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ABSTRACT Microplastic is a contaminant of concern worldwide. Rivers are implicated as major pathways of microplastic transport to marine and lake ecosystems, and microplastic ingestion by

freshwater biota is a risk associated with microplastic contamination, but there is little research on microplastic ecology within freshwater ecosystems. Microplastic uptake by fish is

likely affected by environmental microplastic abundance and aspects of fish ecology, but these relationships have rarely been addressed. We measured the abundance and composition of

microplastic in fish and surface waters from 3 major tributaries of Lake Michigan, USA. Microplastic was detected in fish and surface waters from all 3 sites, but there was no correlation

between microplastic concentrations in fish and surface waters. Rather, there was a significant effect of functional feeding group on microplastic concentration in fish. _Neogobius

melanostomus_ (round goby, a zoobenthivore) had the highest concentration of gut microplastic (19 particles fish−1) compared to 10 other fish taxa measured, and had a positive linear

relationship between body size and number of microplastic particles. Surface water microplastic concentrations were lowest in the most northern, forested watershed, and highest in the most

southern, agriculturally dominated watershed. Results suggest microplastic pollution is common in river food webs and is connected to species feeding characteristics. Future research should

focus on understanding the movement of microplastic from point-source and diffuse sources and into aquatic ecosystems, which will support pollution management efforts on inland waters.

SIMILAR CONTENT BEING VIEWED BY OTHERS MICROPLASTIC BURDEN IN MARINE BENTHIC INVERTEBRATES DEPENDS ON SPECIES TRAITS AND FEEDING ECOLOGY WITHIN BIOGEOGRAPHICAL PROVINCES Article Open access

04 December 2023 CURRENT LEVELS OF MICROPLASTIC POLLUTION IMPACT WILD SEABIRD GUT MICROBIOMES Article Open access 27 March 2023 FIELD EVIDENCE FOR MICROPLASTIC INTERACTIONS IN MARINE BENTHIC

INVERTEBRATES Article Open access 22 October 2021 INTRODUCTION In the mid-1900s, plastic became an integral component of human cultures and commerce globally1. Plastic contributes to ~10%

of all municipal waste2 and 50–80% of waste on beaches and in the oceans3. Plastic litter is an emerging concern in ecosystems worldwide. Approximately 20 million tons of plastic enters the

marine environment each year4, and plastic litter is predicted to outweigh fish in the ocean by the year 20505. Plastic is abundant in the most remote, un-inhabited parts of the world such

as the Barents Sea (Artic Ocean)6, Henderson Island (South Pacific)7, and the deep ocean8,9. Sources of plastic litter to aquatic ecosystems include wastewater treatment plant effluent10,11,

industrial production12, synthetic textiles13, and the breakdown of larger anthropogenic litter (AL; trash) into smaller pieces14,15,16. While a growing body of research shows plastic is

ubiquitous globally, its biological and ecological effects are less well known. Microplastic (particles <5 mm) is a focus of research on interactions between plastic and biota, including

microbes, invertebrates, fish, birds, and aquatic mammals17,18,19,20. Microplastic can adsorb hydrophobic compounds such as persistent organic pollutants and contaminants of emerging concern

(_e.g_. Triclosan and polyaromatic hydrocarbons (PAHs))21,22,23. Once ingested, compounds can be desorbed in the anaerobic environment of the gut and absorbed by animal tissues21,23. This

may accelerate bioaccumulation of microplastic and adsorbed compounds as they move through food webs via trophic transfer24. For example, up to 60 ng g−1 dry weight of pyrene (a PAH) was

measured in the gill tissue of a filter-feeding mussel (_Mytilus galloprovincialis_ Lamarck) after consuming microplastic exposed to pyrene22. Ingestion of microplastic could also reduce

nutrient assimilation via digestive tract blockages and irritation of epithelial lining25,26. Finally, microplastic supports unique microbial communities compared to natural habitats and

substrates20,27 and could facilitate pathogenic ‘hitch-hikers’ such as the bacteria Campylobacteraceae and _Vibrio_10,19,20. Although microplastic ecology is a rapidly growing field of

research, most studies have focused on marine organisms and habitats, with fewer studies conducted in rivers. Understanding the abundance, movement, and biological interactions of

microplastic in freshwaters is critical to documenting its global impacts28. In addition, because freshwater ecosystems are closely connected to the terrestrial landscape and have much

smaller volumes of water than the oceans, freshwaters represent important sites for prevention and management of microplastic. Landscape features in a watershed (_e.g_., land-use, riparian

