key: cord-0816532-93epotrn authors: Razeghi, Nastaran; Hamidian, Amir Hossein; Wu, Chenxi; Zhang, Yu; Yang, Min title: Microplastic sampling techniques in freshwaters and sediments: a review date: 2021-05-18 journal: Environ Chem Lett DOI: 10.1007/s10311-021-01227-6 sha: dbf84f26f0cdd1ed8c984954d6994470d401f02b doc_id: 816532 cord_uid: 93epotrn 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. Ecosystems and their different organisms have been widely impacted by anthropogenic activities such as the discharge of pollutants Mirzajani et al. 2016 Mirzajani et al. , 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. ). 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; 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 lm 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 jaggededged 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 . 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 (Mojoudi et al. , 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 TiO 2 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? Environmental Chemistry Letters e. What are the advantages and disadvantages of sampling methods? 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. 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 Environmental Chemistry Letters 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. 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) . 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. 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 ). 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. 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 Environmental Chemistry Letters Environmental Chemistry Letters (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. 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 . 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-lm mesh compared to a 330-lm mesh (Dris et al. 2018) . Double neuston net trawl (500 lm 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 ). 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 netbased sampling and bulk sampling seems to be very helpful in estimating the missing fractions and enables a greater spatial resolution (Fischer et al. 2016) . 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 lm 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-lm plankton net. Bulk water sampling was used for anthropogenic fiber analysis and 300-lm-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 . 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 . 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. 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 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 Environmental Chemistry Letters 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) . 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. 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 . Care must also be taken to homogenize the sample during further processing. 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. 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). 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 Environmental Chemistry Letters 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 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 Environmental Chemistry Letters 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. 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 Environmental Chemistry Letters 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. Conflicts of interest The authors have no conflict of interests to disclose. Consent for publication All authors agreed with the content and all gave explicit consent to submit, and they obtained consent from the responsible authorities. Human exposure to microplastics: a study in Iran The first evaluation of microplastics in sediments from the complex lagoon-channel of Bizerte (Northern Tunisia) Microplastic distribution in surface water and sediment river around slum and industrial area (case study: Ciwalengke River, Majalaya district, Indonesia) Study on age-related bioaccumulation of some heavy metals in the soft tissue of rock oyster (Saccostrea cucullata) from Laft Port -Qeshm Island Freshwater, Glossary of Meteorology Microplastic contamination in lake Winnipeg Microplastics in the marine environment Microplastics distribution, abundance and composition in sediment, fishes and benthic organisms of the Gorgan Bay. Caspian sea Plastic debris in 29 Great Lakes tributaries: relations to watershed attributes and hydrology Sources and sinks of microplastics in Canadian Lake Ontario nearshore, tributary and beach sediments A watershed-scale, citizen science approach to quantifying microplastic concentration in a mixed land-use river Grab vs. neuston tow net: a microplastic sampling performance comparison and possible advances in the field Microscopy and elemental analysis characterisation of microplastics in sediment of a freshwater urban river in Scotland Plastic levels in sediments closed to Cecina river estuary Plastic pollution in freshwater ecosystems: macro-, meso-, and microplastic debris in a floodplain lake Identification of microplastics in fish ponds and natural freshwater environments of the Carpathian basin Microplastics in river suspended particulate matter and sewage treatment plants Microplastics Analytics: Sampling, Preparation and Detection Methods. Discussion Paper within the scope of the research focus. Plastics in the Environment Sources Sinks Solutions. Discussion paper. German Federal Ministry of Education and Research Microplastics as vector for heavy metal contamination from the marine environment Microplastics: a novel method for surface wate sampling ad sample extraction in Elechi Creek, Rivers State, Environmental Chemistry Letters Nigeria Spatial patterns of plastic debris along estuarine shorelines Microplastics and their possible