The following explanatory variables were selected for analysis: surface microplastic concentration (g km 2 ), distance from river (m), rainfall -up to ten days before, including sample day(mm), chlorophyll a -average of neuston tows (µg L -1 ), total population agglomeration (n), total metropolitan population (n), fishery catch (tonnes), aquaculture accumulative (tonnes) and end member contribution (%). The end member contribution represents the percentage of river water at the sampling location. It was calculated using the salinity measured at the sampling location and assuming a salinity of 0 for the river water.