vegetation, and geomorphology) influence particle transport and concentration in rivers29,30,31,32,33, but few studies have examined how landscape features influence the abundance and

biological interactions of microplastic in freshwaters. Microplastic originates from point and non-point sources. Earlier studies have focused on point sources of microplastic pollution such

as WWTP effluent10,34,35, while non-point sources of pollution are less well understood. Recent evidence suggests non-point sources include application of biosolids to agricultural

fields36, atmospheric deposition37, and stormwater runoff38. Landscape features that serve as sinks for other types of fine particles most likely promote deposition of microplastic, such as

dams, lakes, and low-velocity zones39. Understanding the composite effect of landscape sources and sinks on microplastic abundances in freshwater food webs requires comparisons across

watersheds of different land-use types40. The rapidly growing field of microplastic ecology has allowed for developments in methodology and thereby facilitated new approaches to data

collection and processing41,42,43. Many studies of microplastic use neuston or plankton nets to collect samples44. However, Barrows _et al_.42 compared ‘grab’ samples of 1 L of seawater to

the conventional neuston net approach and found grab samples collected 3 orders of magnitude (5.9 ± 4.4 SD L−1) more microplastic than the net approach (0.005 ± 0.004 SD L−1). The authors

concluded most microplastic escaped the neuston net (0.363 mm mesh) and that grab samples collected a wider size range of microplastic, which better reflect microplastic abundance in the

environment42. In addition to changes in data collection procedures, contamination of samples with microplastic is common. Sources include airborne particles in the lab and field, as well as

microplastic in reagent chemicals and filtered lab water45,46. Previous research accounts for contamination in each step10, but no previous studies have quantified the size, shape, and

color of contamination particles compared to environmental samples. This comparison will help isolate the sources of contamination and more accurately account for contaminants in field

samples. In this study, we measured microplastic abundance in fish and water from three major tributaries of Lake Michigan, which spanned a land-use gradient. We expected _(H_1) that

microplastic abundance in fish would increase with body size and trophic position, and _(H_2) that a positive relationship would be observed between microplastic concentrations in fish

digestive tissue and the water column. In addition, we hypothesized _(H_3) that microplastic would predominantly be small or medium sized fibers across river sites and fish taxa, with a

similar composition as a recent study using the same grab sample approach in the Gulf of Maine43. Finally, _(H_4) we predicted that analytical controls (to account for contamination) would

show substantially fewer microplastic fibers and would be distinct in size, color, and relative composition compared to environmental samples. METHODS STUDY SITES The Muskegon River (MG),

Milwaukee River (MK), and St. Joseph River (SJ) are major tributaries of Lake Michigan, USA (Fig. 1). We selected these sites to span a land-use gradient dominated by forest (MG),

urban-agriculture (MK), and agriculture (SJ; Table 1). Dominant land-use categories were determined by calculating relative abundance of land-use in forest, urban, and agriculture categories

within each watershed using the 2011 National Land Cover Dataset in ArcMap Geographic Information Systems (GIS)47. All field work was conducted upriver of each river mouth to ensure surface

water current was unidirectional toward Lake Michigan. The Muskegon River was sampled within Muskegon State Park, 1.1 km upstream from the river mouth in Muskegon, MI (43°13′56.2″N,

86°19′40.1″W). Milwaukee River field work was conducted 1.5 km upstream from the river mouth, just downstream of the confluence of the MK and Menomonee Rivers in Milwaukee, WI (43°01′49.8′N,

87°54′27.9″W). The St. Joseph River field site was 0.93 km upstream from the river mouth in Benton Harbor and St. Joseph, MI (42°06′44.4″N, 86°28′41.7″W). All field work was conducted with

approval of state and local officials. FISH AND WATER COLLECTION Fish were opportunistically collected with wading seine nets adjacent to MG and SJ water collection sites and 1.5 km

downstream from the MK water collection site. Fish were preliminarily identified in the field and up to 15 fish per taxa were euthanized with MS-222 (Tricaine-S; 0.25 g L−1) while the

remaining fish were released. Fish were preserved in 70% ethanol in the field and transported to the laboratory where they were identified to species or genus48 and processed for

microplastic gut content. All methods were carried out in accordance with ethical guidelines and regulations and approved by Loyola University Chicago’s Institutional Animal Care and Use

Committee. Microplastic was collected from surface water habitats via grab samples with 2 L glass bottles (_n_ = 4 bottles per site) from the MG, MK, and SJ Rivers during summer 2016.

