Long-Term Trends in Microplastic Accumulation in Urban Riverine Ecosystems: A 10-Year Study

Microplastic contamination of freshwater ecosystems represents an emerging environmental threat with poorly understood long-term trajectories. We report a 10-year longitudinal study (2015-2025) monitoring microplastic accumulation across 12 sampling sites in three Brazilian urban rivers (São Paulo metropolitan region). Microplastic concentrations increased by 340% over the study period, with polyethylene (42%) and polypropylene (26%) comprising the dominant polymer types. Particle size distribution shifted toward smaller fragments over time, consistent with ongoing fragmentation. Seasonal analysis reveals peak accumulation following monsoon events (r=0.84 with precipitation intensity), implicating stormwater runoff as the primary transport vector. Biodiversity indices correlate negatively with microplastic concentration (r=-0.71, p<0.01).