http://orcid.org/0000-0002-6027-3823
Figures
Abstract
Microplastics are an environmental contaminant of growing concern, but there is a lack of information about microplastic distribution, persistence, availability, and biological uptake in freshwater systems. This is especially true for large river systems like the Colorado River that spans multiple states through mostly rural and agricultural land use. This study characterized the quantity and morphology of microplastics in different environmental compartments in two large reservoirs along the Colorado River: Lakes Mead and Mohave, within Lake Mead National Recreation Area. To assess microplastic occurrence, surface water and surficial sediment were sampled at a total of nine locations. Sampling locations targeted different sub-basins with varying levels of anthropogenic impact. Las Vegas Wash, a tributary which delivers treated wastewater to Lake Mead, was also sampled. A sediment core (33 cm long, representing approximately 19 years) was extracted from Las Vegas Bay to assess changes in microplastic deposition over time. Striped bass (Morone saxatilis), common carp (Cyprinus carpio), quagga mussels (Dreissena bugensis), and Asian clams (Corbicula fluminea) were sampled at a subset of locations to assess biological uptake of microplastics. Microplastic concentrations were 0.44–9.7 particles/cubic meter at the water surface and 87.5–1,010 particles/kilogram dry weight (kg dw) at the sediment surface. Sediment core concentrations were 220–2,040 particles/kg dw, with no clear increasing or decreasing trend over time. Shellfish microplastic concentrations ranged from 2.7–105 particles/organism, and fish concentrations ranged from 0–19 particles/organism. Fibers were the most abundant particle type found in all sample types. Although sample numbers are small, microplastic concentrations appear to be higher in areas of greater anthropogenic impact. Results from this study improve our understanding of the occurrence and biological uptake of microplastics in Lake Mead National Recreation Area, and help fill existing knowledge gaps on microplastics in freshwater environments in the southwestern U.S.
Citation: Baldwin AK, Spanjer AR, Rosen MR, Thom T (2020) Microplastics in Lake Mead National Recreation Area, USA: Occurrence and biological uptake. PLoS ONE 15(5): e0228896. https://doi.org/10.1371/journal.pone.0228896
Editor: Zhi Zhou, Purdue University, UNITED STATES
Received: September 27, 2019; Accepted: January 24, 2020; Published: May 4, 2020
This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.
Data Availability: All sample results are available online in the USGS ScienceBase Catalog at https://doi.org/10.5066/P9V1MNHH.
Funding: Funding was provided to AKB by the Water Quality Partnership Program of the U.S. Geological Survey and the U.S. National Park Service. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Introduction
Our understanding of microplastics in the environment has increased dramatically over the past decade. Defined as plastic particles < 5 mm in diameter, microplastics come from a wide variety of sources including fibers from textiles, preproduction pellets and powders, microbeads from personal care products, and breakdown of primary plastics such as bags, bottles, wrappers, packing foam, and car tires [1–5]. Microplastics reach aquatic environments via numerous and diverse pathways including littering, stormwater runoff, domestic and industrial wastewater, atmospheric deposition, and direct loss from buoys, boats, and other aquatic equipment [6–8]. Recent studies have highlighted the ubiquity of microplastics in virtually all compartments of the environment, from the ocean surface to its deepest trenches, in freshwater rivers and lakes, in precipitation, and in alpine snowpack [6,9–13]. Ingestion of microplastics has been observed across a wide range of aquatic organisms, from zooplankton to birds to whales [14–16]. However, biological risk assessment is complicated by the diversity of microplastic particle sizes, morphologies, and chemical compositions [17]. A growing body of evidence points to adverse biological effects, mostly at the sub-organismal level, but toxicological benchmarks have yet to be established, and organismal and community-level impacts remain largely unclear [18–21].
North American freshwater studies of microplastics have primarily focused on the Great Lakes region [6,9,22–24]. There is a general lack of information on microplastic occurrence and biological uptake in freshwater in the western portion of the continent, and especially the arid southwestern U.S. This investigation presents the first assessment of microplastic occurrence and biological uptake in two large reservoirs in the Mojave Desert, Lakes Mead and Mohave, located in Lake Mead National Recreation Area (LMNRA), a unit of the U.S. National Park Service. We report microplastic concentrations in water, surficial sediment, fish, and shellfish across a gradient of anthropogenic impacts to test the hypothesis that higher microplastic concentrations will be found in areas with higher anthropogenic impact. Additionally, we report microplastic concentrations from a sediment core representing ~19 years of deposition to test the hypothesis that rates of microplastic deposition have increased over time. Results from this study provide an initial baseline for microplastic occurrence and biological uptake in two large reservoirs along the Colorado River, in addition to an initial understanding of how human population density influences microplastic concentrations in a largely undeveloped watershed.
Methods
Study area
Lakes Mead and Mohave are reservoirs along the Colorado River on the Arizona-Nevada border in the arid southwestern U.S. (Fig 1). Together these reservoirs cover 759 km2 [25] within the 6,070 square kilometer LMNRA. The reservoirs support a diverse population of benthic invertebrates, wintering bald eagles and other aquatic dependent birds, endangered fishes, and sportfish, which serve as an important food source for aquatic birds and are a major draw for park visitors. Lake Mead supports one of the world’s only self-sustaining populations of the critically endangered razorback sucker (Xyrauchen texanus) [25].
Lake boundaries are from the U.S. Geological Survey National Hydrography Dataset and are based on full-pool conditions, giving the false appearance that the Las Vegas Wash sampling location was in Lake Mead; at the time of sampling, the water level in Lake Mead was ~43 m below full pool, and the Las Vegas Wash sampling location was riverine. Hillshades are from U.S. Geological Survey 100-meter digital data. LV, Las Vegas.
Lakes Mead and Mohave span a gradient of anthropogenic impacts. The Colorado River and its tributaries make up one of the major river basins of western North America. The basin upstream of Lake Mead includes portions of Wyoming, Colorado, Utah, New Mexico, Nevada, and Arizona. The Colorado River enters Lake Mead at Gregg Basin after flowing through Lake Powell and Glen Canyon dam in Glen Canyon National Recreation Area, and through Grand Canyon National Park. These are areas of sparse population but extremely high and increasing recreational use. For example, in 2018, Glen Canyon had 4.2 million visitors, and the Grand Canyon had 6.4 million visitors [26]. The Virgin and Muddy Rivers enter Lake Mead at Overton Arm. There are numerous small towns located along the Virgin and Muddy Rivers, but in general the basins are sparsely populated. The Virgin River flows through Utah, Arizona, and Nevada, and was designated Utah’s first wild and scenic river in 2009.
