconductors are used as robust and environmentally friendly techniques. Microplastics ingestion by organisms is also discussed as a removal strategy. However, sorption and filtration processes coupled with membrane bioreactors lead to higher microplastics removal compared to other methods (Hamidian et al. 2021; Padervand et al. 2020). Othman and coworkers reviewed microplastics degradation through enzymatic processes (Othman et al. 2021). ZnO nanorod photocatalysts excited by visible light were used to degrade low density polyethylene film in water (Tofa et al. 2019).

Inland waters and marine environments are facing similar issues related to microplastics presence. However, some differences like physical and chemical characteristics of water cannot be ignored. Here we review techniques for sampling microplastics in waters and sediments with focus of the following issues:

• a. What is the evolution of the number of scientific studies on microplastics in freshwater and sediment?

• b. Which freshwater compartments are more commonly investigated for microplastics?

• c. Which sampling matrix, water or sediment, is most frequently studied?

• d. What are the most common sampling methods in water and sediment studies?

• e. What are the advantages and disadvantages of sampling methods?

Data acquisition

Literature was gathered through online search in the ISI Website of Knowledge, Science Direct and Google Scholar using keywords and phrases including “microplastic” OR AND “freshwater”, OR AND “plastic particle”, OR AND “plastic fragment”, OR AND “pellets” OR AND “river” OR AND “estuary” OR AND “lake”. The retrieved articles were then screened by study area, of which studies in water and sediment of inland water systems were selected including rivers, estuaries, lakes, reservoirs, estuaries, etc. After identifying candidate research, the abstracts of all studies reporting microplastics in freshwater ecosystems were studied. It is worthy noticing that microplastic research solely on microplastics in freshwater species was excluded. However, a combination of water or sediment studies with biota or all three (water, sediment and biota) were included simultaneously. A total of 150 published pieces of research between 2010 and 2020 were retrieved and evaluated. Details of each study were recorded in an EXCEL spreadsheet for subsequent analysis. This information was used to determine the extent and depth of current microplastic research and to identify important data gaps.

Microplastics presence in freshwater environments

Research papers with an emphasis on microplastics in inland water bodies are mostly published in the last ten years. Microplastics have been recorded along shorelines of the Tamar estuary, UK (Browne et al. 2010). In two urban rivers (the Los Angeles River and the San Gabriel River), Southern California, microplastics were found 16 times more abundant than macroplastics and three times heavier than the bigger particles (Moore et al. 2011). Zbyszewski and Corcoran (2011) scrutinized the distribution and degradation of plastic particles along the beaches of Lake Huron, Canada (Zbyszewski and Corcoran 2011). The first ecosystemic review, assessing microplastics in different compartments, including water, sediment and biota, was reported by Faure et al. in Lake Geneva, Switzerland. Plastics were found on every beach and in the surface layer of Lake Geneva. However, no plastics were observed in biota in this study (Faure et al. 2012). Studies detecting microplastics in different freshwater compartments across continents and even in remote areas (Free et al. 2014; Zhang et al. 2016) are summarized in Tables 1–4.

Table 1 Microplastic studies in water of inland water bodies around the world

Table 2 Microplastic studies in sediment of inland water bodies around the world

Table 3 Microplastic studies in water and sediment of inland water bodies around the world

Table 4 Microplastic studies in water, sediment and biota of inland water bodies around the world

The numbers of microplastic studies in freshwater environments increased rapidly from four in 2013 to 37 studies in 2019 and 27 papers as of September 2020 (Fig. 2).

