Seasonal microplastic ingestion by carnivorous chaetognaths in Jiaozhou Bay, China: Field evidence revealing microplastic trophic transfer.

Microplastics (MPs) are widely distributed in marine environments and ingested by marine organisms, especially zooplankton. Chaetognaths, typical carnivorous zooplankton, are pivotal in the food chain from secondary producers, such as copepods, to higher trophic level species. However, little is known about their MP ingestion. In this study, based on field observation data, for the first time, we studied seasonal characteristics and risks of MPs ingested by chaetognaths in Jiaozhou Bay and assessed effects of key prey copepods on MP ingestion by chaetognaths. MP/chaetognath values in February, May, August, and November were 0.19, 0.17, 0.15, and 0.39, respectively, showing no significant seasonal variation. Chaetognaths predominantly ingested MPs that were fiberous in shape, 101-400 µm in size and polyester in polymer type, with no significant seasonal variations. The risk of MP load in chaetognaths was low, but there are higher polymeric hazards and potential ecological risks. MP/chaetognath values were positively correlated with the copepod abundance and MP/copepod values. The characteristics of MPs ingested by chaetognaths were also highly similar to those of MPs ingested by copepods. However, the overall risk of biomagnification in the copepod-chaetognath food chain was low. This study provided field evidence for MP transfer in the planktonic food chain.

Impact of various microplastics on the morphological characteristics and nutrition of the young generation of beech (Fagus sylvatica L.).

Microplastics have the capacity to accumulate in soil due to their high resistance to degradation, consequently altering soil properties and influencing plant growth. This study focused on assessing the impact of various types and doses of microplastics on beech seedling growth. In our experiment, we used polypropylene and styrene granules with diameter of 4.0 mm in quantities of 2.5% and 7%. The hypothesis was that microplastics significantly affect seedlings' nutritional status and growth characteristics. The research analysed seedlings' nutrition, root morphological features, above-ground growth, and enzymatic activity in the substrate. Results confirmed the importance of microplastics in shaping the nutritional status of young beech trees. Microplastic type significantly impacted N/P and Ca/Mg stoichiometry, while microplastic quantity influenced Ca/Al and Ca+K+Mg/Al stoichiometry. Notably, only in the case of root diameter were significantly thicker roots noted in the control variant, whereas microplastics played a role in shaping the leaves' characteristics of the species studied. The leaf area was significantly larger in the control variant compared to the variant with polypropylene in the amount of 2.5% and styrene in the amount of 7%. Additionally, the study indicates a significant impact of microplastics on enzyme activity. In the case of CB and SP, the activity was twice as high in the control variant compared to the variants with microplastics. In the case of BG, the activity in the control variant was higher in relation to the variants used in the experiment. Research on the impact of microplastics on the growth of beech seedlings is crucial for enhancing our understanding of the effects of environmental pollution on forest ecosystems. Such studies are integral in shaping forestry management practices and fostering a broader public understanding of the ecological implications of plastic pollution.

First Evidence of Microplastic Ingestion by Riverine Fish From the Freshwater of Northwest Peninsular Malaysia.

In a baseline study, we investigated microplastic contamination in fish from the Pinang and Kerian Rivers in Northwest Peninsular Malaysia. In recognition of the growing concern over microplastic pollution in aquatic environments, we aimed to assess the abundance and characteristics of microplastics ingested by various fish species. Fish samples were collected from local fishermen, followed by a digestion process using 10% potassium hydroxide (KOH). Microplastics were isolated and analyzed through visual examination and attenuated total reflectance Fourier transform infrared spectroscopy. The results revealed a high prevalence of microplastics, with Johnius borneensis and Oreochromis sp. exhibiting the highest abundance, averaging 48.6 and 42.8 microplastics/g, respectively. The predominant shapes were fibers (55.6%) and fragments (25.9%), with colors primarily transparent (48.19%) and black (30.12%). Our results indicate significant contamination levels in freshwater fish, emphasizing the need for further research and effective mitigation strategies. These findings provide crucial baseline data on microplastics in Malaysian freshwater ecosystems. Environ Toxicol Chem 2024;00:1-10. © 2024 SETAC.

Plastic ingestion by odontocetes from the Western South Atlantic: A particular concern to a threatened species.