Annual visitation to the LMNRA was 7.5 million people in 2018 [27]. Visitation is heaviest in Boulder Basin and Las Vegas Bay in the western portion of Lake Mead, nearest the Las Vegas Metropolitan Area. Boulder Basin is home to the largest marina in the LMNRA, the Las Vegas Boat Harbor. The northern and easternmost portions of Lake Mead, Overton Arm and Gregg Basin, respectively, are farther from population centers and receive fewer visitors.
The majority of sediment entering Lake Mead comes from the Colorado River, but most dissolved or particle-bound contaminants entering Lake Mead come from Las Vegas Wash. The Las Vegas Wash is a tributary which delivers urban runoff, stormwater, and effluent from four wastewater treatment plants (719 million liters/day) in Las Vegas Valley to Las Vegas Bay [25]. Previous studies have shown that the relatively high contaminant loads from Las Vegas Wash have resulted in endocrine disruption of fish that primarily reside in Las Vegas Bay [28–34], although enhancements of the wastewater treatment plants since the early 2000s have reduced the overall chemical load [34,35]. However, none of these previous studies have examined the role of microplastics in the Lake Mead foodweb.
Sample collection
Samples were collected in March 2017 and March 2018 (sediment core) at eight reservoir locations (seven in Lake Mead and one in Lake Mohave), and one tributary location (Las Vegas Wash; Fig 1, Table 1). The Las Vegas Wash sampling location appears to be in Lake Mead in Fig 1, but because of the low reservoir level was actually a riverine environment at the time of sampling. A total of 48 samples were collected: water and surficial sediment samples were collected at all locations; sediment cores, fish, and shellfish were collected at a subset of locations. Sample collection protocols were approved under National Park Service Research Permit LAKE-2017-SCI-0001.
Water samples.
Water samples were collected once at each of the nine locations using methods and equipment similar to previous studies [6]. A 70 x 40 x 260 cm (width x height x length) microplastics net (HYDRO-BIOS) with 100 μm polyamide mesh was towed at the water surface for fifteen minutes (Fig 2A), except at Las Vegas Wash where the net was held at a fixed location (described below) and sampling was stopped after 5 minutes because the net began to clog. Floats on each side of the net’s frame kept the frame at a consistent depth through the duration of sample collection and across sites. The average submerged depth of the net was 22 ± 2.8 cm. For boat samples, the net was towed at 0.6–0.9 m/s alongside the boat beyond the bow wake using a fixed metal pole. At Las Vegas Wash, two people held the net in the stream and care was taken to keep the opening of the net upstream of where they were standing. A flow meter (General Oceanics, Inc, model 2030R) suspended in the mouth of the net was used to measure the average velocity of water entering the net. The total volume of water filtered (sampled) was calculated from the width and height of the submerged portion of the net, the sampling duration, and the average velocity. Total filtered volumes ranged from 10 to 122 cubic meters (m3).
(A) Water sample collection net. (B) Sediment from Ponar sampler.
Following sample collection, the net was suspended and sprayed with site water using a 70 pounds/inch2 wash down pump (Johnson Pumps model 10-13399-0311) to wash the sampled material down into the detachable polyamide mesh bucket at the bottom of the net. The net was sprayed primarily from the outside for ~2 minutes. The volume of site water used to wash the net was determined to equal ~19 L, which is equivalent to 0.02 to 0.2% of the sampled volume. Thus, any microplastic contribution to the sample from washing the net with site water was negligible. After washing the net, the sample was transferred from the mesh bucket to a glass jar using a squirt bottle and site water. The sample was preserved with isopropyl alcohol.
Surficial sediment samples.
Surficial sediment was collected once at each of the nine locations using a standard Ponar sampler (6”W x 6”L Stainless Steel Ponar Grab sampler). Water depths at sediment sampling locations were 0.3 m at Las Vegas Wash and 3.6–99 m elsewhere. Water depth at Las Vegas Bay Shallow was 16 m, and at Las Vegas Bay Deep was 51 m. Ponar sediments were emptied into a stainless-steel pan and the sample was taken from the top ~3 cm using a stainless-steel spoon (the stratigraphy of the sediment remained generally intact during transfer to the pan; Fig 2B). At Las Vegas Wash, surficial sediment was collected from the top ~3 cm from depositional areas by wading and using a stainless-steel spoon. Approximately 400 mL of surficial sediment was collected at each location, composited into a glass jar and preserved with isopropyl alcohol. The Ponar, pan, and spoon were rinsed with site water between each location.
Sediment cores.
A sediment core was collected at Las Vegas Bay Shallow using a gravity corer with polycarbonate barrel measuring 1.2 meters long and 6.8 cm inner diameter. The core penetrated to a depth of 33 cm. After collection, the core was capped and taken to shore for extrusion and sectioning into four 8–9 cm intervals. Core sections were collected and stored in glass jars at room temperature.
Fish.
Fish were collected by boat electroshocking as part of routine fish population monitoring at two locations to assess potential differences in microplastic ingestion with differences in anthropogenic impact. The location considered to have relatively high anthropogenic impact was Las Vegas Bay, and the relatively low impact site was Overton Arm. Both benthic- and pelagic-feeding fish were targeted to assess potential differences in microplastic ingestion related to feeding strategy. Striped bass were chosen to represent pelagic feeders and common carp were chosen to represent benthic feeders. A total of 21 fish were collected: 8 striped bass and 6 common carp at Las Vegas Bay, and 7 striped bass at Overton Arm. No common carp were available at Overton Arm. Fish were euthanized by a sharp blow to the base of the skull. Fish lengths and weights are provided in S1 Table. The complete gastrointestinal tract (esophagus, stomach, and intestines) was removed, wrapped in aluminum foil, and stored frozen in a plastic bag until analysis.
Shellfish.
Quagga mussels were first detected in Lake Mead in 2007 and have become abundant on all hard surfaces and in soft sediment [36]. Asian clams have been present in Lake Mead since at least the 1970s, but their introduction to Lake Mead is unknown [37]. Since the introduction of quagga musssels, Asian clam abundance has fallen and, therefore, quagga mussels were the target shellfish species for collection. As with fish, quagga mussels were collected at two locations, Las Vegas Bay (n = 11) and Overton Arm (n = 10), to assess potential differences related to adjacent land use. Quagga mussels were collected using a Ponar sampler (6”W x 6”L). Although not targeted, Asian clams were found in the surficial sediment samples at two locations (Las Vegas Boat Harbor and Las Vegas Bay) and were kept for analysis. Shellfish were euthanized by freezing, then were removed from their shells and stored frozen until analysis. Because of their small size, the quagga mussels from each site were composited for analysis, whereas the Asian clams were analyzed individually. Shellfish sizes are provided in S1 Table.
