Slow degradation of compostable plastic carrier bags in a stream and its riparian area

There is no place on Earth where plastic debris could not be found. Impacts of plastics on aesthetics, biota and ecosystems are dependent on how long plastic items last, and what degradation products are released, in recipient environments. As bio-based plastics tend to replace petroleum-based plastics in everyday life, it is important to upgrade knowledge on the degradation of new polymers in natural environments. Single-use plastic carrier bags are nowadays made of bio-plastics certified as biodegradable and compostable. It is unclear, however, whether claims of biodegradability and compostability can be taken as evidence of rapid degradation of plastic bags outside recycling/composting facilities. This study sought to provide quantified information about the degradation of compostable plastic carrier bags in streams and riparian zones. We found that plastic samples enclosed in different types of mesh bags lost weight at extremely slow rates, albeit significant when submerged in a stream. 95% of initial plastic mass remained after 77 days spent in water whereas alder leaf litter allowed to decompose under the same condition had completely disappeared before the end of the study. Determination of respiration rate and invertebrate abundance in plastic samples showed a greater decomposer activity in the stream than in the riparian environment. However, biotically-mediated degradation by decomposers was probably overridden by dissolution processes in mediating plastic mass loss. Our findings suggest that mismanaged plastic carrier bags could impact recipient ecosystems even when they are claimed as biodegradable or compostable.