The ingestion of debris by marine fauna is a growing threat to biodiversity. This study aimed to evaluate and characterize litter ingestion by odontocetes from the Western South Atlantic. Between 2018 and 2022, 154 stomachs from six species were collected from stranded individuals and incidental captures. Stomach contents were analyzed with the naked eye and items of anthropic origin found were counted and physically/chemically characterized. Generalized Linear Models were used to evaluate the influence of biological factors on the presence/absence of litter in stomachs, and for Pontoporia blainvillei only, the influence of these factors on the number of ingested items was also tested; additionally, a temporal analysis of ingestion was done for this species (1994-2022). A total of 156 items, mainly macro-sized plastics made of polypropylene, were found in 52 stomachs of four species: Tursiops spp. (FO% = 3.3%), Steno bredanensis (10.0%), Delphinus delphis (28.6%) and P. blainvillei (47.5%). The presence/absence of litter was explained only by species (χ2 = 28.29 and p < 0.001). For P. blainvillei, a threatened species in the region, the number of items was positively influenced by individual size (χ2 = 6.01 and p = 0.01) and sex (χ2 = 7.93 and p = 0.005). There was an increase in plastic ingestion by this species over the years (χ2 = 121.6 and p < 0.001) and it was estimated that 75% of P. blainvillei stomachs will contain plastic by 2040. The ingestion of litter by odontocetes from the Western South Atlantic was confirmed and the potential risks posed by this type of pollution were evidenced, especially since these species also face other anthropic pressures. These results further demonstrate the increasing threat of litter in the ocean and highlight the importance of circularity of plastics and proper waste management.

Anthropogenic debris pollution in yellow-legged gull (Larus michahellis atlantis) nests in biosphere reserves of the Canary Islands.

Anthropogenic debris, particularly plastic pollution, has emerged as a significant environmental threat to biodiversity. Given that seabirds interact with artificial debris through ingestion, entanglement, and nest incorporation, it is particularly important to quantify the quantity, origins, and chemical composition of these debris items. In this work, it was evaluated for the first time the occurrence of anthropogenic debris in nests of yellow-legged gull (Larus michahellis atlantis) in biosphere reserves of the Canary Islands (Spain). A total of 48 abandoned nests were collected from five remote and hardly accessible sampling areas, revealing that 81.3 % contained anthropogenic waste, with plastic accounting for 34.7 % of the debris, followed by metal (33.6 %) and paper (19.6 %). On average, 32.8 ± 40.9 items were found per nest. Regarding the origin, food packagings (47.8 %), personal hygiene products (21.7 %), and textiles (15.8 %) were identified as the predominant sources. Furthermore, the polymer composition of the plastics was characterised by means of Fourier-transform infrared spectroscopy analysis, being polyester the most abundant (38.2 %), followed by polyethylene (25.6 %) and rayon (10.3 %). The incorporation of anthropogenic debris into nest construction may result from outdoor human activities carried out far from nesting areas.

An equilibrium criterion for plastic debris fate in wave-driven transport.

The nearshore zone turns out to be the area with the higher concentration of plastic debris and, for this reason, it is important to know the processes that affect the transport and the fate of this type of litter. This study focuses on investigating the dynamics of various plastic types under several hydrodynamic conditions primarily induced by waves. 2D tests were carried out at the Hydraulic Laboratory of the University of Messina reproducing the main phenomena that occurred during the wave propagation on a planar beach. More than 200 different conditions were tested changing the wave characteristics, the water depth, the plastic debris characteristics (density and shape), and the roughness of the fixed bottom. In general, it can be observed that the reduction in particle displacement occurs due to: i) a decrease in wave steepness; ii) an increase in depth; iii) an increase in particle size; iv) an increase in plastic density. However, the experimental investigation shows that some plastic characteristics and bed roughness, even when hydraulically smooth, can alter these results. The experimental data analysis identified a criterion for predicting the short-term fate of plastic debris under wave action. This criterion to determine equilibrium conditions, based on an empirical relationship, takes into account the wave characteristics, the bed roughness and slope, and the weight of the debris.

Macro-and micro-plastics detected in razorbill Alca torda in the western Mediterranean Sea.

The Mediterranean Sea is a hotspot for plastic pollution and, consequently, an area at high risk of plastic exposure for oceanic seabirds. In this work we retrieved plastic items from different tissues/organs of razorbills Alca torda. This is a piscivorous species of the Alcidae family usually thriving along the North Atlantic coasts. In the winter 2022-2023, some individuals were observed in the Mediterranean area up to the Italian tyrrhenian coasts. Among the five carcasses examined, three contained plastic debris, mainly in the digestive system, in the form of fragments and fibers. The latter were also found in the pectoral muscles and, in one individual, in the liver. Polyethylene was the most represented polymer (55.2 %), followed by polypropylene (24.1 %) and cellulose (10.4 %). Previous reports hypothesized that North Atlantic razorbills are less exposed to plastic pollution because of their feeding strategy. Our results contradict this hypothesis, highlighting their susceptibility to plastic contamination.