Sample analysis
For sediment and water samples, a modified version of the National Oceanic and Atmospheric Administration's (NOAA) microplastic analysis method [38] was used to isolate and count microplastics. Briefly, for water, 1) whole samples were sieved into a single size class of 355 to 5,600 μm to remove small and large material; 2) organic non-plastic material was removed via a wet peroxide oxidation (WPO; Fenton’s reaction) digestion using 20 mL of 30% hydrogen peroxide and an iron catalase, additional hydrogen peroxide was frequently added to remove additional organic material; 3) remaining material was then sieved into two size classes, 355 to 1,000 μm and 1,000 to 5,600 μm; 4) each size class was density separated with lithium metatungstate (1.6 g/mL) and the less dense fraction retained on a sieve; lastly, 5) plastics and remaining “indigestible” material were transferred from the sieve onto a 47 mm gridded cellulose acetate filter paper using water and vacuum filtration and visually identified using a stereoscope capable of 40x magnification. The use of a 355 μm sieve size is a departure from the NOAA method, which used a 300 μm sieve size. As a result, microplastic concentrations reported in this study are likely slightly lower than they would be with a 300 μm sieve, which may be an important consideration for comparison with other studies.
Identified plastics were categorized based on their morphology as fragments, fibers, foams, beads/pellets, and films [9]. Both color and counts were recorded. Sediment samples were processed with the same methods as water samples with three additional initial steps: 1) sediments were dried at 90°C to obtain a sediment dry weight; 2) 500 mL of potassium metaphosphate solution (5.5 g/L) was stirred with dry sediment to disaggregate sediment; and 3) an initial density separation was performed in lithium metatungstate to separate plastics and organic material from the sediment. Only material with a density of less than 1.6 g/mL was retained for subsequent digestion and size separation.
Tissue samples were also processed via visual counting with stereoscope and classification of their morphology and color recorded. For fish, processed tissues included the complete gastrointestinal tract; for mussels, the whole organism was processed, excluding the shell. To isolate plastics, mussel and fish tissue were placed into a solution of 30% potassium hydroxide (KOH) and allowed to digest for at least 24 hours [39]. Additional time was often needed to digest depending on the amount of tissue. For mussels, digestate was first filtered with a 125 μm sieve to remove KOH and then transferred to a 47 mm gridded cellulose acetate filter paper using water and vaccum filtration for plastic identification and counting. Both striped bass and common carp tissue samples had a large amount of shell and rock material in their GI tract requiring removal. After removal of KOH with a 125 μm sieve, fish digestate was treated with a dilute solution of 5% hydrochloric acid to dissolve shells followed by a density separation in 1.6 g/mL LMT solution to isolate plastics from rocks and transferred to a 47 mm gridded cellulose acetate filter paper for plastic identification and counting. KOH digestion was used due to its efficacy at dissolving organic tissues and the fact that most plastics are resistant to degradation from KOH [39]. The size of microplastics counted in mussels and fish was larger than 125 μm and no size separation was completed due to the relatively small amount of plastic in tissues and no large macroplastics (>5.6 mm) present.
Data analysis
Plastic particle concentrations are reported in particles/m3 in water samples, particles per kilogram dry weight (kg dw) in sediment samples, and particles per organism in fish and shellfish. Because quagga mussels were composited by site, results are averages across 10–11 organisms.
Comparisons of fish lengths and microplastic concentrations between different locations and species were done by two-sample unpaired Wilcoxon Rank Sum tests with a significance level (p value) of 0.05 using the wilcox.test function in R [40]. Normality of fish lengths and fish microplastic concentrations was assessed using the Shapiro-Wilk’s test in R [40]. Relations between fish length and microplastic concentration were assessed by site and species using Spearman correlation with a p value of 0.05. Spearman correlations were calculated in R using the rcorr function in the Hmisc package [41].
All sample results are available online in the USGS ScienceBase Catalog [42].
Quality assurance and quality control
Quality assurance and quality control included field replicates, field blanks, and laboratory blanks. Field replicates of water and sediment samples were collected at two locations. The relative percent difference (RPD) in total microplastic concentration between replicate pairs in water samples was 9.5% (Overton Arm) and 24.9% (Las Vegas Bay shallow), and in sediment samples was 57.4% (Overton Arm) and 25.1% (Las Vegas Bay shallow).
Field blanks consisted of sample jar blanks and a field equipment blank. The purpose of the sample jar blanks was to assess potential contamination from the glass sampling jars and from the atmosphere while the sampling jars were open. The sample jar blanks were collected by leaving a clean, unrinsed sample jar open to the outdoor atmosphere for approximately five minutes. Four sample jar blanks were collected for this and concurrent studies, with an average ± standard deviation (SD) of 3.5 ± 5.7 microplastic particles per jar (range 0–12; S2 Table). The contribution of fibers from the atmosphere versus what was in the jar to begin with (i.e., from the factory) is not known. Based on these results we recommend future studies pre-rinse sample jars to minimize this potential contamination source. The purpose of the field equipment blank was to assess the potential contamination from using site water to wash the nets, as well as potential contamination from the net, plastic components in the pump, and carryover from the previously-collected sample. The field equipment blank was collected by spraying site water from Las Vegas Bay through the net bucket for 5 minutes (greater than twice the time and volume typically used to wash a net), then washing any collected material from the net bucket into a sampling jar. Two plastic fibers were reported in the field equipment blank (S2 Table). The contribution of fibers from the net versus the atmosphere versus the jar is not known. The small number of microplastic particles in the field equipment blank indicate that the net and the pump were not major sources of contamination, and that the net was being adequately rinsed between sites.
Laboratory blanks consisted of process blanks and equipment blanks. Laboratory process blanks were collected to determine the potential contamination of samples from atmospheric exposure at the laboratory. Fifty laboratory process blanks were collected concurrent with the analysis of regular samples by setting an open petri dish alongside the analyst for the duration of sample processing. The number of microplastic particles in laboratory process blanks ranged from 0 to 17, with a median of 1.0 and an average ± SD of 2.6 ± 3.5 (S2 Table). Laboratory equipment blanks were collected by analyzing samples of deionized water to confirm clean handling of samples, processing solutions, and equipment, and potential crossover contamination between samples. Seven laboratory equipment blanks were run. The number of microplastic particles in laboratory equipment blanks ranged from 2 to 6, with a median of 3 and an average ± SD of 3.6 ± 1.4 (S2 Table).
In summary, results from field and laboratory blanks indicate a combined contamination estimate of 11.7 microplastic particles per sample, on average (median 7; range 4–37). For context, environmental water and sediment samples averaged 110 and 114 total particles, respectively (water samples ranged from 38 to 243 particles, sediment samples ranged from 28 to 425 particles). Environmental results were not blank-subtracted based on laboratory or field blank results [17,43–45]. Rather, blank results provide the range of potential contamination during field and laboratory protocols.