sources: the example of Ofanto river in Southeast Italy Microplastics in the gastrointestinal tracts of fish and the water from an urban prairie creek Polystyrene spherules in coastal waters Plastics on the Sargasso Sea surface Microplastic pollution in St. Lawrence river sediments Microplastics in Irish freshwaters: a preliminary study Plastic particles in surface waters of the northwestern Atlantic Advancing the quality of environmental microplastic research Factors controlling the distribution of microplastic particles in Benthic sediment of the Thames River Hidden plastics of Lake Ontario, Canada and their potential preservation in the sediment record Fluorescence sensing of microplastics on surfaces Distribution, Abundance, and Spatial Variability of Microplastic Pollution in Surface Waters of Lake Superior. Dissertation Studies of the effects of microplastics on aquatic organisms: what do we know and where should we focus our efforts in the future? Factors influencing microplastic abundances in nearshore, tributary and beach sediments along the Ontario shoreline of Lake Erie Occurrence of microplastic fragments in the Pasig River Manuscript prepared for submission to environmental toxicology and pharmacology pollution in drinking water source areas: Microplastics in the Danjiangkou Reservoir Microplastics in surface waters and sediments of the Three Gorges Reservoir Microplastic pollution in streams spanning an urbanisation gradient Accumulation, tissue distribution, and biochemical effects of polystyrene microplastics in the freshwater fish red tilapia (Oreochromis niloticus) Microplastics in surface waters and sediments of the Wei River, in the northwest of China The rapid increases in microplastics in urban lake sediments Microplastic contamination in an urban area: a case study in Greater Paris Synthetic and nonsynthetic anthropogenic fibers in a river under the impact of Paris Megacity: Sampling methodological aspects and flux estimations Suspended microplastics and black carbon particles in the Jade system, southern North Sea 2020) Occurrence, distribution and size relationships of plastic debris along shores and sediment of northern Lake Victoria Tracing microplastics in aquatic environments based on sediment analogies Spatiotemporal distribution and annual load of microplastics in the Nakdong River, South Korea Microplastic pollution in the surface waters of the Laurentian Great Lakes Influence of wastewater treatment plant discharges on microplastic concentrations in surface water Distribution, sedimentary record, and persistence of microplastics in the Pearl River catchment Trophic level transfer of microplastic: Mytilus edulis (L.) to Carcinus maenas (L.) Pollution due to plastics and microplastics in Lake Geneva and in the Mediterranean Sea Plastic pollution in Swiss surface waters: nature and concentrations, interaction with pollutants Microplastic pollution in the sediment of Jagir Estuary, Surabaya City Microplastic pollution in lakes and lake shoreline sediments-a case study on Lake Bolsena and Lake Chiusi (central Italy) High-levels of microplastic pollution in a large, remote, mountain lake Microplastics in the Solent estuarine complex, UK: an initial assessment Production, use, and fate of all plastics ever made Back to plastic pollution in COVID times Microplastic in two South Carolina Estuaries: occurrence, distribution, and composition Microplastics entering northwestern Lake Ontario are diverse and linked to urban sources Effects of microplastics on European flat oysters, Ostrea edulis and their associated benthic communities A review on the characteristics of microplastics in wastewater treatment plants: a source for toxic chemicals Spatial distribution of arsenic in groundwater of Iran, a review Heavy metal bioaccumulation in sediment, common reed, algae and blood worm from the Shoor River Distribution of microplastics in surface water of the lower Yellow River near estuary Microplastics as vectors for environmental contaminants: exploring sorption, desorption, and transfer to biota Abundance, distribution patterns, and identification of microplastics in Brisbane river sediments Microplastic abundance and composition in western Lake superior as determined via microscopy, Pyr-GC/MS, and FTIR Microplastics in the marine environment: a review of the methods used for identification and quantification Microplastic pollution in estuaries across a gradient of human impact Plastics and other anthropogenic debris in freshwater birds from Canada Microplastics in freshwater and terrestrial environments: Evaluating the current understanding to identify the knowledge gaps and future research priorities Microplastics in small waterbodies and tadpoles from Yangtze River Delta Coupled effects of urbanization level and dam on microplastics in surface waters in a coastal watershed of Southeast China Instrumental analysis of microplastics-benefits and challenges Microplastic contamination of river beds significantly reduced by catchment-wide flooding Contamination of beach sediments of a subalpine lake with microplastic particles Pigments and plastic in limnetic ecosystems: a qualitative and quantitative study on microparticles of different size classes Variation in plastic abundance at different lake beach zones-a case study An unintended challenge of microplastic pollution in the urban surface water system of Lahore Physicochemical properties of antibiotics: a review with an emphasis on detection in the aquatic environment Microplastics in the surface sediments from the Beijiang River littoral zone: composition, abundance, surface textures and interaction with heavy metals Microplastic pollution in the rivers of the Tibet Plateau Microplastics in sediment and surface water of west dongting lake and south dongting lake: abundance, source and composition Microplastic hotspots in the