Persistent plastic: Insights from seawater weathering and simulated whale gut.

Single-use plastics make up 60-95 % of marine plastic pollution, including common commodity films used for packaging and bags. Plastic film breaks down as a function of environmental variables like wave action, wind, temperature, and UV radiation. Here, we focus on how films degrade in cold waters across depths, time, and simulated mammal digestion. Five types of single-use film plastics (HDPE thin & thick, LDPE, PP, PE) were weathered for eight months in temperate waters at surface and depth in the Salish Sea, WA, USA, and subsequently exposed to a laboratory-simulated gray whale stomach. None of the types of plastics examined here fully degraded during the course of this 8 months study. Weathering time and depth significantly impacted many of the physical attributes of plastics, while exposure to a simulated whale gut did not. If unable to degrade plastics through digestion, whales risk long-term exposure to physical and chemical attributes of plastics.

Abundance, characteristics and ecological risk assessment of microplastics in ship ballast water in ports around Liaodong Peninsula, China.

The development of the shipping industry has led to a large volume of ballast water discharge annually. This accelerates pollutants' transfer and dispersion, such as microplastics. Currently, empirical data on microplastics in ballast water are rarely available. This study innovatively investigated the abundance, morphological characteristics (particle size, shape, and color), and polymer composition of microplastics in ballast water from ports surrounding the Liaodong Peninsula. The results revealed that the average abundance of microplastics in 13 ships' ballast water was 6071.30 ± 1313.85 items/m3. Notably, the small microplastics (0.06-2.50 mm) were most abundant, accounting for 94.52 % of the total microplastics. Transparent, fiber, and polyethylene glycol terephthalate were the most prevalent color, shape, and polymer composition of microplastics detected in the ballast water. The risk assessment indicated that these microplastics present ecological risks to organisms. These findings suggest that ship ballast water is the potential "hotspot" for marine microplastics transport.

Small microplastics have much higher mass concentrations than large microplastics at the surface of nine major European rivers.

Understanding the fates and impacts of microplastics requires information on their sizes, polymer types, concentrations, and spatial and temporal distributions. Here, we focused on large (LMPs, 500 µm to 5 mm) and small (SMPs, 25 to 500 µm) microplastics sampled with the exact same protocol in nine of the major European rivers during the seven months of the Tara Microplastic Expedition. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and pyrolysis coupled with gas chromatography and mass spectrometry (Py-GC-MS) analyses were used to determine the microplastics contents by number and mass. The median LMP concentration was 6.7 particles m-3, which was lower than those in other regions of the world (America and Asia). The SMP mass concentration was much higher to the LMP concentrations, with SMP/LMP ratios up to 1000 in some rivers. We did not observe a systematic positive effect of urban areas for the two size classes or polymers; this could be explained by the fact that the transport of microplastic is highly heterogeneous in rivers. We believe that this study has important implications for predictive models of plastics distribution and fate in aquatic environments.

Untargeted metabolomic insights into plastisphere communities in European rivers.

Every year, rivers introduce a staggering amount of hundred kilotons of plastic into the Oceans. This plastic is inhabited by microorganisms known as the plastisphere, which can be transferred between different ecosystems through the transport of microplastics. Here, we simulated the microbial colonization of polyethylene-based plastic pellets that are classically used to manufacture large-scale plastic products. The pellets were immersed for 1 month in four to five sampling stations along the river-to-sea continuum of nine of the major European rivers. This study presents the first untargeted metabolomics analysis of the plastisphere, by using ultra high-performance liquid chromatography coupled to high-resolution mass spectrometry (UHPLC-HRMS). The plastisphere metabolomes were similar in the Rhine and Rhone rivers, while being different from the Tiber and Loire rivers, which showed greater similarity to the Thames and Seine rivers. Interestingly, we found a clear distinction between plastisphere metabolomes from freshwater and marine water in most of the river-to-sea continuum, thus suggesting a complete segregation in plastisphere metabolites that is not consistent with a major transfer of microorganisms between the two contrasted ecosystems. Putative annotations of 189 discriminating metabolites suggested that lipid metabolism was significantly modulated. These results enlightened the relevance of using environmental metabolomic as complementary analysis to the current OMICs analysis.