Fig. 2

Frequency and trend of studies (n = 150) on the presence of microplastic particles in freshwater environment in different matrix including water, sediment, water + sediment, water or sediment + biota and water + sediment + biota

In the early stages, it was suggested that the chemical types of microplastics in freshwater seem to be less diverse compared to those collected from the marine salty environment. This observation was attributed to the higher density of seawater, which enables more types of plastic materials with different densities to float on the surface of the water (Zhang et al. 2015). However, this is not always the case, because the processes of controlling distribution and exposure to plastics particles are not necessarily restricted to a specific environmental compartment. Polymers with higher density (density > 1.0 g mL⁻¹) were observed in freshwater environments (Moore et al. 2011; Zhou et al. 2020). Negatively buoyant particles (e.g., polyester, rayon, nylon and cellulose acetate) may remain suspended in water (Baldwin et al. 2016). There may be differing degrees of physical and chemical characteristics, such as storms and wave action and saline water in marine systems. But plastics in freshwater systems still experience physical and chemical degradation (Andrady et al. 2011). It was suggested that polymer density alone is not the most significant control on microplastic particle fate within the aquatic environment. Microplastic morphology, incorporation into copolymeric materials during manufacturing and inclusion within aggregates of varying overall densities may play major roles in microplastics distribution (Hendrickson et al. 2018).

The ability to capture plastic particles from water or sediment matrix and separating them from organic and mineral material are challenging. Identifying types of plastics in the samples and on different surfaces is also important (Costa et al. 2021). It is suggested that microplastics in freshwater systems are similar to those in marine environments, and they are exposed to similar threats (Holland et al. 2016). Therefore, microplastic characteristics, detection methods, methods of analysis and impacts on biota are suggested to be similar.

Sampling procedure and tools

Choice of preservation techniques in different stages of microplastic studies largely depend on the research question (Lusher et al. 2017), economic proportionality of the methods and also the study compartment. Microplastics have now been reported in a range of freshwater environments, including surface water, water column, benthic sediments, littoral sediments and aquatic biota. Three main strategies are identified for sampling. They include selective sampling, volume-reduced sampling and bulk sampling. Different sampling strategies may be selected when the type of matrix to be examined for microplastics (water or sediment or biota) has been taken into account. Selective sampling in field consists of direct collection of items from the environment which are recognizable by the naked eye. This method is usually used on the surface of shore sediments and is more practical for large microplastics (1–5 mm). Bulk samples refer to samples where the whole volume of the sample is taken without reducing it during the sampling process. Volume-reduced samples in both sediment and water samples refer to samples where the volume of the bulk sample is usually reduced during sampling. Only a portion of the sample is preserved in this method, and it is mostly used for water samples (Hidalgo-Ruz et al. 2012).

Water compartment

In water samples, microplastic burden in the measuring units is much lower compared to that in sediment samples. Consequently, analysis of water samples requires higher sampling volumes (Huppertsberg and Knepper 2018). Volume-reduced methods are on-site filtration by nets or sieving. They are more suitable for water samples as they give a promising specimen volume without the need to transfer the whole to the laboratory. Therefore, resulting in a relatively small concentrated final sample. Here we discus three main water sampling methods, including trawls, pump samplers and grab samples.

Trawls and nets

Different types of trawls and nets like manta, neuston or plankton nets and bongo net are used (Fig. 3). Trawl is usually deployed off of a boat, submerged and towed on a linear course at a low speed for a set time or distance (Hidalgo-Ruz et al. 2012; Sighicelli et al. 2018). The area of each sampling is calculated by multiplying the towing distance with the width of the trawl. The volume of water through the net uses either a flow meter or calculations based on the distance traveled by boat at a constant speed (Sadri and Thompson 2014). However, because the net's immersion depth changes constantly with waves, wind and boat movement, it is difficult to estimate the exact volume of water being filtered.

Fig. 3

Microplastics sampling tools in freshwater studies; a manta trawl, b plankton net, c Petite ponar grab d Van Veen garb, e Ekman grab sampler, f box corer, g sediment corer, h metal pail, i showel, j trowel

Campanale et al. (2020) collected microplastics by three surface plankton nets fixed in the middle of Ofanto River, in order to reduce the spatial and temporal variability (Campanale et al. 2020). In order to ensure that the most representative body of water is being sampled, factors such as time, location and length of trawls in relation to the strength of tides should be carefully considered. Furthermore, the trawling distance using nets varies depending on the abundance of floating microplastics. It should further consider whether the trawl direction with reference to the prevailing wind direction could have an effect on the abundance and size of particles captured within the trawl (Zhang et al. 2018). In these methods, nets are limited by a single mesh size that is sometimes clogged by suspended material (e.g., organic matter or phytoplankton) (Liedermann et al. 2018; Sadri and Thompson 2014).

