Quantification, characterization, and source identification of macro- and mesoplastics in the water column of Rivers Sabaki and Tana.

Five sampling campaigns were conducted in the water columns of River Sabaki and Tana in Kenya, Between October 2021 and January 2023, covering a 1-year cycle, at four sites in River Sabaki (2.5 km, 3.05 km, 3.51 km, and 4.52 km) and River Tana (1.5 km, 1.8 km, 2.0 km, and 2.5 km) distant from the river mouth. The ebb and flood tides were sampled to calculate net plastic litter fluxes. Two 6350-µm seine nets were deployed in two replicates per sampling point. Factor and cluster analysis were used to investigate plastic litter sources for both rivers. The influences of rainfall on plastic abundance and mass were explored using permutational linear models. A total of 15,318 plastic litter items weighing 1.37 kg were recorded in River Sabaki, and 3741 plastic litter items weighing 0.95 kg in River Tana. The top ten captured plastic litter types sorted by abundance and mass were mostly plastic fragments. The annual net plastic litter flux to the ocean through River Sabaki amounted to 1,277,120.63 items year by abundance and 22.30 kg year by mass. For River Tana, the same fluxes were 207,550.76 items year, and 28.09 kg year, respectively. In River Sabaki, significant impacts of rainfall on plastic abundance and mass were found. River Sabaki's pollution sources included upstream reaches, fishing activities, and littering by locals and tourists. River Tana's major pollution sources were illegal dumpsites, littering, fishing, and recreational activities. This research can guide combat plastic pollution in the rivers and ultimately the ocean.