Microplastics (<5 mm) can have negative and deleterious environmental effects. However, the scarce information on the levels of microplastics and the factors underlying their input in wetlands, limits the understating of the vulnerability of these ecosystems to microplastics. To contribute closing this important information gap, the concentration (items/m−2), size and the variables related to the distribution of microplastics in fluvial sediments of the Cuiabá River, a tributary of the Pantanal, the world largest flooded Savanna and a UNESCO World Heritage were investigated. The mean microplastic level was 576.8 ± 577.8 items/m−2 of sediments which were mainly fragments (309.0 ± 319.6 items/m−2) and fibers (267.8 ± 339.9 items/m−2). The maximum level was 1938.8 items/m−2 found in the urban stretch of the sister cities of Cuiabá and Várzea Grande, the largest urban agglomeration in the study area. Unexpectedly, the composition of the microplastic with highest average level (240.7 ± 310.4 items/m−2), a blue synthetic fiber, could not be unequivocally determined by RAMAN spectrometry. Its complex composition includes Pb, Li and Nb, suggesting it may derive from electronic waste. Polypropylene (159.2 ± 133.4 items/m−2) and Polyvinyl Chloride (104.6 ± 217.2 items/m−2) were the second and third polymers with higher average levels. The average size of microplastics particles in sampling sites ranged from 0.18 mm to 1.74 mm. Microplastic concentration was positively related to urban areas. Similarly, microplastics particles size decreased with distance from urban areas. These results suggest that urban areas along the Cuiabá River, with poor waste management, are a major source of microplastic pollution in the Pantanal floodplain and that electronic waste might contribute significantly to this contamination. This study highlights the importance of adequate waste management in cities to limit microplastic contamination in areas of ecological importance, such as the Pantanal.

Microplastics are an emerging global threat to freshwater ecosystems. There is a growing number of microplastic studies in wetlands and floodplains such as the Pantanal. Microplastics in the Pantanal’s water courses, lakes, and floodplains were sampled with a plankton net in 16 locations in the Pantanal lowlands at varying distances from their potential urban sources over 2 years. Microplastics from the water samples were separated from sediments and organic matter and examined under a stereomicroscope to be identified and counted. Visual determination was then confirmed by RAMAN spectrometry. We found 730 microplastics. The average ± std microplastic concentration was 38 ± 16 x/m3, ranging between 10 and 100 x/m3. Fibres, fragments, and films represented 68%, 28%, and 4%, respectively, of the total microplastics. Surprisingly, a synthetic blue fibre of non-identified composition accounted for 51.0% of the microplastics. Other identified polymers included PP (18.2%), PET (16.9%), PVC (5.1%), PVA (4.4%) and LDPE (4.4%). The blue fibres, PP, and PET were found at all sampling locations. Microplastic concentrations varied from 27 ± 12 x/m3 to 48 ± 21 x/m3 across the sampling sites. Microplastics’ average size was 0.51 ± 0.21 mm, ranging from 0.06 to 0.97 mm. In the study area, the average seasonal concentrations varied from 34 ± 11 to 43 ± 18 x/m3 during the 2 years study period (2018–2019). In lentic environments, the median concentrations (30 x/m3) were significantly lower than in the lotic ones (40 x/m3). Urban human activities are a potential source of microplastics as the concentration of these increased significantly with the population of the nearest human agglomeration. These results indicate widespread contamination of the Pantanal waters by microplastics. Furthermore, a microsynthetic of indeterminate composition is found everywhere in the Pantanal, and more research is urgently needed to determine its composition and sources. The contamination by micropolymers and other microsynthetics will likely impact the Pantanal biota and, eventually, humans living in these floodplains.