Snake and Lower Columbia rivers: a journey from the greater yellowstone ecosystem to the Pacific ocean Comparison between manta trawl and in situ pump filtration methods, and guidance for visual identification of microplastics in surface waters Assessment of the sources and inflow processes of microplastics in the river environments of Japan Plastic debris in the ocean Occurrence and spatial distribution of microplastics in river shore sediments of the Rhine-Main area in Germany Microplastic as a vector for chemicals in the aquatic environment: critical review and model-supported reinterpretation of empirical studies Macroplastic and microplastic contamination assessment of a tropical river Microplastics in a freshwater environment receiving treated wastewater effluent The Danube so colourful: a potpourri of plastic litter outnumbers fish larvae in Europe's second largest river Microplastics en route: field measurements in the Dutch river delta and Amsterdam canals, wastewater treatment plants, North Sea sediments and biota Microplastics contamination in different trophic state lakes along the middle and lower reaches of Yangtze River Basin A methodology for measuring microplastic transport in large or medium rivers Occurrence and distribution of microplastics in an urban river: a case study in the Pearl River along Guangzhou City Microplastics in urban and highway stormwater retention ponds Comparison of microplastic pollution in different water bodies from urban creeks to coastal waters Freshwater microplastics in Norway: A first look at sediment, biota and historical plankton samples from Lake Mjøsa and Lake Femunden. Norwegian Insititute for Water Research Sampling, isolating and identifying microplastics ingested by fish and invertebrates Repeated detection of polystyrene microbeads in the lower Rhine River Seasonal microplastics variation in nival and pluvial stretches of the Rhine River-From the Swiss catchment towards the North Sea Microplastics profile along the Rhine River Microplastic pollution in benthic midstream sediments of the Rhine River Bioaccumulation and elimination rate of cobalt in Capoeta fusca under controlled conditions He Q (2020a) Distribution and characteristics of microplastics in the Yulin River, China: Role of environmental and spatial factors Microplastics in the surface water of Wuliangsuhai Lake, northern China Impact of microplastic fibers from the degradation of nonwoven synthetic textiles to the Magdalena river water column and river sediments by the city of Neiva Pelagic plastic pollution within the surface waters of Lake Michigan, USA Microplastic fibers in the intertidal ecosystem surrounding Halifax Harbor Microplastic is an abundant and distinct microbial habitat in an urban river Microplastic in surface waters of urban rivers: concentration, sources, and associated bacterial assemblages The presence of microplastic in freshwater systems: Snake river and Palisades Reservoir. Department of Health and Science Microplastic in riverine fish is connected to species traits Distribution of microplastic and small macroplastic particles across four fish species and sediment in an African lake Guidelines for Harmonizing Ocean Surface Microplastic Monitoring Methods. Version 1.1. Ministry of the Environment Mountains to the sea: river study of plastic and non-plastic microfiber pollution in the northeast USA A systems approach to understand microplastic occurrence and variability in Dutch riverine surface waters Distribution and abundance of fish in the southwest of Caspian Sea coastal waters Metal bioaccumulation in representative organisms from different trophic levels of the Caspian Sea Possible effect of Balanus improvisus on Cerastoderma glaucum distribution in the south-western Caspian Sea Effective removal of heavy metals from aqueous solution by porous activated carbon/thiol functionalized graphene oxide composite Synthesis and evaluation of Activated Carbon/Nanoclay/ Thiolated Graphene Oxide Nanocomposite for Lead (II) Removal from Aqueous Solution Quantity and type of plastic debris flowing from two urban rivers to coastal waters and beaches of Southern California Plastic pollution in five urban estuaries of KwaZulu-Natal Identification of microplastics in surface water and Australian freshwater shrimp Paratya australiensis in Victoria Interaction of freshwater microplastics with biota and heavy metals: a review Microplastics and attached microorganisms in sediments of the Vitória bay estuarine system in SE Brazil A first survey on the abundance of plastics fragments and particles on two sandy beaches in Kuching The effects of natural and anthropogenic microparticles on individual fitness in Daphnia magna Microplastics in a stormwater pond Microbial degradation of microplastics by enzymatic processes: a review Environmental life cycle assessments of emerging anode materials for Li-Ion batteries-metal oxide NPs Removal of microplastics from the environment Status of microplastic pollution in aquatic ecosystem with a case study on Cherating River Characterization of microplastic litter in the gastrointestinal tract of Solea solea from the Adriatic Sea Microplastics in freshwater river sediments in Shanghai, China: a case study of risk assessment in mega-cities Microplastics in sediments of the Changjiang Estuary Plastics-The facts 2018. An analysis of European plastics production, demand and waste data /9658/Plastics_the_facts_2018_AF_web.pdf. Accessed Microplastics in sediments of River Yongfeng from Maanshan city Scientific studies on microplastics pollution in Iran: an in-depth review of the published articles Micro-plastic ingestion by waterbirds from contaminated wetlands in South Africa Assessing ground and surface water scarcity indices using ground and surface water footprints in the Tehran province of Iran Microplastics pollution in different aquatic