Bottles were rinsed three times with DI water filtered through a 0.363 µm mesh in the lab pre-sampling. Collection bottles were filled with water directly below the river surface along the

left and right side of river channels (_n_ = 2 bottles per in-stream river location), capped immediately to prevent atmospheric contamination of microplastic, and transported to the

laboratory for microplastic processing42. MICROPLASTIC QUANTIFICATION AND CHARACTERIZATION In the laboratory, the body length of each fish was measured, the digestive tracts were dissected,

and the tissue was placed in individual clean beakers. Digestive tissue was dried at 75 °C for at least 24 h and underwent wet peroxide oxidation (0.05 M Fe(II) and 30% H2O2) at 75 °C to

dissolve organic material49. Microplastic is resistant to wet peroxide oxidation49. Samples were filtered onto gridded 0.45 µm filters (WhatmanTM, Pittsburgh, Pennsylvania, USA). Filters

were examined at 25–50 × magnification under dissecting microscopes and checked two separate times to confirm microplastic counts were consistent and conservative. Microplastic was counted

and categorized as either fiber, fragment, bead, foam, or film and classified into a color category20,50. Due to the high abundance of fibers, length was measured on a randomly selected

sub-sample of fibers on each filter. Fiber length was measured along the longest dimension with an ocular micrometer or estimated using the filter grid width (3.2 mm)42. We measured size and

color of 554 fibers from fish (_n_ = 819 counted). Fish were classified into functional feeding groups (FFG) and assigned a trophic fraction based on collective data available on FishBase51

coupled with visual identification of macroinvertebrates in gut content post wet peroxide oxidation (Table 2). A total of 17 fish were classified as detritivores, 30 as omnivores, and 27 as

zoobenthivores (Table 2). Microplastic concentration in fish taxa and FFG were calculated as the No. microplastic fish−1. River surface water samples were vacuum filtered onto gridded 0.45 

µm filters (~300 mL filter−1) and oven dried at 75 °C for 24 h42. Microplastic was quantified as explained above. Microplastic concentration was calculated by dividing the total number of

microplastic in each sample by the sample’s total volume (L). Microplastic polymer type was identified using Fourier Transform Infrared Spectroscopy (FTIR) on randomly selected microplastic

from environmental samples. This technique produces infrared absorption bands that are unique to each polymer type. The small size of most of the microplastic and impurities (_e.g_., organic

material and minerals) that can adhere to the microplastic is challenging for FTIR14,52,53. Of 160 number of fibers analyzed by FTIR, 5.6% were successfully identified. QUANTIFICATION OF

LABORATORY MICROPLASTIC CONTAMINATION We carried out filter controls to account for microplastic contamination associated with surface water samples. We placed gridded 0.45 µm filters onto

the vacuum filtration apparatus and rinsed the collection cup with 0.363 µm filtered DI water (_n_ = 10 filter controls). In addition, a second set of controls were completed to account for

microplastic contamination from the digestion and filtering processes (_i.e_., ‘digestion control’) that fish samples were exposed to. Twenty mL of 30% H2O2 and 20 mL of 0.05 M Fe(II)

solution were added to a clean beaker, heated at 75 °C, and vacuum filtered onto a gridded 0.45 µm filter (_n_ = 10). Microplastic was quantified and characterized as explained for surface

water and fish samples. Microplastic contamination (mean No. filter−1) consisted of 2.3 (±0.63 SE) and 4 (±0.39 SE) fibers for filter and digestion controls, respectively. Mean fiber

contamination from each control was accounted for in each sample type (_i.e_., digestion control was used for fish and filter control was used for grab samples). We corrected fiber color and

size category by removing fibers from fish (4) and surface water samples (2) following the color and size class frequencies recorded on control filters. Finally, we compared the abundance,

category, size, and color of contamination fibers to those found in environmental samples. STATISTICAL ANALYSES We compared microplastic concentration in fish among sites and taxa using both

one-way ANOVA and Kruskal-Wallis analyses with _anova(lm())_ and _kruskal.test()_ using the R statistical program54. We used the same test to compare surface water microplastic

concentrations among sites. Presentation of both non-parametric and parametric analyses were included to balance the interpretation of the lower power non-parametric test with the robust

parametric analyses since data sets were a mix of both normal and non-normal Gaussian distributions, which were similar to analyses conducted with our previous work55. To identify which

sites were significantly different from one another, pairwise post-tests were conducted between sites with both _pairwise.t.test()_ with a Bonferroni correction (parametric) and

