Abundances of microplastics (MPs) are often used as pollution indicators. However, ignoring sedimentary processes can lead to MPs abundance estimation biases. This study evaluates the relationship between sediment granulometry and MPs accumulation, and validates this relationship in two river systems of the Ecuadorian Amazon. Six zones along the upper Tena River and twelve in the Pano River were identified using the Van Zuidam slope classification system selecting sites below 8 degrees. Thirty sediment samples were collected from the Tena River and sixty for the Pano River, sampling the sediment material deposited after a single strong rainfall event. MPs were quantified using 2.5 g of sediment per sample and the granulometry was determined sieving 100 g separating the material into seven fractions (>2 mm, 1 mm, 500 μm, 250 μm, 125 μm, >63 μm, and <63 μm). A total of 564 and 567 MPs were recorded for the Tena and Pano Rivers respectively. A significant positive correlation was found between MPs abundance and mud content (r = 0.46, p = 0.009) of the Tena River. These results were validated using data from the Pano River that showed a significant positive correlation between MPs and mud content (r = 0.412, p = 0.00244). The consistency of results across two independent rivers suggests that sediment granulometry is a predictive factor that must be reported when estimating MPs accumulation. Neglecting granulometric analysis can result in abundance differences of up to an order of magnitude. This will happen even when MPs analysis is conducted using the same sediment but taken in areas of the river that differ in flow.
Keywords
Abundance, Ecuadorian Amazon, Heavy rainfall events, Sediment content, River systems.