Aging of microplastics in a subtropical river system in Florida, USA

Rivers are dynamic conduits for microplastics, yet the interplay between polymer chemistry, environmental exposure, and microbial colonization remains poorly understood. We conducted a 24 month in situ study in the Hillsborough River, Florida (USA) assessing the aging of five polymer types— low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene, polystyrene (PS), and polyethylene terephthalate (PET)—across five environmental layers: air, surface water, middle water, deep water, and sediment. Particles were analyzed using scanning electron microscopy, flow cytometry, Fourier transform infrared spectroscopy, and image-based shape metrics. Aging patterns were strongly layer- and polymer type-dependent: PET and PS exhibited the highest chemical weathering (carbonyl indices up to 12.7 and 4.9, respectively) and microbial colonization (average cell counts of 1.5 × 10 ^5 ± 1.2 × 10 ^5 and 1.3 × 10 ^5 ± 1.0 × 10 ^5 , respectively), while LDPE and HDPE supported substantial biofilm formation (average biomass of 4.6 ± 4.1 mg and 6.5 ± 7.6 mg, respectively) with limited photooxidation (average carbonyl indices of 0.7 ± 0.6 and 0.7 ± 0.7, respectively). Air-exposed particles underwent continuous photooxidation, surface and middle water layers promoted dense microbial biofilms (average biomass of 5.6 ± 5.2 mg and 4.1 ± 3.8 mg, respectively), and deep water and sediment showed slower colonization and lower chemical alteration (average carbonyl indices of 2.1 ± 3.0 and 2.1 ± 3.6, respectively). Mass and particle shape remained largely stable, highlighting microplastic persistence under natural riverine conditions. Computer simulations corroborated colonization trends, though sediment dynamics were underestimated, emphasizing the need for refined modeling. These results demonstrate that polymer type and environmental context govern early aging processes and suggest that targeted management strategies should account for both microplastic polymer type and riverine layer-specific transport.