Navigation-Induced Turbulence as a Driver of Plastic Fragmentation

Globally, plastic pollution in aquatic environments has been considered one of the major contemporary environmental concerns. In recent years, an increasing number of studies reported environmental consequences and concentrations of plastic particles in freshwater systems. The observed abundance of plastic particles in ecosystems may be influenced by factors such as the properties of plastic, including size and shape. Plastic items can be differentiated according to their size range and therefore identified by adopting the prefixes macro (> 25 mm), meso (> 5 mm < 25 mm), and micro (< 5 mm). Once large plastics accumulate in the natural environment, they are subject to multiple weathering processes that drive fragmentation and degradation. Plastic items suspended in the water column may be exposed to the strong hydrodynamic forces generated by vessel motion, for instance, turbulent water flow created by the propellers as the vessel moves through the water. Facing that, this study experimentally quantifies the forces needed to fragment plastic items collected in the Rhine River, subsequently, it assesses the likelihood of drag forces exerted by the propeller jet of moving vessels as causes of plastic fragmentation. By examining the forces applied to different categories of plastic items, valuable insights will be gained on the mechanical fragmentation of plastics in a highly navigated river, contributing to better predictions of the spread and transport of plastic items and their risks to wildlife and humans. Among the observed categories, “Plastic film 2.5–50 cm (soft)” exhibited the lowest median force to break compared to all other categories (3.8N), being more susceptible to fragmentation under high jet-induced velocities generated by vessels, showing consistently higher breakage probabilities, exceeding 50% at velocities ≥ 5 m.s⁻¹ and reaching values up to ~80% at 10 m.s⁻¹. On the contrary, the top three categories that exhibited the highest resistance to breaking, with high median values, corresponded to “Plastic cups” (14.09N), “ Sanitary/ wet wipes” (11.24N), and “ Plastic cotton swabs” (11.08N).