Results
Water
Microplastic concentrations in water samples were 0.44–1.99 particles/m3, except at Las Vegas Wash, where the concentration was 9.7 particles/m3 (Figs 3A and 4). Fibers comprised 68.9% of the microplastic particles in water samples, on average, followed by fragments (15.6%), films (8.9%), foams (6.5%), and beads/pellets (0.1%). Non-fibrous particles (namely fragments, films, and foams) were primarily found in samples from Las Vegas Wash, Las Vegas Bay, and Gregg Basin. Most sampled particles (73.1%) were in the 355–1,000 μm size range, while 26.5% were in the 1,000–5,600 μm size range and 0.4% were >5,600 μm. The predominant colors of microplastic particles in water samples were clear (33.4% average), white (18.7%), black (17.1%), blue (14.7%), and red (6.7%).
Total microplastic concentrations in (A) water and (B) surficial sediment samples, Lake Mead and Lake Mohave, 2017. Las Vegas Wash (LV Wash) is a stream sample, all others are lake samples. Lake boundaries are from the U.S. Geological Survey National Hydrography Dataset. dw, dry weight.
dw, dry weight; LV, Las Vegas.
Surficial sediment
Microplastic concentrations in surficial sediment samples were highest at Gregg Basin (1,010 particles/kg dw), followed by Las Vegas Boat Harbor (440 particles/kg dw; Figs 3B and 4). Elsewhere, surficial sediment concentrations were 87.5–283 particles/kg dw. On average, 80.3% of particles in surficial sediment samples were fibers, followed by fragments (8.9%), films (7.7%), foams (1.4%), and “other” particles (1.7%). Fragments were primarily found in samples from Las Vegas Bay, Las Vegas Boat Harbor, and Lake Mohave, whereas films were found at low concentrations at all sites. The “other” particles were only found at the Las Vegas Boat Harbor, where they made up 15.5% of the sample. These were black, rubbery particles that were suspected of being tire wear particles, though that was not confirmed. Tires are used extensively in the Las Vegas Boat Harbor as boat bumpers and to form a large, approximately 1.5 km-long floating breakwall around the harbor.
Most sampled particles (63.7%) were in the 355–1,000 μm size range, while 36.3% were in the 1,000–5,600 μm size range. The predominant colors of microplastic particles in surficial sediment samples were clear (37.8% average), black (26.2%), blue (24.3%), and red (6.5%).
Las Vegas Bay sediment core
Based on a previously published sediment mass accumulation rate of ~1.0 g/cm2/year in Las Vegas Bay [46], the 33 cm-long sediment core represented approximately 19 years of deposition (2000–2018). Concentrations of microplastics in the four sectioned samples of the core ranged from 220 to 2,040 particles/kg dw, with no consistent trend with depth/time, although the highest concentration was in the deepest sample (Fig 5A). Virtually all (99.3%) of the microplastic particles in core samples were fibers. Most sampled particles (72.0%) were in the 355–1,000 μm size range, while 28.0% were in the 1,000–5,600 μm size range. On average, 77.3% were clear, 10.2% were blue, 8.2% were black, and 4.2% were red and other colors. The predominance of clear particles was relatively consistent with depth, varying from 67.3% to 82.8% (Fig 5B).
Concentrations of different microplastic types (A) and relative abundance of microplastic colors (B) in sediment core samples from Las Vegas Bay, Lake Mead, 2018. dw, dry weight.
Fish
Microplastic concentrations in striped bass across both sampling locations ranged from 0–19 particles/organism (mean 4.2, median 2.0; Fig 6). Striped bass from Las Vegas Bay (mean 6.3, median 4.0 particles/organism) had higher (not statistically significant) microplastic concentrations per fish than striped bass from Overton Arm (mean 1.9, median 2.0 particles/organism). There was no significant difference in fish lengths between the two locations (p = 0.41). Common carp at Las Vegas Bay (mean 11.0, median 12.0 particles/organism) had higher microplastic concentrations per fish than striped bass at the same location (not statistically significant). Although the common carp were significantly larger than the striped bass at Las Vegas Bay, species length was not correlated with microplastic concentration. Fibers made up on average 90.7% of all microplastic particles in fish samples. The predominant colors of microplastic particles in fish samples were clear (37.2% on average), blue (29.5%), black (17.1%), and red (9.3%).
Common carp were not sampled at Overton Arm. LV, Las Vegas.
Shellfish
Concentrations of microplastics in Asian clams ranged from 18–105 particles/organism (mean 51.7 particles/organism, n = 3; Fig 7). In quagga mussels, microplastic concentrations were higher at Las Vegas Bay (13.0 particles/organism averaged over 11 composited individuals) than at Overton Arm (2.7 particles/organism averaged over 10 composited individuals), despite the Overton Arm quagga mussels being approximately double in their size (shell long axis length of 0.6–1.4 cm at Las Vegas Bay versus 1.8–2.2 cm at Overton Arm; S1 Table).
*Average from 10–11 individuals, composited.
The relative abundances of particle types in shellfish were similar to those in surficial sediment: on average, fibers made up 80.9% of particles, followed by films (11.4%), fragments (6.3%), and foams (1.4%)). The predominant colors of microplastic particles in shellfish samples were clear (42.5%), blue (23.1%), black (17.8%), and red (4.9%).
Discussion
Results from this study show that microplastics were present in water and sediment throughout the study area and that ingestion by aquatic organisms was common. Microplastic concentrations (including concentrations in biota) appeared to be higher at locations with greater direct anthropogenic use and input (e.g., Las Vegas Wash, Las Vegas Bay, Las Vegas Boat Harbor), similar to results from studies looking at dissolved and particulate endocrine disrupting compounds in water, sediment, and fish [25,32–34,46,47]. An exception to this was surficial sediment from Las Vegas Wash, which had the lowest concentration among the sampled locations. We hypothesize that the flow velocity and turbulence at the Las Vegas Wash location kept most microplastic particles in suspension, thereby minimizing deposition. Another exception was Gregg Basin, a remote arm of Lake Mead far from Las Vegas or other large population centers, which had the highest microplastic concentration in surficial sediment, as well as high relative abundances of fragments and films in water. Much of the Colorado River Basin upstream of Gregg Basin is undeveloped, but upstream areas such as Grand Canyon National Park and Glen Canyon National Recreation Area receive heavy visitation, including water-based recreationists. Gregg Basin is where the flow of the Colorado River slows as it enters Lake Mead, so it is likely that any plastic particles in suspension will fall out and accumulate here as the river water slows and the depth of the water column deepens. We would expect a similar process to occur in Las Vegas Bay, where Las Vegas Wash enters the lake and velocities slow. Surficial sediment concentrations were comparatively low in Las Vegas Bay, however, perhaps simply due to choice of sampling locations.