Survival and transfer potential of Salmonella enterica serovar Typhimurium colonising polyethylene microplastics in contaminated agricultural soils.

Agricultural environments are becoming increasingly contaminated with plastic pollution. Plastics in the environment can also provide a unique habitat for microbial biofilm, termed the 'plastisphere', which can also support the persistence of human pathogens such as Salmonella. Human enteric Salmonella enterica serovar Typhimurium can enter agricultural environments via flooding or from irrigation with contaminated water. Using soil mesocosms we quantified the ability of S. Typhimurium to persist on microplastic beads in two agriculturally relevant soils, under ambient and repeat flood scenarios. S. Typhimurium persisted in the plastisphere for 35 days in both podzol and loamy soils; while during multiple flood events was able to survive in the plastisphere for up to 21 days. S. Typhimurium could dissociate from the plastisphere during flooding events and migrate through soil in leachate, and importantly could colonise new plastic particles in the soil, suggesting that plastic pollution in agricultural soils can aid S. Typhimurium persistence and facilitate further dissemination within the environment. The potential for increased survival of enteric human pathogens in agricultural and food production environments due to plastic contamination poses a significant public health risk, particularly in potato or root vegetable systems where there is the potential for direct contact with crops.

First-of-Its-Kind: Nationwide meta-analysis of microplastic pollution and risk assessment in Thailand.

Thailand ranks as the sixth largest contributor to global microplastic pollution, which is exacerbated by extensive plastic use. Despite rising concerns, no comprehensive review is available on microplastic contamination and its potential risk in Thailand. This review synthesised data on microplastic abundance and characteristics within the country from 118 peer-reviewed publications (2017-2024). We found predominant microplastic presence in crustaceans (1.69-160.15 items/g), followed by Mollusca (0.03-9.5 items/g) and fishes (0.01-28.17 items/g), with higher abundances in wastewater (4 × 102 to 6.09 × 105 items/m3) compared to that in freshwater (1.44-2.92 × 106 items/m3) and seawater (2.70 × 10-1 to 6.25 × 104 items/m3). Marine sediments (48.3-2.13 × 104 items/kg) also showed significantly higher microplastic concentrations than terrestrial sediments (3-2.92 × 103 items/kg). Predominant microplastics were identified as fibers (59.36% and 35.05% for biological and environmental samples, respectively) and fragments (24.14%, 30.68%) in blue (25.95%, 18.64%), and colourless/transparent (20.01%, 14.47%), primarily composed of polyethylene terephthalate (19.46%, 9.19%), nylon (3.23%, 9.99%), polypropylene (19.78%, 24.23%), and polyethylene (14.81%, 11.66%). The potential ecological risk was low in all ecosystems except for wastewater. Shrimp and fish were more susceptible to microplastics compared to other studies in the region. Additionally, the sources, transport, and pathways of microplastic pollution in Thailand's aquatic territories and the current measures and policies implemented by the government to address plastic pollution are discussed. This review has compiled up-to-date insights into the prevalence, distribution, and risks associated with microplastics, which is instrumental in formulating effective strategies for contaminant control and ultimately reducing plastic pollution.

Floating microplastics in Svalbard fjords: High spatial variability requires methodological consistency in estuarine systems.

Microplastic pollution was studied in surface waters of Isfjorden, Svalbard in July 2021 as a part of an international regional harmonisation exercise. Surface microplastics (0.5-5 mm) were sampled with a neuston net in triplicate per study site in several branches of Isfjorden, covering populated and unpopulated fjords. High spatial variability of microplastic abundance (0-32,700 items/km2) was observed within a single fjord resulting from the hydrodynamic pattern formed through the interaction of surface currents, freshwater runoff, and wind conditions. Maximum microplastic abundance was not correlated with the distance from the local source and was instead defined by local small-scale hydrodynamics. Future recommendations for correct assessment of surface microplastics concentration in estuarine environments are presented.

Plastic nests: The first record of the Yellow-rumped Cacique (Cacicus cela) and the Great Kiskadee (Pitangus sulphuratus) building nests with discarded fishing material on the Amazon Coast.

The Amazonian coastal ecosystems face a significant threat from plastic contamination, posing a major risk to their biodiversity. Here, we provide the first evidence of using plastics in nest building by Yellow-cacique Cacique (Cacicus cela) and Greater Kiskadee (Pitangus sulphuratus). During 2023 and 2024, 15 nests built with ropes and fishing lines were recorded in the Brazilian Amazon Coast, a colony of 12 nests in the State of Amapá and one nest in Pará for C. cela, and one nest for P. sulphuratus was observed in Maranhão during the fieldwork, covering all Brazilian Amazon Coast. These nests were observed on the edge of mangroves, built with solid waste from fishing activities, pointing out how environmental pollution can be incorporated into wildlife this issue. Our records emphasize the effect of plastic pollution on two passerine species associated with mangrove forests and the need for conservation measures and further research.