Types of microplastics are closely related to the mesh size of tools used for specimen collection. For instance, smaller-sized mesh used in some studies could increase plastic particles of certain shapes (e.g., fibers) to a concentration several orders of magnitude higher than those collected using nets with a larger mesh size. On that account, the abundance of microplastics is largely underestimated by researchers who used a trawl for sample collection (Z.F.Wang et al. 2018b). By using manta trawl, a significant fraction of actual small microplastic particles is very likely to be underestimated because they might pass through the net.

Regarding the limitations of obtaining sufficient water volumes while avoiding net clogging, it was strongly recommended to use tandem nets with different mesh sizes. This helps to better characterize smaller microplastics (Anderson et al. 2017). Dris et al. (2018) had a 250-times higher probability of sampling fibers when using an 80-µm mesh compared to a 330-µm mesh (Dris et al. 2018). Double neuston net trawl (500 µm mesh size) was used as sampling tool in assessing microplastics in surface waters of Lake Superior. No difference was detected between the paired net samples, suggesting that single net sampling produces a representative estimate of microplastic particle condition within a body of water (Cox 2018). A comparison study was conducted between a manta net and a neuston net for microplastics in ocean surface water. Results showed that the manta net tended to have slightly higher densities of microplastics than those of the neuston net. However, no statistical difference was observed. Neuston net is relatively stable in rough water although efforts are needed to maintain the net in submerged depth. Manta net tends to jump in rough water (Michida et al. 2019). Sampling with the manta trawl to function properly requires relatively calm conditions (Anderson et al. 2017). Modified BfG basket sampler used for the Austrian Danube River, clearly showed the necessity of a strong and stable equipment carrier. The nets were positioned on the surface, in the middle of the water column, and at the bottom of the river and with different mesh sizes (Liedermann et al. 2018).

Nets alone may fail to deliver the overall pattern of microplastic pollution in an area, because there does not seem to be sufficiently retaining fibers and small microplastics.

Pump samplers and grab samples

Volume reduction pump sampler and grab samples are also used in some of the research papers. Pump sampling consist of pumping water manually or using a motor through an inline filter. Grab sampling method includes using a bucket to collect water and sieve the water in the field (Han et al. 2020; Miller et al. 2017; Y. Mao et al. 2020a). A fixed amount of bottle is also submerged, filling with surface water for laboratory analysis (Barrows et al. 2018; Dubaish and Liebezeit 2013). Water collected using pumps or bulk samplers is taken from different depths with different volumes. Due to the high variability of microplastic spatial distribution, the sampling area covered is limited and using a pump or bulk sampler may not be representative. Therefore, taking multiple replicates is suggested (Zhang et al. 2018). However, pumps can be used to collect large volumes of water, which may be advantageous in areas where the density of microplastics is suspected to be low (Crawford and Quinn 2017). Water volume could be variable from 5 mL to 500 L (Braun et al. 2018). In addition, they do not possess the limitation caused by pacific sampling mesh tool. Based on literature reviews, significantly more microplastic particles are present in smaller size ranges. A combination of volume-reduced net-based sampling and bulk sampling seems to be very helpful in estimating the missing fractions and enables a greater spatial resolution (Fischer et al. 2016).

Comparison between different water sampling methods

In the comparison of manta trawling and pump sampling methods in microplastic sampling from water of Lake Tollense, Germany, different results were observed in the abundance of microplastics, microplastics shape and size. It was suggested that manta trawl is not sufficient in retaining fibers and small microplastics from water samples. Therefore, the pump sampling approach with the filtration of large water volumes is necessary to generate reliable results. However, the pump sampling covers small microplastics. Small plastic particles are greater in number. Volume-reduced sampling covers large microplastics, being less abundant but still important. Fibers detected in the manta samples were unevenly spread across the whole size range. Fibers found in the pump samples showed distinct positively skewed distribution peaking at > 500–600 µm in length. The most abundant polymer composition in manta trawl samples was polyethylene and polyethylene terephthalate for the pump sampling method (Tamminga et al. 2020).