Across all sample types, the most common color of microplastic particles was clear, making up 33.4–42.5% (on average) of particles in water, surficial sediment, fish, and shellfish, and 77.3% of particles in the sediment core. This abundance of clear particles may be attributed in part to the loss of color during environmental degradation and (or) bleaching during the WPO digestion of the sample in the laboratory [45]. Blue particles were also common across sample types, making up 10.2 to 29.5% of particles, on average. The source of blue particles is unclear but their prevalence has been noted by studies worldwide [44,45,48,49]. White particles were fairly common in the water samples (18%) but were rare or absent in other compartments. This difference may be because of polymer type (e.g., white particles tend to be a certain polymer type which is buoyant and therefore remains at the water surface), or it may be that in sediment and biota the once-white particles have degraded to clear.
As with other studies of microplastics in freshwater [6,50–52], in this study fibers were the most abundant microplastic type in all environmental compartments. Whereas fragments, films, and foams were more commonly found at locations with greater anthropogenic impact, fibers were ubiquitous across all locations. The widespread presence of fibers across the study area, including at relatively remote locations (e.g., Overton Arm, Virgin Basin), suggests a diffuse source. Increasingly, the atmosphere is being investigated as an important pathway for microplastic fiber transport to and deposition in remote locations. Recent studies in both urban areas and remote mountain environments have documented atmospheric deposition of microplastic fibers and other particle types (e.g., fragments) [8,11,49,53]. The atmospheric contribution of microplastics to Lakes Mead and Mohave is unclear but may be significant, particularly as the area is downwind of Las Vegas and other large metropolitan areas in California.
There is clear evidence of microplastics entering the food chain. The biological component of this study focused on a limited number of fish and shellfish species, but microplastic ingestion by other organisms is also likely. This includes other fish species, such as the endangered razorback sucker, as well as piscivorous birds such as osprey and bald eagles. Results here are conservative due to the relatively large size fraction of microplastics identified. Given the propensity for microplastic abundance to increase with smaller size fractions [54], it is likely that the number of ingested micro- and nanoplastics also increases with smaller size fractions (<150 μm). The biological effects of microplastic ingestion in the study area are unknown because toxicological benchmarks have yet to be established.
Comparisons to previous studies
Comparison of results between microplastic studies is complicated by a lack of standardized field and laboratory methods [55–57]. In this section, comparisons to other studies are focused on freshwater studies which used methods similar to the current study. Where possible, we also compare results to those from the St. Croix National Scenic Riverway (hereafter St. Croix) and the Mississippi National River and Recreation Area (hereafter Mississippi) in Wisconsin and Minnesota (USA) [58]. The St. Croix and Mississippi are riverine rather than reservoir environments, but are relevant because they are the only other U.S. National Park Service waterbodies which have been sampled for microplastics using similar methods. More comprehensive reviews of microplastics in freshwater environments are available elsewhere [13,55,56,59].
Concentrations of microplastics in water samples in the current study (0.44–9.70 particles/m3) are somewhat lower than a reported value for Brownlee Reservoir, another large reservoir in the western U.S. (13.7 particles/m3; Snake River, Idaho/Oregon) [52]. Concentrations in water samples from the St. Croix and Mississippi Rivers (0.80–4.84 particles/m3) [58], and from the much more urban Milwaukee Harbor and nearshore Lake Michigan (Wisconsin, USA; 0.21–5.23 particles/m3) [60], were similar to those in the current study. However, the St. Croix/Mississippi and Milwaukee Harbor/Lake Michigan studies used a larger mesh size (333 μm) than the current study, potentially biasing their concentrations low.
Concentrations in surficial sediment samples in the current study (87.5–1,010 particles/kg dw) are similar to those reported in the Milwaukee Harbor and nearshore Lake Michigan (Wisconsin, USA; 39.5–319 particles/kg dw) [60]; nearshore Lake Ontario (Canada; 40–4,270 particles/kg dw) [23]; and an urban lake in London (England; 539 particles/kg dw) [61]. The overlap in sediment microplastic concentrations between Lake Mead NRA and these more urbanized locations may point to a dispersed pathway such as atmospheric deposition of microplastic particles on the landscape [8,11,53].
A review of microplastic ingestion by wild freshwater fish around the world found that 74% of studies (n = 23) reported mean concentrations of fewer than 3 particles/organism, and only 13% reported mean concentrations greater than 6 particles/organism (maximum average concentration 19.2 particles/organism) [62]. Based on these ranges, concentrations in fish in the current study are typical at Overton Arm (average 1.9 particles/organism in striped bass), but are somewhat high at Las Vegas Bay (average 6.3 particles/organism in striped bass, 11.0 particles/organism in common carp). However, this comparison is complicated by differences in fish species, sizes, trophic levels, and feeding habits among the different studies. In smallmouth bass from the St. Croix and Mississippi Rivers, concentrations were somewhat higher than the current study, averaging 17.5 particles/organism (range 1–111 particles/organism) [58].
Studies of microplastics in wild quagga mussels are lacking, but a study of Asian clams at 21 lake, river, and estuary locations in the Middle-Lower Yangtze River Basin (China) reported averages of 0.4–5.0 particles/organism [45]. Asian clam concentrations in the current study were considerably higher, averaging 51.7 particles/organism (range 18–105). However, the current study included only three Asian clams, all collected from areas of relatively high anthropogenic impact, and thus not likely representative of Lake Mead NRA in general. Concentrations in three-ridge mussels from the St. Croix and Mississippi Rivers averaged 6.7 particles/organism (range 1–18), similar to concentrations in quagga mussels in the current study (average of 2.7–13.0 particles/organism).
Constraints on interpretations
Water and sediment were sampled only once at each location, and fish and shellfish sample sizes were also relatively small (especially those of Asian clams, which were not targeted but sampled opportunistically). RPDs of water and sediment sample replicates show considerable variability, highlighting the constraints on interpretations based on small sample numbers. Further study is needed to better understand the spatial and temporal variability in microplastic concentrations in LMNRA.
In addition to small sample sizes, another constraint on our interpretations is the sole reliance on visual microscopy for detection/identification of microplastics in samples. Studies have shown that microplastic identification using visual microscopy is susceptible to false-positives due to the difficulty in deciphering between plastic and natural materials, especially at sizes <100 μm [63,64]. As a result, spectroscopic methods such as Raman micro-spectroscopy or Fourier Transform Infrared Spectroscopy (FTIR) are increasingly being used, typically on a subset of particles, to confirm determinations made by visual microscopy. In a comparison of visual microscopy to Raman micro-spectroscopy, Lenz et al. [63] found that the false-positive rate using visual microscopy decreased with increasing particle sizes. For that study, at sizes <50 μm, 63% of particles that had been visually identified as plastic were confirmed using spectroscopic methods, but the confirmation rate increased to 83% for particles >100 μm. A study comparing visual microscopy to FTIR reported that fibers were overestimated using visual microscopy but that fragments were underestimated, and that total microplastic particle counts were actually lower using visual microscopy compared to FTIR [65]. Although spectroscopic confirmation was not used in the current study, we considered only particles >355 μm in sediments and water and >150 μm in tissues to minimize false-positives, despite water samples having been collected using a 100 μm net. Based on Lenz et al. (2015), this conservative approach should result in a relatively low rate of false positives, perhaps in the range of 15–20%.