environments and biota: a review of recent studies Spatial and temporal distribution of microplastics in water and sediments of a freshwater system Microplastic contamination in an urban estuary: abundance and distribution of microplastics and fish larvae in the Douro estuary On the quantity and composition of floating plastic debris entering and leaving the Tamar Estuary Microplastics in sediment from Skudai and Tebrau river, Malaysia: a preliminary study Ingestion and transfer of microplastics in the planktonic food web Microplastic pollution, a threat to marine ecosystem and human health: a short review Microplastics in freshwater sediments of atoyac river basin, puebla city Microplastic pollution in the surface waters of Italian Subalpine Lakes River Deltas as hotspots of microplastic accumulation: The case study of the Ebro River (NW Mediterranean) The distribution of microplastic pollution in the Mohawk River. Mohawk Watershed Symposium Microplastic pollution in Vembanad Lake, Kerala, India: the first report of microplastics in lake and estuarine sediments in India Using the Asian clam as an indicator of microplastic pollution in freshwater ecosystems Microplastics in Taihu lake Microplastics contamination in a high population density area of the Chao Phraya River On the representativeness of pump water samples versus manta sampling in microplastic analysis Microplastics and associated PAHs in surface water from the Feilaixia Reservoir in the Beijiang River Plastics, the environment and human health: current consensus and future trends Lost at sea: Where is all the plastic? Abundance, distribution, and drivers of microplastic contamination in urban river environments Visible light photocatalytic degradation of microplastic residues with zinc oxide nanorods Microplastics in freshwater environment: the first evaluation in sediments from seven water streams surrounding the lagoon of Bizerte A temporal sediment record of microplastics in an urban lake SFRA0025: Identification and Assessment of Riverine Input of (Marine) Litter. Report for Michail Papadoyannakis Microplastics in the sediments of a UK urban lake Microplastic abundance and distribution in the open water and sediment of the Ottawa River, Canada, and its tributaries Microplastic particles in sediments of Lagoon of Venice, Italy: first observations on occurrence, spatial patterns and identification Microplastics profile in a typical urban river in Beijing Occurrence and pollution characteristics of microplastics in surface water of the Manas River Basin Microplastic contamination in freshwater: first observation in Lake Ulansuhai Microplastics flowing into Lake Winnipeg: densities, sources, flux, and fish exposures The effect of dams on river transport of microplastic pollution A case study investigating temporal factors that influence microplastic concentration in streams under different treatment regimes Microplastic pollution in surface sediments of urban water areas in Changsha, China: abundance, composition, surface textures Abundance and characteristics of microplastics in beach sediments: insights into microplastic accumulation in northern Gulf of Mexico estuaries Microplastics pollution in inland freshwaters of China: a case study in urban surface waters of Wuhan Microplastics in surface waters of dongting lake and hong lake Preferential accumulation of small (\ 300 lm) microplastics in the sediments of a coastal plain river network in eastern China Microplastic distribution at different sediment depths in an urban estuary Microplastic pollution in inland waters focusing on Asia Spatialtemporal distribution of microplastics in surface water and sediments of Maozhou River within Guangdong-Hong Kong-Macao Greater Bay Area Occurrence and fate of microplastic debris in middle and lower reaches of the Yangtze River-from inland to the sea Sources and distribution of microplastics in China's largest inland lake-Qinghai Lake Microplastic risk assessment in surface waters: A case study in the Changjiang Estuary, China Microplastics in sediments from an interconnected river-estuary region Microplastic abundance, distribution and composition in the Pearl River along Guangzhou city and Pearl River estuary Microplastic pollution in surface water of urban lakes in Changsha, China Comparison of the abundance of microplastics between rural and urban areas: a case study from East Dongting Lake Microplastics in four estuarine rivers in the Chesapeake Bay, USA Occurrence of microplastics in the beach sand of the Chinese inner sea: the Bohai Sea Microplastic abundance, distribution and composition in water, sediments, and wild fish from Poyang Lake, China Distribution and degradation of fresh water plastic particles along the beaches of Lake Huron Comparison of the distribution and degradation of plastic debris along shorelines of the Great Lakes North America The hydro-fluctuation belt of the three Gorges reservoir: source or sink of microplastics in the water? Accumulation of floating microplastics behind the three Gorges Dam Microplastic pollution in China's inland water systems: A review of findings, methods, characteristics, effects, and management Microplastic pollution of lakeshore sediments from remote lakes in Tibet plateau Occurrence and characteristics of microplastic pollution in Xiangxi Bay of Three Gorges Reservoir Microplastics' pollution and risk assessment in an urban river: a case study in the Yongjiang River Analysis of suspended microplastics in the Changjiang Estuary: Implications for riverine plastic load to the ocean Microplastic in three urban estuaries Suspended microplastics in the surface water of the Yangtze Estuary System, China: first Environmental Chemistry Letters observations on occurrence, distribution Distribution and characteristics of microplastics in urban waters of seven cities in the Tuojiang River basin