Abstract
Microplastic (MP) pollution represents a significant global environmental threat due to its widespread presence in aquatic ecosystems. However, research on MP contamination in Mexican rivers is currently limited. The objective of this study was to assess the spatial and temporal distribution of MPs in surface water at 18 sites along the Guanajuato River basin, including both influent and effluent from a wastewater treatment plant (WWTP) located in the middle basin. The presence of MPs in the feces of the American white pelican (Pelecanus erythrorhynchos) and the neotropic cormorant (Nannopterum brasilianum), two species residing in the lower basin, was also investigated. MP abundance in surface water varied significantly among zones and between seasons. Abundance peaked in the urban middle basin, where it was higher during the rainy season (809.3 ± 751.2 particles L−1) than during the dry season (449.4 ± 361.5 particles L−1), whereas the upper basin showed the opposite pattern. The WWTP removed 97.0–98.8% of the MPs entering the plant. Fragments and fibers comprised 51.6% and 48.3% of the MPs in surface water, respectively. The most common polymer types in surface water were polypropylene–polyethylene terephthalate (PP–PET, 23.5%) and polycarbonate (PC, 17.3%). The frequency of occurrence of MPs in fecal samples was 97.7% for pelicans and 94.9% for cormorants. Fibers dominated the fecal samples, and the polymer types identified in feces were also detected in surface water. These results highlight the extent of MP contamination in the urban sections of the Guanajuato River and underscore potential risks to local aquatic bird species.