Results may be biased low for cosmetic microbeads. A limited number of polymers are susceptible to degradation from the temperatures reached during processing (e.g., WPO and sediment drying). Specifically cosmetic microbeads that have non-plastic wax components [66] are likely to be removed during processing. WPO and temperatures up to 100°C have not been found to alter other common polymers (e.g. polyethylene, polystyrene, or nylon) [66].
Conclusion
This study is a first look at microplastic occurrence in LMNRA, and helps fill existing knowledge gaps on microplastics in freshwater environments in the southwestern U.S. The results presented here provide a basic understanding of microplastic occurrence in different environmental compartments in Lakes Mead and Mohave, and they provide insights into sources, transport, and fate across a gradient of anthropogenic impacts. Further study is needed to better understand these processes. Although sample sizes are limited, results indicate greater uptake of microplastics by fish and shellfish in areas with greater anthropogenic impacts. The ecosystem effects of the observed microplastic pollution remain unclear, but will be increasingly important as the human population living and recreating in the Colorado River Basin continues to grow.
Supporting information
S1 Table. Locations and sizes of fish and shellfish samples.
NA, not available (was not weighed because of size).
https://doi.org/10.1371/journal.pone.0228896.s001
(XLSX)
S2 Table. Summary of results from laboratory process and equipment blank samples.
https://doi.org/10.1371/journal.pone.0228896.s002
(XLSX)
Acknowledgments
The authors gratefully acknowledge field support by USGS personnel Erin Orosco, Jon Wilson, Afan Tarar, Mike Pavelko, and Guy DeMeo, and NPS personnel Ben Smith, Taylor Senegal, Heather Whitesides, and Mark Sappington. We also thank Lisa Ozborn and Casey Korby at the Nevada Department of Wildlife for fish collection, and Tanner Sareela, Eric Brown, Stephen Sissel, and Shannon Black at the USGS for assistance in the laboratory. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
References
- 1. Gregory MR. Plastic scrubbers’ in hand cleansers: A further (and minor) source for marine pollution identified. Mar Pollut Bull. 1996;32(12):867–71.
- 2. Browne MA, Galloway T, Thompson R. Microplastic—an emerging contaminant of potential concern? Integr Environ Assess Manag. 2007;3(4):559–61. pmid:18046805
- 3. Cole M, Lindeque P, Halsband C, Galloway TS. Microplastics as contaminants in the marine environment: A review. Mar Pollut Bull. 2011;62(12):2588–97. pmid:22001295
- 4. Hartline NL, Bruce NJ, Karba SN, Ruff EO, Sonar SU, Holden PA. Microfiber Masses Recovered from Conventional Machine Washing of New or Aged Garments. Environ Sci Technol. 2016 Nov 1;50(21):11532–8. pmid:27689236
- 5. Kole PJ, Löhr AJ, Van Belleghem FGAJ, Ragas AMJ. Wear and Tear of Tyres: A Stealthy Source of Microplastics in the Environment. Int J Environ Res Public Health [Internet]. 2017 Oct [cited 2019 Mar 29];14(10). Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5664766/ pmid:29053641
- 6. Baldwin AK, Corsi SR, Mason SA. Plastic Debris in 29 Great Lakes Tributaries: Relations to Watershed Attributes and Hydrology. Environ Sci Technol. 2016 Sep 14;50(19):10377–85. pmid:27627676
- 7. Mason SA, Garneau D, Sutton R, Chu Y, Ehmann K, Barnes J, et al. Microplastic pollution is widely detected in US municipal wastewater treatment plant effluent. Environ Pollut. 2016;218:1045–54. pmid:27574803
- 8. Dris R, Gasperi J, Saad M, Mirande C, Tassin B. Synthetic fibers in atmospheric fallout: A source of microplastics in the environment? Mar Pollut Bull [Internet]. 2016 [cited 2016 Feb 17]; Available from: http://www.sciencedirect.com/science/article/pii/S0025326X16300066
- 9. Eriksen M, Mason S, Wilson S, Box C, Zellers A, Edwards W, et al. Microplastic pollution in the surface waters of the Laurentian Great Lakes. Mar Pollut Bull. 2013;77(1–2):177–82. pmid:24449922
- 10. Peng X, Chen M, Chen S, Dasgupta S, Xu H, Ta K, et al. Microplastics contaminate the deepest part of the world’s ocean. Geochem Perspect Lett. 2018;1–5.
- 11. Allen S, Allen D, Phoenix VR, Roux GL, Jiménez PD, Simonneau A, et al. Atmospheric transport and deposition of microplastics in a remote mountain catchment. Nat Geosci. 2019;1.
- 12.
Wetherbee GA, Baldwin AK, Ranville JF. It is raining plastic. US Geological Survey; 2019.
- 13. Xu S, Ma J, Ji R, Pan K, Miao A-J. Microplastics in aquatic environments: Occurrence, accumulation, and biological effects. Sci Total Environ. 2020 Feb 10;703:134699. pmid:31726297
- 14. Lavers JL, Bond AL, Hutton I. Plastic ingestion by flesh-footed shearwaters (Puffinus carneipes): Implications for fledgling body condition and the accumulation of plastic-derived chemicals. Environ Pollut. 2014;187:124–9. pmid:24480381
- 15. Besseling E, Foekema EM, Van Franeker JA, Leopold MF, Kühn S, Bravo Rebolledo EL, et al. Microplastic in a macro filter feeder: Humpback whale Megaptera novaeangliae. Mar Pollut Bull. 2015 Jun 15;95(1):248–52. pmid:25916197
- 16. Desforges J-PW, Galbraith M, Ross PS. Ingestion of Microplastics by Zooplankton in the Northeast Pacific Ocean. Arch Environ Contam Toxicol. 2015 Oct 1;69(3):320–30. pmid:26066061
- 17. Rochman CM, Brookson C, Bikker J, Djuric N, Earn A, Bucci K, et al. Rethinking microplastics as a diverse contaminant suite. Environ Toxicol Chem. 2019;38(4):703–11. pmid:30909321
- 18. Rochman CM, Browne MA, Underwood AJ, van Franeker JA, Thompson RC, Amaral-Zettler LA. The ecological impacts of marine debris: unraveling the demonstrated evidence from what is perceived. Ecology. 2016;97(2):302–12. pmid:27145606
- 19. Foley CJ, Feiner ZS, Malinich TD, Höök TO. A meta-analysis of the effects of exposure to microplastics on fish and aquatic invertebrates. Sci Total Environ. 2018 Aug 1;631–632:550–9. pmid:29529442
- 20. Adam V, Yang T, Nowack B. Toward an ecotoxicological risk assessment of microplastics: Comparison of available hazard and exposure data in freshwaters. Environ Toxicol Chem. 2019;38(2):436–47. pmid:30488983
- 21. Ribeiro F, O’Brien JW, Galloway T, Thomas KV. Accumulation and fate of nano- and micro-plastics and associated contaminants in organisms. TrAC Trends Anal Chem. 2019 Feb 1;111:139–47.