Lahens and colleagues utilized a bucket and 300-µm plankton net. Bulk water sampling was used for anthropogenic fiber analysis and 300-µm-mesh size plankton net exposition for fragment analysis (Lahens et al. 2018). In the study of Su and colleagues, the average abundance of microplastics was found to be higher in plankton net samples rather than bulk surface water samples (Su et al. 2016). In general, water sampling volumes depend on the solid richness and the target microplastic size range. Barrows et al. (2017) compared grab samples to the conventional neuston net approach. Grab samples collected three orders of magnitude more microplastics than the net approach (Barrows et al. 2017).

In comparison between manta trawl and in situ pump filtration methods, it was found that the pump sampling method is more accurate in volume measurement and versatile for point sampling and filter size choice. However, due to the lower sampling volume, it might be more suitable for sampling in areas with a higher level of contamination. On the other hand, the trawling method has the ability to cover and sample a larger area and therefore overcomes some of the problems related to patchiness (Karlsson et al. 2020). A combination of volume-reduced net-based sampling and bulk sampling seems to be very effective in comprehensive monitoring of microplastic in aquatic environment.

Sediment compartment

Because of such characteristics as buoyancy and extreme durability, synthetic polymers are present in rivers, lakes and oceans and accumulate in sediments all over the world. Microplastic durability makes it highly resistant to degradation from decades to millennia in its polymer forms (Mathalon and Hill 2014). Small plastic particles are easily accessible to a wide range of aquatic organisms, accumulating in their cells and tissues and ultimately transferred through the food web. Most plastics are extremely durable and persistent (Sharma and Chatterjee 2017).

Microplastics in water compartment may be diluted due to seasonal variation in water volume and water dynamic behavior. For sediment compartments, with a static environment, dilution barely happens, and sediments can easily act as accumulation environments. Sediments are a site of microplastics accumulation and the habitat of benthic organisms, which are key components of food webs. In microplastic scientific assessment, sediment samples are taken from both the subtidal and benthic part of freshwater bodies. This issue affects the life quality of the organisms in ecosystems, both in benthic and littoral sediment zone. For example, microplastics were recorded in fecal samples and feathers of waterbirds from contaminated wetlands in South Africa. Plastic particles can fill the gizzard and possibly block the pyloric valve leading into the intestine (Reynolds and Ryan 2018). Microplastics were present in the benthic fish species and benthic organisms of the Caspian Sea, and the abundance of plastic particles in animals near the shore was greater than in the central part (Bagheri et al. 2020). High doses of microplastics led to fewer species and fewer juvenile isopods and periwinkles in European flat oysters and their associated benthic communities (Green 2016).

Sampling tools are selected with regard to sampling places. Sampling is performed in various directives with respect to the analysis of nutrients or pollutants, such as metal ions or persistent organic substances (Braun et al. 2018).

Manual grab samplers

Sediment manual grab methods utilize tools such as hand spades and stainless steel spoon for littoral and beach environments (Fig. 3). As sampling of sediments is facilitated compared to that of the water column, monitoring shore sediments appears to be advantageous. Moreover, non-buoyant particles can be analyzed in sediment samples rather than in water surface samples (Klein et al. 2015). Therefore, sampling from different compartments can give a comprehensive outlook of microplastic pollution problem.

Deep sediment samplers

Different kinds of grabs and corers are suggested to be suitable for deeper sediment sampling. Ekman and Van Veen grab samplers are deployed to study benthic sediment (Merga et al. 2020; Neto et al. 2019; Sruthy and Ramasamy 2017). Deep sediment sampler can provide a look at the changing abundance and microplastic debris in lake sediments that span a century to present day. These particle can contribute to the discussion on plastic wastes as stratigraphic markers for the Anthropocene (Turner et al. 2019; Vaughan et al. 2017). For specimen transition, the use of glass bottle is recommended. However, plastic or aluminum foil bags can also be used. In the case of plastic container utilization, blank control should be included to prevent bias in the study results (Zhang et al. 2018). Care must also be taken to homogenize the sample during further processing.

