High abundances of microplastics in a tropical urban stream, negatively related to precipitation.

Abstract Most studies on microplastic (MP) pollution in freshwaters consider large rivers and lakes, with streams relatively poorly studied. We analysed the temporal variation in abundance of MP particles in an urban stream draining commercial and residential areas, sampling on seven occasions during the course of nine months. We filtered samples through a nylon net of 68µm mesh size, carried out digestion with a novel method, namely WPO (H2O2 + Fe) followed by KOH, and analysed MP particles quantitatively and qualitatively by light microscopy. All samples contained MPs, with concentrations varying between 1.1 and 11.3 particles.L− 1. The most abundant forms were transparent filaments (found in 67% of the samples) and black fragments (found in 47% of the samples). MP concentrations were significantly positively related to the absence of rainfall. Mean transport was calculated to be 5.75x1011 items.year− 1. These abundances are equal or superior to those recorded in large rivers draining industrial areas, suggesting that small streams can be an important source of MP pollution.