- 22. Corcoran PL, Norris T, Ceccanese T, Walzak MJ, Helm PA, Marvin CH. Hidden plastics of Lake Ontario, Canada and their potential preservation in the sediment record. Environ Pollut. 2015 Sep;204:17–25. pmid:25898233
- 23. Ballent A, Corcoran PL, Madden O, Helm PA, Longstaffe FJ. Sources and sinks of microplastics in Canadian Lake Ontario nearshore, tributary and beach sediments. Mar Pollut Bull. 2016 Sep 15;110(1):383–95. pmid:27342902
- 24. Hoellein TJ, McCormick AR, Hittie J, London MG, Scott JW, Kelly JJ. Longitudinal patterns of microplastic concentration and bacterial assemblages in surface and benthic habitats of an urban river. Freshw Sci. 2017;000–000.
- 25.
Rosen MR, Turner K, Goodbred SL, Miller JM. A Synthesis of Aquatic Science for Management of Lakes Mead and Mohave [Internet]. USGS Circular 1381; 2012. Available from: http://pubs.usgs.gov/circ/1381/
- 26.
U.S. National Park Service. National Park Service Visitor Use Statistics [Internet]. 2019 [cited 2019 Sep 20]. Available from: https://irma.nps.gov/Stats/Reports/Park/
- 27.
U.S. National Park Service. Fact Sheet—Lake Mead National Recreation Area [Internet]. 2019 [cited 2019 Jun 26]. Available from: https://www.nps.gov/lake/learn/news/facts.htm
- 28.
Bevans HE, Goodbred SL, Miesner JF, Watkins SA, Gross TS, Denslow ND, et al. Synthetic organic compounds and carp endocrinology and histology in Las Vegas Wash and Las Vegas and Callville Bays of Lake Mead, Nevada, 1992 and 1995 [Internet]. U.S. Dept. of the Interior, U.S. Geological Survey,; 1996 [cited 2019 Aug 18]. (Water-Resources Investigations Report). Report No.: 96–4266. Available from: http://pubs.er.usgs.gov/publication/wri964266
- 29. Patiño R, Goodbred SL, Draugelis‐Dale R, Barry CE, Foott JS, Wainscott MR, et al. Morphometric and Histopathological Parameters of Gonadal Development in Adult Common Carp from Contaminated and Reference Sites in Lake Mead, Nevada. J Aquat Anim Health. 2003;15(1):55–68.
- 30. Leiker TJ, Abney SR, Goodbred SL, Rosen MR. Identification of methyl triclosan and halogenated analogues in male common carp (Cyprinus carpio) from Las Vegas Bay and semipermeable membrane devices from Las Vegas Wash, Nevada. Sci Total Environ. 2009 Mar 1;407(6):2102–14. pmid:19054547
- 31. Patiño R, Rosen MR, Orsak EL, Goodbred SL, May TW, Alvarez D, et al. Patterns of metal composition and biological condition and their association in male common carp across an environmental contaminant gradient in Lake Mead National Recreation Area, Nevada and Arizona, USA. Sci Total Environ. 2012 Feb 1;416:215–24. pmid:22206697
- 32. Goodbred SL, Patiño R, Torres L, Echols KR, Jenkins JA, Rosen MR, et al. Are endocrine and reproductive biomarkers altered in contaminant-exposed wild male Largemouth Bass (Micropterus salmoides) of Lake Mead, Nevada/Arizona, USA? Gen Comp Endocrinol. 2015 Aug 1;219:125–35. pmid:25733205
- 33. Patiño R, VanLandeghem MM, Goodbred SL, Orsak E, Jenkins JA, Echols K, et al. Novel associations between contaminant body burdens and biomarkers of reproductive condition in male Common Carp along multiple gradients of contaminant exposure in Lake Mead National Recreation Area, USA. Gen Comp Endocrinol. 2015 Aug 1;219:112–24. pmid:25583583
- 34. Jenkins JA, Rosen MR, Draugelis-Dale RO, Echols KR, Torres L, Wieser CM, et al. Sperm quality biomarkers complement reproductive and endocrine parameters in investigating environmental contaminants in common carp (Cyprinus carpio) from the Lake Mead National Recreation Area. Environ Res. 2018 May 1;163:149–64. pmid:29438900
- 35.
Wessells SM, Rosen MR. Lake Mead—Clear and Vital, USGS General Information Product 148 [Internet]. 2013 [cited 2019 Aug 19]. Available from: https://pubs.usgs.gov/gip/148/
- 36. Wittmann ME, Chandra S, Caires A, Denton M, Rosen MR, Wong WH, et al. Early invasion population structure of quagga mussel and associated benthic invertebrate community composition on soft sediment in a large reservoir. Lake Reserv Manag. 2010 Nov 19;26(4):316–27.
- 37. Peck S, Pratt W, Pollard J, Paulson L, Baepler D. Benthic invertebrates and crayfish of Lake Mead. Lake Mead Limnol Res Cent Environ Res Cent Univ Nev Las Vegas. 1987;93.
- 38. Masura J, Baker J, Foster G, Arthur C. Laboratory methods for the analysis of microplastics in the marine environment: recommendations for quantifying synthetic particles in waters and sediments. NOAA Technical Memorandum NOS-OR&R-48. 2015.
- 39. Kühn S, van Werven B, van Oyen A, Meijboom A, Bravo Rebolledo EL, van Franeker JA. The use of potassium hydroxide (KOH) solution as a suitable approach to isolate plastics ingested by marine organisms. Mar Pollut Bull. 2017 Feb 15;115(1):86–90.
- 40.
R Core Team. R: A Language and Environment for Statistical Computing, version 3.6.1 [Internet]. Vienna, Austria: R Foundation for Statistical Computing; 2015. Available from: http://www.R-project.org/
- 41.
