Effects of rheologically stratified seawater during algal bloom on sinking dynamics of microplastics

The settling behaviour of microplastics (MPs) governs their residence time and bioavailability in the water column. Despite significant research efforts on the hydrodynamics of MPs in various aquatic environments, little attention has been given to MPs sinking in seawater enriched with mucus formed by exopolymers (EPSs) excessively secreted by microorganisms during algal blooms. These blooms often develop in density-stratified seawater and can rheologically modify the system by introducing a non-Newtonian EPS-rich layer to the Newtonian seawater column with a typical content of EPSs. In this study, we investigated the previously unaddressed role of rheological stratification that forms at the periphery of the algal bloom region in MPs sinking process. Our laboratory experiments revealed that variations in viscosity and viscoelasticity between Newtonian and non-Newtonian layers significantly impact MPs sinking velocity and orientation, often outweighing the effects of density stratification. Results demonstrated that the sinking velocity of MPs decreased in the non-Newtonian layer leading to residence times up to five times longer than in the Newtonian layer, showing a clear dependence on the viscosity difference between the two layers. Meanwhile, we identified the orientation instabilities of MPs enhanced with increasing EPS content. This study revealed previously unaddressed hydrodynamic effects in rheologically stratified seawater that may intensify MPs accumulation and their interactions with the ecosystem. This could help explain MPs fate in algal bloom-afflicted regions.