Export of Plastic Debris by Rivers into the Sea

A substantial
fraction of marine plastic debris originates from land-based sources
and rivers potentially act as a major transport pathway for all sizes
of plastic debris. We analyzed a global compilation of data on plastic
debris in the water column across a wide range of river sizes. Plastic
debris loads, both microplastic (particles <5 mm) and macroplastic
(particles >5 mm) are positively related to the mismanaged plastic
waste (MMPW) generated in the river catchments. This relationship
is nonlinear where large rivers with  population-rich catchments
delivering a disproportionately higher fraction of MMPW into the sea.
The 10 top-ranked rivers transport 88–95% of the global load
into the sea. Using MMPW as a predictor we calculate the global plastic
debris inputs form rivers into the sea to range between 0.41 and 4
× 106 t/y. Due to the limited amount of data high
uncertainties were expected and ultimately confirmed. The empirical
analysis to quantify plastic loads in rivers can be extended easily
by additional potential predictors other than MMPW, for example, hydrological
conditions.