Frank E Harrell Jr et al. Hmisc: Harrell Miscellaneous [Internet]. 2015. Available from: http://CRAN.R-project.org/package = Hmisc
- 42. Baldwin AK, Spanjer AR, Rosen MR, Thom T. Microplastics in Lake Mead National Recreation Area, 2017–2018. US Geol Surv Data Release [Internet]. 2019; Available from: https://doi.org/10.5066/P9V1MNHH
- 43. Barrows APW, Neumann CA, Berger ML, Shaw SD. Grab vs. neuston tow net: a microplastic sampling performance comparison and possible advances in the field. Anal Methods [Internet]. 2016 Oct 4 [cited 2016 Oct 20]; Available from: http://pubs.rsc.org/en/content/articlelanding/2016/ay/c6ay02387h
- 44. Dris R, Gasperi J, Rocher V, Tassin B. Synthetic and non-synthetic anthropogenic fibers in a river under the impact of Paris Megacity: Sampling methodological aspects and flux estimations. Sci Total Environ. 2018 Mar 15;618:157–64. pmid:29128764
- 45. Su L, Cai H, Kolandhasamy P, Wu C, Rochman CM, Shi H. Using the Asian clam as an indicator of microplastic pollution in freshwater ecosystems. Environ Pollut. 2018 Mar 1;234:347–55. pmid:29195176
- 46. Rosen MR, Van Metre PC. Assessment of multiple sources of anthropogenic and natural chemical inputs to a morphologically complex basin, Lake Mead, USA. Palaeogeogr Palaeoclimatol Palaeoecol. 2010 Aug;294(1–2):30–43.
- 47. Rosen MR, Alvarez DA, Goodbred SL, Leiker TJ, Patino R. Sources and distribution of organic compounds using passive samplers in Lake Mead National Recreation Area, Nevada and Arizona, and their implications for potential effects on aquatic biota. J Environ Qual. 2009;39(7–8):11611172.
- 48. Peters CA, Bratton SP. Urbanization is a major influence on microplastic ingestion by sunfish in the Brazos River Basin, Central Texas, USA. Environ Pollut. 2016 Mar;210:380–7. pmid:26807984
- 49.
Wetherbee GA, Baldwin AK, Ranville JF. It is raining plastic [Internet]. Reston, VA: U.S. Geological Survey; 2019 [cited 2019 May 14]. (Open-File Report). Report No.: 2019–1048. Available from: http://pubs.er.usgs.gov/publication/ofr20191048
- 50. McCormick A, Hoellein TJ, Mason SA, Schluep J, Kelly JJ. Microplastic is an abundant and distinct microbial habitat in an urban river. Environ Sci Technol. 2014;48(20):11863–71. pmid:25230146
- 51. Dris R, Gasperi J, Rocher V, Saad M, Renault N, Tassin B. Microplastic contamination in an urban area: a case study in Greater Paris. Environ Chem. 2015;12(5):592–9.
- 52. Kapp KJ, Yeatman E. Microplastic hotspots in the Snake and Lower Columbia rivers: A journey from the Greater Yellowstone Ecosystem to the Pacific Ocean. Environ Pollut. 2018 Oct 1;241:1082–90. pmid:30029316
- 53. Cai L, Wang J, Peng J, Tan Z, Zhan Z, Tan X, et al. Characteristic of microplastics in the atmospheric fallout from Dongguan city, China: preliminary research and first evidence. Environ Sci Pollut Res. 2017 Nov 1;24(32):24928–35.
- 54. Song YK, Hong SH, Jang M, Kang J-H, Kwon OY, Han GM, et al. Large Accumulation of Micro-sized Synthetic Polymer Particles in the Sea Surface Microlayer. Environ Sci Technol. 2014 Aug 19;48(16):9014–21. pmid:25059595
- 55. Meng Y, Kelly FJ, Wright SL. Advances and challenges of microplastic pollution in freshwater ecosystems: A UK perspective. Environ Pollut. 2020 Jan 1;256:113445. pmid:31733965
- 56. Koelmans AA, Mohamed Nor NH, Hermsen E, Kooi M, Mintenig SM, De France J. Microplastics in freshwaters and drinking water: Critical review and assessment of data quality. Water Res. 2019 May 15;155:410–22. pmid:30861380
- 57. Twiss MR. Standardized methods are required to assess and manage microplastic contamination of the Great Lakes system. J Gt Lakes Res. 2016;42(5):921–5.
- 58. Baldwin AK, King K, Hoellein TJ, Kim L, Karns B. Data release for microplastics in water, sediment, fish, and mussels in the St. Croix National Scenic Riverway and Mississippi National River and Recreation Area, Wisconsin and Minnesota, 2015. US Geol Surv Data Release [Internet]. 2017; Available from: https://www.sciencebase.gov/catalog/item/58e7d00ae4b09da6799c0f8a
- 59. Li J, Liu H, Paul Chen J. Microplastics in freshwater systems: A review on occurrence, environmental effects, and methods for microplastics detection. Water Res. 2018 Jun 15;137:362–74. pmid:29580559
- 60. Lenaker PL, Baldwin AK, Corsi SR, Mason SA, Reneau PC, Scott JW. Vertical Distribution of Microplastics in the Water Column and Surficial Sediment from the Milwaukee River Basin to Lake Michigan. Environ Sci Technol. 2019 Nov 5;53(21):12227–37. pmid:31618011
- 61. Turner S, Horton AA, Rose NL, Hall C. A temporal sediment record of microplastics in an urban lake, London, UK. J Paleolimnol. 2019 Apr 1;61(4):449–62.
- 62. Collard F, Gasperi J, Gabrielsen GW, Tassin B. Plastic particle ingestion by wild freshwater fish: a critical review. Environ Sci Technol [Internet]. 2019 Oct 30 [cited 2019 Nov 7]; Available from: https://doi.org/10.1021/acs.est.9b03083
- 63. Lenz R, Enders K, Stedmon CA, Mackenzie DMA, Nielsen TG. A critical assessment of visual identification of marine microplastic using Raman spectroscopy for analysis improvement. Mar Pollut Bull. 2015 Nov 15;100(1):82–91. pmid:26455785
- 64. Silva AB, Bastos AS, Justino CIL, da Costa JP, Duarte AC, Rocha-Santos TAP. Microplastics in the environment: Challenges in analytical chemistry—A review. Anal Chim Acta. 2018 Aug 9;1017:1–19. pmid:29534790
- 65. Song YK, Hong SH, Jang M, Han GM, Rani M, Lee J, et al. A comparison of microscopic and spectroscopic identification methods for analysis of microplastics in environmental samples. Mar Pollut Bull. 2015;93(1–2):202–9. pmid:25682567
- 66. Munno K, Helm PA, Jackson DA, Rochman C, Sims A. Impacts of temperature and selected chemical digestion methods on microplastic particles. Environ Toxicol Chem. 2018;37(1):91–8. pmid:28782833