Rose or Red, but Still under Threat: Comparing Microplastics Ingestion between Two Sympatric Marine Crustacean Species (Aristaeomorpha foliacea and Parapenaeus longirostris).

Increasing plastic contamination poses a serious threat to marine organisms. Microplastics (MPs) ingestion can represent a risk for the organism itself and for the ultimate consumer. Through the analysis of the gastrointestinal tract, coupled with stable isotope analysis on the muscle tissue, this study provides insights into the relationship between MPs pollution and ecology in two commercial marine species caught in the Central Tyrrhenian Sea: Aristaeomorpha foliacea and Parapenaeus longirostris. Stable isotope analysis was conducted to determine the trophic position and the trophic niche width. The gastrointestinal tracts were processed, and the resultant MPs were analysed under FT-IR spectroscopy to estimate the occurrence, abundance, and typology of the ingested MPs. The trophic level of the species was similar (P. longirostris TP = 3 ± 0.10 and A. foliacea TP = 3.1 ± 0.08), with an important trophic niche overlap, where 38% and 52% of P. longirostris and A. foliacea has ingested MPs, respectively. Though species-level differences may not be evident regarding MP's abundance per individual, a high degree of dissimilarity was noted in the typologies of ingested particles. This research provides valuable insights into how MPs enter marine trophic webs, stressing that isotopic niche analysis should be combined with other methods to explain in detail the differences in MPs ingestion.

Microplastics predominantly affect gut microbiota by altering community structure rather than richness and diversity: A meta-analysis of aquatic animals.

The impacts of microplastics on the gut microbiota, a crucial component of the health of aquatic animals, remain inadequately understood. This phylogenetically controlled meta-analysis aims to identify general patterns of microplastic effects on the alpha diversity (richness and Shannon index), beta diversity, and community structure of gut microbiota in aquatic animals. Data from 63 peer-reviewed articles on the Web of Science were synthesized, encompassing 424 observations across 31 aquatic species. The analysis showed that microplastics significantly altered the community structure of gut microbiota, with between-group distances being 87.75% higher than within-group distances. This effect was significant even at environmentally relevant concentrations (≤1 mg L-1). However, their effects on richness, Shannon index, and beta diversity (community variation) were found to be insignificant. The study also indicated that the effects of microplastics were primarily dependent on their concentration and size, while the phylogeny of tested species explained limited heterogeneity. Furthermore, variations in gut microbiota alpha diversity, beta diversity, and community structure were correlated with changes in antioxidant enzyme activities from the liver and hepatopancreas. This implies that gut microbiota attributes of aquatic animals may provide insights into host antioxidant levels. In summary, this study illuminates the impacts of microplastics on the gut microbiota of aquatic animals and examines the implications of these effects for host health. It emphasizes that microplastics mainly alter the community structure of gut microbiota rather than significantly affecting richness and diversity.

Using feces to indicate plastic pollution in terrestrial vertebrate species in western Thailand.