Comparison between different sediment sampling methods

Surface sediment in shore zone could reflect long-term interfacial interaction between waters and terrestrial environment (Yu et al. 2016). Shore sediment sampling consists of multiple transects at a right angle from the water line and the placement of quadrats along the transects. Transects may be visually scanned for bigger microplastics in the field. The surface layer can be removed to a proximate depth and sieved or transferred to the laboratory for separation steps (Ballent et al. 2016; Egessa et al. 2020). Variation in plastic abundance at different natural beach zones (water line, drift line and high-water line) in Lake Garda was investigated. Results showed that the water line contained the lowest level of plastic particles, whereas the highest proportion of plastic debris was observed in the drift line and high-water line (Imhof et al. 2018). Core samples have the advantage of being able to see depth profile and to study potential microplastic trends in considerable time periods. Surface analysis of these microplastics may show higher degradation effects due to longer time period. However, surface particles may be more exposed to degradation factors.

Sediment sampling tools are selected with regard to sampling places and sampling purposes, as they may show different aspects.

Sample preservation

Samples were are usually preserved with 5% methyl aldehyde and stored at 4 °C before analysis (Zhang et al. 2015) or fixed in 2.5% formalin (Zhao et al. 2014) or submerged in ~ 40% ethanol (EtOH) (Mani and Burkhardt-Holm 2020).

Discussion

No standardized methods exist for selecting mesh size, sampling, clean up, enrichment and detection, making the comparison of different studies complicated. Improving methods is needed to save time and effort in identifying microplastics in different compartments. To the best of our knowledge, water compartments are the most investigated matrix, assessed for microplastics in freshwater environments (102/150). Rivers and estuarine systems are the most frequently studied compartments for microplastic detection, both in water and in sediment (85/150). This may be due to the reported importance of rivers and estuaries as a vector for microplastics transfer to seas and oceans. Littoral or shore sediments and bottom sediment are frequently assessed for microplastics in reviewed studies. Many microplastic studies used Manta nets to collect surface water samples (Fig. 4). Van Veen grabs and simple hand tools like trowels and stainless steel spoons were the most frequently used tools for the bottom and littoral sediments, respectively (Fig. 5). They are also the most common sediment sampling tools from benthic and beach zones and seem to be appropriate for microplastic studies. Regarding sampling tool characteristics, both false positive and false negative results in analyses of small microplastics occur. In recent studies, there is a tendency to detect microplastics in both water and sediment at the same time. The simultaneous detection of microplastics in the water and sediment compartment gives a better perspective of the situation in the ecosystem. The lack of uniformity in reporting the numbers of microplastics, mostly due to employing different units, is considerably noticeable in reviewing papers. This makes the comparison of results difficult and challenging. Some studies have reported microplastic numbers or weights per volume of sampled water or per total dry matter for sampled sediments (particles/kg); the latter is highly recommended.

Fig. 4

Frequency of water sampling tools, used in microplastic freshwater studies

Fig. 5

Frequency of sediment sampling tools, used in microplastic freshwater studies

Conclusion

Employing suitable and reliable sampling, treatment and identification methods is crucial to evaluate microplastic pollution. Sampling and experimental techniques should be standardized to more effectively assess microplastics. Although a smaller mesh size is more appropriate, the choice of trawl or sieve mesh size depends greatly on the study purpose. The kind of environment being studied, e.g., a dynamic river with high water velocity or a calm eutrophic lake or wetland, is also of importance. The exact sampling volume, place and depth must be chosen carefully to ensure that samples represent water body characteristics. Sample volumes should be large enough to minimize overestimation induced by scaling up results, especially for water samples. The pump sampling approach with the filtration of large water volumes is necessary to generate reliable results in the spatial association between microplastic pollution in the surface waters and sediments. The trawling method has the ability to cover a larger area during sampling. To cover different microplastic size and shape, it is advantageous to combine both volume reduction and bulk sampling methods for surface water. More research is required to extend the understanding of representative in the study of microplastics as a key factor for the potential development of reliable data.