_post.hoc.kruskal.nemenyi.test()_ (non-parametric) in ‘PMCMR’ R56. Microplastic concentration categorized by fish FFG (_i.e_., detritivore, omnivore, zoobenthivore) was non-normal and

analyzed with the Kruskal-Wallis test54 followed by _post.hoc.kruskal.nemenyi.test()_ to determine which FFG was significantly different from one another. We conducted linear regression

analyses between fish microplastic concentration and surface water microplastic concentration using _lm()_. We also conducted linear regression analyses between fish traits (_i.e_., body

size and trophic fraction) and microplastic concentrations in fish with linear regression models to determine if microplastic abundance in fish increased with increasing fish body size and

trophic fraction. To determine if larger fish ingested larger fibers, fiber length and fish body length were also analyzed with linear regression models. All fish trait regression models

were conducted with pooled and individual species data, with the exception of fish trophic fraction (_i.e_., each taxa was assigned one trophic fraction). Microplastic category, size class,

and color patterns were analyzed with Chi-square test of independence after data were converted to ratios for each sampling site with _chisq.test()_. This analysis also included data

collected from the Gulf of Maine using the same grab sampling and laboratory processing methods as used in our study42. These analyses allowed for a test of independence to determine if

microplastic category, size class, or color patterns were independent of sampling sites57. To identify if microplastic patterns in the environment were similar to patterns found in

laboratory controls, Chi-square test of independence analyses were conducted with microplastic category, size class, and color data (surface water and fish) and their respective controls

(_i.e_., filter and digestion). Microplastic concentration comparisons between environmental data and controls were analyzed with student’s t-test (_t.test()_) and Wilcoxon rank-sum test,

(_wilcox.test()_), after data were found to have mixed normality post Shapiro-Wilk normality tests54. All analyses were conducted using the ‘base’ package in R version 3.3.0 unless stated

otherwise. DATA AVAILABILITY Data will be made available when requested from corresponding author. ETHICAL STATEMENT All methods were carried out in accordance with ethical guidelines and

regulations and approved by Loyola University Chicago’s Institutional Animal Care and Use Committee. RESULTS MICROPLASTIC ABUNDANCE A total of 74 fish spanning eleven taxa were collected

throughout the study, with 10 taxa (85% of individuals) containing microplastic in their digestive tracts (Table 3). Microplastic abundance in fish was not significantly different among the

three study sites (_P_ > 0.05) and ranged from 10 (±2.3) to 13 (±1.6) microplastic particles fish−1 (Table 3; Fig. 2a). However, there was a significant difference in microplastic

concentration across fish species (Table 3; Fig. 3). Round goby (_Neogobius melanostomus_) microplastic concentration was significantly greater than fathead minnow (_Pimephales promelas_)

and white sucker (_catostomus commersonii_) (all _P_ < 0.05), with no microplastic in gizzard shad (_Dorosoma cepedianum_) (Fig. 3). Round goby was the only fish present across all three

sites, and gobies from SJ River had approx. 50% less microplastic concentration compared to those collected from the MG and MK Rivers (Suppl. Table 1). Microplastic concentrations in surface

waters were significantly different among the 3 sites (Fig. 2b, Table 3), and abundance in fish was unrelated to patterns in the water (_r_2 = −0.011, _P_ > 0.05). There was also a

significant effect of fish FFG on microplastic concentration in fish regardless of river site (Table 3), with zoobenthivores greater in microplastic concentration than detritivores (_P_ <

 0.001). Zoobenthivore fish had significantly greater microplastic compared to the omnivore FFG in the MK River (_X_2 = 10.92, df = 2, _P_ < 0.01), whereas there was no difference in

microplastic abundance among FFG present in other river sites (all _P_ > 0.05; Suppl. Fig. 1). Omnivore fish concentration was significantly influenced by site (F = 4.92, df = 2, _P_ = 

