Plastic litter is rapidly bioeroded in mangrove forests

Due to their architectural and hydrodynamic properties, mangrove forests are emerging as global hotspots for plastic sequestration. Mangroves encroached by coastal cities contain up to two orders of magnitude more plastic than their non-urban counterparts. In urban mangroves, plastic substrata are often used as microhabitats, but the consequences of this interaction for the degradation process of plastics in the environment are unknown. Hence, we hypothesized that plastics are differentially colonized and transformed by distinct macrobenthic assemblages in urban vs. wild mangrove forests. To test this hypothesis, plastic sheets (low-density polyethylene, LDPE; polypropylene, PP; polyethylene terephthalate, PET and expanded polystyrene, EPS) were placed in two positions (on-ground and above ground), and the colonizing biota assessed after four months in urban and wild mangrove forests in the southern Colombian Caribbean. We found 19 plant and animal taxa scraping, burrowing into, biting, fracturing, etching, and boring through plastic sheets, demonstrating that bioerosion is a critical degradation pathway that influence the fate of plastic litter in the environment. As hypothesized, there were significant differences in the structure of macrobenthic biota and functional groups of bioeroders between urban and wild forests. Moreover, the bioerosion rate of EPS and PP was faster in urban than in wild mangrove forests. These findings challenge the view that the biological transformation of plastics is a slow process, mainly driven by microorganisms.