Revealing microplastic dynamics: the impact of precipitation and depth in urban river ecosystems

Research on microplastics in Latin America is limited compared to a global perspective. Brazil plays a significant role in this context, as it possesses 12% of the world's freshwater reserves, constituting 53% of South America's water resources. There has been growing concern regarding the plastic pollution of the country's freshwater systems in recent years. Therefore, this study investigated the impact of plastic pollution on the Guaíba River, a significant watercourse in the southern region of Brazil that is subjected to high anthropogenic pressure. Additionally, we examined correlations between the presence of microplastics and key factors influencing their distribution in the river. Thus, freshwater was collected in seven sampling campaigns from 2019 to 2020, totaling 66 samples. The microplastics were quantified and characterized according to their color, shape, and polymeric composition. The concentration of microplastics varied between 2.9 and 53.8 items m^-3, and the distribution and transport were positively influenced by the population density, precipitations, and depth of each sampling point. White-transparent color category (51%) and fragment shape (89%) were predominant among the found particles. Polyethylene (PE) and polypropylene (PP) represented 37% and 57% of the analyzed particles. The non-metric multidimensional scaling (nMDS) analysis indicated that similar contamination sources, such as domestic sewage, could influence three out of ten sampled points. Several microplastics presented the formation of cracks, with sizes smaller than 10 μm on their surface, which might indicate an erosion process, resulting in the formation of nanoplastics. The color fading observed in microplastics suggests that the particles were subjected to environmental stressors, leading to the leaching or degradation of the dye on the plastic. The results confirmed the ubiquity of microplastics in Guaíba River, highlighting the importance of improving the regulations on plastic waste disposal in the country to prevent the contamination of freshwater bodies.