0.015), with omnivore fish from the MG River significantly greater in microplastic concentration compared to omnivore fish from the MK River (_P_ = 0.018) but was similar in concentration to

fish from the SJ River (_P_ > 0.05). Zoobenthivore fish in the MK River had the greatest microplastic concentration compared to zoobenthivore fish from MG and SJ Rivers; however, this

comparison was not statistically significant (F = 2.169, df = 2, _P_ > 0.05). Fiber polymer composition from fish digestive tissues consisted of polyethylene, polyacrylonitrile,

polyacetal, and polyvinyl acetate (Suppl. Table 2). There were some links between fish body size and trophic fraction and the abundance of microplastic in digestive tissue. For example, gut

microplastic in round goby increased with body length (_r_2 = 0.393; _P_ = 0.010; Suppl. Table 3; Fig. 4b), but fish body size was not related to microplastic abundance in other taxa or when

taxa were pooled (Suppl. Table 3; Fig. 4a). Microplastic abundance in fish was also positively related to an increase in fish trophic fraction, although the model explained a small portion

of the variation in the data (_r_2 = 0.050; _P_ = 0.032; Fig. 4c). Microplastic abundance in zoobenthivores was positively related to increasing fish body length (_r_2 = 0.188; _P_ = 0.014;

Suppl. Table 3), which was most likely driven by the round goby (classified as zoobenthivore; Table 2). There were no such patterns observed for the other FFG (Suppl. Table 3). Fiber length

within fish digestive tracts had no relationship with fish body length, indicating fibers of all size classes were found equally in small and large fish (Suppl. Table 4). CHARACTERIZATION OF

MICROPLASTIC Microplastic category, size class, and color patterns were significantly different in surface water river sites and the Gulf of Maine42 (Fig. 5, Suppl. Table 5, Suppl. Figs 2

and 3). Fibers comprised approximately 97–100% of all microplastic found in the water and fish (Suppl. Fig. 2). Fragments comprised 1.5–3% of microplastic collected from water in the MK and

SJ Rivers respectively. Foam was only found in the MK River and accounted for 0.4% of the water column microplastic at that site. Fragments were also rare in fish and accounted for

approximately 2.5–3% of the microplastic (Suppl. Fig. 2). The relative abundance of fiber colors significantly depended on river site (_X_2 = 66.06, df = 6, _P_ < 0.001), with clear

fibers dominant at the SJ River (approx. 80%) and blue fibers (approx. 44%) most common at the MG River (Suppl. Fig. 3a). In contrast, fiber color patterns were similar in fish across sites,

with clear and blue fibers predominant (Suppl. Fig. 3b). A total of 526 (out of 980) fibers were classified as small (<1.5 mm), medium (1.6–3.2 mm), or large (>3.3 mm). Small fibers

were the most common size found across water samples and fish (Fig. 5). However, medium sized fibers were common from water samples at the SJ River but not at the other sites (Fig. 5a).

Surface water microplastic category, color, and fiber size class patterns in the Gulf of Maine were similar to data from the MG, MK, and SJ Rivers (Fig. 5a; Suppl. Figs 2a and 3a).

CHARACTERIZING MICROPLASTIC IN LABORATORY CONTROLS Analyses of microplastic in control procedures could help reveal potential sources of microplastic contamination. Microplastic

concentrations were significantly lower in laboratory controls compared to environmental samples (Fig. 5a; tRiver = −3.544, df = 72.5, _P_ < 0.001; tFish = 9.563, df = 37.4, _P_ < 

0.001). Microplastic concentrations on surface water (approx. 16 ± 4 pieces filter−1) and fish filters (approx. 13 ± 0.7 pieces filter−1) were 8× and 3× greater than filter controls (2 ± 0.4

pieces filter−1) and digestion controls (4 ± 0.63 pieces filter−1; Fig. 6a) respectively. Microplastic composition in filter and digestion controls was different in size class and color

patterns compared to water column and fish results (Suppl. Table 6; Fig. 6b,c). Fibers were the dominant microplastic type found in controls, which was similar to surface water and fish

(Fig. 6b). Small fibers were the dominant size class found across environmental samples and controls. However, large fibers were more common in filter controls than surface water samples

(Fig. 6c). Clear was the dominant color in both surface water and filter control samples (Fig. 6d), but purple and red/clear bi-colored fibers were more common in filter controls compared to

surface water samples. Similarly, red and gray fibers were common in the water samples, but not in filter controls (Fig. 6d). Clear and blue fibers were the most common fibers in both fish

and digestion controls, but digestion controls were characterized by more blue/clear fibers compared to fibers from fish, which had more gray fibers (Fig. 6d). DISCUSSION Microplastic

pollution is pervasive worldwide, but research on its abundance, movement, and biological interactions in freshwater ecosystems is newly emerging. In this study, we present evidence that

microplastic is present in major tributaries of Lake Michigan, including fish digestive tissue and surface waters. Understanding the factors which drive microplastic patterns in freshwater