Plastic pollution is a widespread and growing concern due to its transformation into microplastics (MPs), which can harm organisms and ecosystems. This study, aimed to identify plastic pollution in the feces of terrestrial vertebrates using convenience sampling both inside and outside protected areas in Western Thailand. We hypothesized that MPs are likely to be detectable in the feces of all vertebrate species, primarily in the form of small black fragments. We predicted varying quantities of MPs in the feces of the same species across different protected areas. Furthermore, we expected that factors indicating human presence, landscape characteristics, scat weight, and the MP abundance in water, soils, and sediments would influence the presence of plastics in feces. Among 12 terrestrial species studied, potential MPs were found in 41.11% of 90 samples, totaling 83 pieces across eight species including the Asian elephant (Elephas maximus), Eld's deer (Rucervus eldii), Dhole (Cuon alpinus), Gaur (Bos gaurus), Sambar deer (Rusa unicolor), Wild boar (Sus scrofa), Northern red muntjac (Muntiacus vaginalis), and Butterfly lizard (Leiolepis belliana). Specifically, 3.61% of all potential MPs (three pieces) were macroplastics, and the remaining 96.39% were considered potential MPs with the abundance of 0.92 ± 1.89 items.scat-1 or 8.69 ± 32.56 items.100 g-1 dw. There was an association between the numbers of feces with and without potential plastics and species (χ2 = 20.88, p = 0.012). Most potential plastics were fibers (95.18%), predominantly black (56.63%) or blue (26.51%), with 74.70% smaller than two millimeters. Although there were no significant associations between species and plastic morphologies, colors, and sizes, the abundance classified by these characteristics varied significantly. FTIR identified 52.38% as natural fibers, 38.10% as synthetic fibers (rayon, polyurethane (PUR), polyethylene terephthalate (PET), polypropylene (PP), and PUR blended with cotton), and 9.52% as fragments of PET and Polyvinyl Chloride (PVC). Human-related factors were linked to the occurrence of potential plastics found in the feces of land-dwelling wildlife. This study enhances the understanding of plastic pollution in tropical protected areas, revealing the widespread of MPs even in small numbers from the areas distant from human settlements. Monitoring plastics in feces offers a non-invasive method for assessing plastic pollution in threatened species, as it allows for easy collection and taxonomic identification without harming live animals. However, stringent measures to assure the quality are necessitated to prevent exogenous MP contamination. These findings underscore the importance of raising awareness about plastic pollution in terrestrial ecosystems, especially regarding plastic products from clothing and plastic materials used in agriculture and irrigation systems.

The plastisphere can protect Salmonella Typhimurium from UV stress under simulated environmental conditions.

Plastic waste is found with increasing frequency in the environment, in low- and middle-income countries. Plastic pollution has increased concurrently with both economic development and rapid urbanisation, amplifying the effects of inadequate waste management. Distinct microbial communities can quickly colonise plastic surfaces in what is collectively known as the 'plastisphere'. The plastisphere can act as a reservoir for human pathogenic bacteria, including Salmonella enterica sp. (such as S. Typhimurium), which can persist for long periods, retain pathogenicity, and pose an increased public health risk. Through employing a novel mesocosm setup, we have shown here that the plastisphere provides enhanced protection against environmental pressures such as ultraviolet (UV) radiation and allows S. Typhimurium to persist at concentrations (>1 × 103 CFU/ml) capable of causing human infection, for up to 28 days. Additionally, using a Galleria Mellonella model of infection, S. Typhimurium exhibits greater pathogenicity following recovery from the UV-exposed plastisphere, suggesting that the plastisphere may select for more virulent variants. This study demonstrates the protection afforded by the plastisphere and provides further evidence of environmental plastic waste acting as a reservoir for dangerous clinical pathogens. Quantifying the role of plastic pollution in facilitating the survival, persistence, and dissemination of human pathogens is critical for a more holistic understanding of the potential public health risks associated with plastic waste.

Spatial distribution and risk assessment of microplastics in surface waters of the St. Lawrence Estuary.

Development of effective prevention and mitigation strategies for marine plastic pollution requires a better understanding of the pathways and transport mechanisms of plastic waste. Yet the role of estuaries as a key interface between riverine inputs of plastic pollution and delivery to receiving marine environments remains poorly understood. This study quantified the concentration and distribution of microplastics (MPs) (50-3200 µm) in surface waters of the St. Lawrence Estuary (SLE) in eastern Canada. Microplastics were identified and enumerated based on particle morphology, colour, and size class. Fourier Transform Infrared (FTIR) spectroscopy was used on a subset of particles to identify polymers. Generalized linear models (Gamma distribution with log-link) examined the relationship between MP concentrations and oceanographic variables and anthropogenic sources. Finally, a risk assessment model, using MP concentrations and chemical hazards based on polymer types, estimated the MP pollution risk to ecosystem health. Mean surface MP concentration in the SLE was 120 ± 42 SD particles m-3; MP concentrations were highest in the fluvial section and lowest in the Northwest Gulf of St. Lawrence. However, MP concentrations exhibited high heterogeneity along the length and width of the SLE. Microplastics were elevated at stations located closer to wastewater treatment plant outflows and downstream sites with more agricultural land. Black, blue, and transparent fibers and fragments ≤250 µm were most commonly encountered. Predominant polymer types included polyethylene terephthalate, regenerated cellulose, polyethylene, and alkyds. While the overall risk to ecosystem health in the entire estuary was considered low, several stations, particularly near urban centres were at high or very high risk. This study provides new insights into the quantification and distribution of MPs and first estimates of the risk of MP pollution to ecosystem health in one of the world's largest estuaries.