Biofouling changes the settling dynamics of macroplastic plates

Plastic pollution transported in rivers remains poorly understood due to the diversity of shapes, sizes, and densities of plastics, as well as their complex interactions with biofilms. While previous studies have explored the settling velocities of plastics and their interactions with biofilms, they often overlook how biofouling alters plastic dynamics and settling behaviour. To address this, over 800 settling experiments were conducted to demonstrate that the dynamics and falling velocities of isotropic (spheres) and anisotropic (square and rectangle plates) macroplastics of different densities (1050 to 2200 kg/m3) are significantly impacted by biofouling. Three-dimensional tracking of plastic trajectories revealed that biofilm colonisation on the surface of anisotropic plastic plates triggered them to exhibit more chaotic trajectories, larger horizontal dispersion and higher oscillatory frequencies. These dynamics reduced the average vertical settling velocity of anisotropic biofouled plates by up to 12%—despite greater plastic densities and considering the multimodal distribution of a plate’s fall velocity—compared to their pristine counterparts. Results highlight the necessity of accounting for the intricate multimodal settling dynamics of plastics, including their interactions with biofilms, to provide more reliable predictions of plastic transport and fate in aquatic environments. The trajectories of plastic particles in settling experiments within a water column are more chaotic and slower by up to 12% when colonised with biofilms, relative to pristine particles, even with larger densities.