food webs will be critical for management policies. MICROPLASTIC IN FISH DIGESTIVE TRACTS WAS DIFFERENT AMONG SPECIES AND FEEDING GROUPS Plastic and other anthropogenic litter is commonly

found in marine fish58, but its abundance, introduction pathway, and physiological effects on freshwater fish are largely unknown. In this study, 85% of fish had microplastic in their

digestive tissues with an average of approximately 13 particles fish−1 with fibers the dominant microplastic. Pazos _et al_.59 found microplastic abundance was approximately 8–55 particles

fish−1, and fibers were also the dominant microplastic category across all fish taxa and from fish collected from the Rio de la Plata estuary, Argentina59, which is comparable to gut

microplastic in fish in our study. In contrast, Lusher _et al_.60 found 11% (84 of 761) of mesopelagic fish (_e.g_., glacial lantern fish (_Benthoseoma glacial_)), spotted barracudina

(_Arctozenus risso_), and lancet fish (_Notoscopelus kroyeri_) from the Northeast Atlantic Ocean had microplastic, with an average of 1.2 particles fish−1. Anthropogenic litter has been

found in 25–28% of fish collected from markets prepared for human consumption (seafood) in the USA and Indonesia ranging from 0.3–5.9 particles fish−1 58. These studies suggest microplastic

abundance in fish could vary across a gradient of aquatic habitats. Microplastic in riverine fish might be higher than marine fish due to lower water volume:surface water area ratio and

proximity to terrestrial microplastic point-sources; however, a systematic comparison has not yet been completed. Fish ecological and morphological traits were linked to gut microplastic

abundance from fish in Lake Michigan major tributaries. As we expected, microplastic abundance was positively related to fish trophic fraction. Zoobenthivores had greater microplastic

abundance compared to omnivores and detritivores, suggesting predator oriented fish may obtain microplastic via trophic transfer from prey items. Farrell _et al_.24 demonstrated the

potential for microplastic trophic transfer from mussels (_Mytilus edulis_) to crabs (_Carcinus maenas_). The authors reported microplastic concentrations were up to 163,111 particles mL−1

of crab haemolymph, which represented 0.24% of the microplastic retained by mussels. Two fish species of particular interest in our study were the round goby and the gizzard shad. The round

goby is invasive in the Great Lakes and is a voracious, opportunistic feeder61,62. This could explain why this species had the highest microplastic concentration in its digestive tissues

(approx. 20 particles fish−1). Microplastic abundance in round gobies increased with fish body length and suggested microplastic in these fish could accumulate with age. The gizzard shad was

the only taxa with no microplastic in its gastrointestinal tract, which could be attributed to its common diet of plants and detritus instead of aquatic fauna63. In contrast, Pazos _et

al_.59 found no relationship between microplastic abundance in fish and fish trophic group and body length in the Rio de la Plata estuary, Argentina. Ferreira _et al_.64 reported 64% of

_Cynoscion acoupa_ (Acoupa weakfish, Lacepéde) collected from the Goiana Estuary (South America) had microplastic in their stomachs, with adult fish containing more microplastic than

juvenile and sub-adult fish, suggesting fish ontogeny may play a role in microplastic abundance in fish. Collective findings from this study suggest fish species traits could help explain

microplastic abundance, but can be species dependent. Future research should include identifying functional traits linked with microplastic abundance in aquatic biota to help identify

wildlife taxa susceptible to microplastic. These results do not indicate the source(s) of microplastic in fish digestive tracts. Microplastic abundance in fish were similar across the 3

study sites, indicating microplastic concentration in the water column was not a reliable predictor of microplastic abundance in fish and may not be the primary source of ingested

microplastic for these taxa. In contrast, Pazos _et al_.59 found fish collected closer to WWTPs had significantly greater microplastic concentrations in their digestive tissues compared to

fish from further locations in the freshwater zone of the Rio de la Plata estuary, Argentina. Most studies have focused on quantifying microplastic abundance in fish and invertebrates65,66,

but little research has been conducted to identify the movement of microplastic between the environment and organisms24. Aquatic biota may ingest microplastic either directly from their

habitat (_i.e_., water column or benthos) or indirectly via trophic transfer. It is unknown the proportion of microplastic each pathway contributes to microplastic abundance in aquatic

biota, or what aspects of gut tissue anatomy may affect internal microplastic retention. Retention of microplastic in the gut could lead to irritation of the epithelial lining and blockage

in the digestive tract due to the shape and filamentous structure of microplastic25, which could impact aquatic biota ingestion and egestion rates. Future research is needed to explore the

pathways in which microplastic is incorporated into aquatic biota and food webs. SOURCES OF MICROPLASTIC TO RIVERS Watersheds with urban and agricultural land-use can have increased point-

and non-point sources of microplastic pollution. We noted higher microplastic concentration was in human-dominated watersheds relative to the forested watershed. Some microplastic sources in

urban and agricultural environments are well documented, while others will require additional research to measure. For example, McCormick _et al_. (2016) reported microplastic flux

downstream of WWTPs in the Chicago region, a densely populated area in the USA, was on average 1.3 million particles d−1 and was higher downstream of WWTP outfalls compared to upstream

locations. Litter can accumulate on freshwater beaches and in riparian zones of rivers27,67 and is similar in composition in benthic habitats28, suggesting urban terrestrial zones are

sources of AL, and possibly microplastic, to rivers. Plasticulture, the use of plastic film/mulch to cover and protect seedbeds, is a worldwide agricultural practice that could also

contribute microplastic to the landscape across large spatial scales. For example, plasticulture is present in 156,900 ha in the Shandong province, China68, creating the potential for

microplastic to be introduced throughout this region as large plastic degrades into smaller particles. Microplastic concentrations in biosolids (_i.e_., WWTP sludge converted to a

fertilizer) from seven WWTPs in Ireland ranged from 4,196–15,385 particles kg−1 dry weight36. Therefore, biosolid application on agricultural fields may be a non-point source of microplastic

pollution to nearby rivers and lakes; however, this has not yet been measured. COMPARING RESULTS TO LITERATURE AND CONSIDERATIONS FOR SCALING Comparing the values for water column

microplastic from this study to published results requires consideration of methodology and location. Previous work using neuston nets in freshwater ecosystems found lower values than those

documented with grab samples. In a study measuring microplastic abundance in 29 Laurention Great Lakes tributaries using neuston nets, microplastic concentrations ranged from 0.05–32

particles m−3 and was positively related to an increase in urban land-use40. The Seine River at Paris had microplastic concentrations of 3–106 particles m−3 11 with the same method. Finally,

McCormick _et al_. (2016) also used neuston nets to report microplastic concentrations downstream from WWTP effluent (0.80–11.22 particles m−3) were greater than concentrations upstream of

WWTPs (0.48–5.92 particles m−3). In contrast, a grab sample approach in the Gulf of Maine estimated microplastic concentrations at 3,400–10,000 particles m−3 42, approximately one order of

magnitude less than our results. Barrows _et al_.42 also reported similar composition of microplastic as our results, where particles were primarily small fibers (<1.5 mm) and most

commonly clear or blue42 (supporting _H_4). These collective results show microplastic pollution is abundant in freshwater habitats and that rivers are sources of microplastic pollution to

downstream ecosystems. Scaling up microplastic results to larger volumes of water and time periods will require careful attention to replication and consideration of distinct river habitats.

We did not extrapolate our water column samples to estimate flux (No. particles day−1) or export (No. particles km−2 d−1) due to the volume and lack of temporal replication in our surface

water collection. Estimates of flux and export are needed to inform global budgets of plastic movement in rivers. To do so, we suggest that more samples should be collected across the width

of a river, with depth-integrated collection, and at multiple times of year. In addition, we propose that investigators simultaneously measure microplastic at the benthic surface, in the

water column, and floating at the water surface. Initial assessments of these habitats in rivers suggest high variation among sampling locations and times, including deposition and

resuspension of microplastic, as is common for naturally occurring particles39. Rigorous sampling regimes will allow for initial budgets of microplastic to be constructed for rivers.

COMPOSITION OF MICROPLASTIC IN CONTROLS Analyses of microplastic in control procedures could help reveal potential sources of microplastic contamination in laboratory settings. Laboratory

microplastic contamination was minimal in this study (2–4 particles filter−1) and similar to other studies20,59 (_e.g_., McCormick _et al_.20, Pazos _et al_.59). As we predicted,

microplastic in controls were typically large fibers and unique in colors (_e.g_., purple) compared to environmental samples. Sources of microplastic contamination could have come from

laboratory technician clothing, atmospheric deposition, and the water supply. In this study, DI water with a 363 µm mesh covering the faucet was used for all solutions and rinsing of

glassware. De-ionized water was used due to greater microplastic contamination in MilliQ and tap water in the laboratory (_personal observation_, McNeish). Microplastic contamination has

been documented in a diversity of commercial salt brands ranging from 1–10 particles kg−1 of salt46, which is important since density separation of microplastic using salts is a common

protocol in the microplastic field49. Possible contamination may have also come from pre-ordered 30% hydrogen peroxide. Although we did not isolate the sources of microplastic contamination,

this study is the first to report microplastic contamination is unique in size and color compared to environmental samples. Polymer identification in small plastic fibers is a major

challenge for this field of research. It is possible some fibers in this study were mis-identified as plastic instead of other anthropogenic sources of fibers (_e.g_., cotton and viscose

fibers). Lenz _et al_.69 reported a 25% error in mis-identification of fibers as plastic when comparing visual versus FTIR identification of fibers as plastic. This suggests the possibility

that 25% of the fibers in this study could not be plastic. However, the fibers that were identified by FTIR in our study were all plastic. Moreover, the patterns observed across sites and

fish taxa would still be the same assuming this error was consistent throughout sample processing. As technology for polymer identification on small fibers develops, this outstanding

question of misidentification will inform research on microplastic pollution across ecosystem types. SUMMARY Microplastic is abundant in rivers and fish connected to Lake Michigan, which

serve as conduits of contamination via river currents and fish movement. Species functional traits may help predict microplastic abundance in fish and could be applied to other fauna.

Understanding traits that make fauna susceptible to microplastic pollution could enhance our understanding of how these organisms interact with microplastic and could enable us to target

species for conservation efforts. Microplastic concentrations in Lake Michigan tributaries were also higher than what has been reported for marine coastal habitats42 and the open ocean70;

although, greater harmony in methodological approaches would be needed for more robust comparisons of microplastic concentrations across large spatial scales. In particular, more research is

needed to pinpoint the landscape features which serve as point and non-point sources of microplastic pollution to freshwaters, and its accumulation in food webs. These collective findings

highlight the need for future research to focus on the movement of microplastic across the terrestrial-aquatic boundary and the importance of focusing pollution management efforts on inland

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particles in surface and sub-surface samples. _Sci. Rep_. 5 (2015). Download references ACKNOWLEDGEMENTS We thank officials from Benton Harbor and St. Joseph, MI, the Milwaukee Park

District, WI, and Muskegon State Park, MI for use of the field sites. Special appreciation to Marty Berg for use of laboratory equipment and to Brenainn Turner, Paul Risteca, Veronica

Lourich, Anthony Overhiser, Anna Vincent and the nine other graduate and undergraduate students at Loyola University Chicago and the University Notre Dame who contributed to field and

laboratory work. This work was supported by a grant from the Illinois-Indiana Sea Grant of the National Oceanic Atmospheric Administration (074483-15907) to JJK, TJH and SAM and by a grant

from the National Science Foundation (CAREER 1553835) to TJH. Any opinions, findings, and conclusions or recommendations expressed are those of the authors and do not necessarily reflect the

views of Sea Grant, the National Oceanic Atmospheric Administration, or the National Science Foundation. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Loyola University Chicago – Biology,

1032 West Sheridan Road, Chicago, IL, 60660, USA R. E. McNeish, L. H. Kim, J. J. Kelly & T. J. Hoellein * Department of Geology and Environmental Sciences, The State University of New

York at Fredonia, 280 Central Ave., Science Complex 340, Fredonia, NY, 14063, USA H. A. Barrett & S. A. Mason Authors * R. E. McNeish View author publications You can also search for

this author inPubMed Google Scholar * L. H. Kim View author publications You can also search for this author inPubMed Google Scholar * H. A. Barrett View author publications You can also

search for this author inPubMed Google Scholar * S. A. Mason View author publications You can also search for this author inPubMed Google Scholar * J. J. Kelly View author publications You

can also search for this author inPubMed Google Scholar * T. J. Hoellein View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS J.J.K., T.J.H.,

and S.A.M. designed the research; R.E.M. conducted the research and statistical analyses; L.H.K. conducted field work and training on sample processing; S.A.M. and H.A.B. conducted FTIR

analysis; All authors contributed to writing and editing of the manuscript. CORRESPONDING AUTHOR Correspondence to R. E. McNeish. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare

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and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE McNeish, R.E., Kim, L.H., Barrett, H.A. _et al._ Microplastic in riverine fish is connected to species traits. _Sci Rep_ 8, 11639 (2018).

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