Non-buoyant microplastics (MPs) sink through the marine water column, adversely affecting the ecosystem. The manner in which MPs influence the water environment depends to a large extent on their settling dynamics, driven by their properties and the physio-chemical characteristics of water column. However, some properties of seawater remain elusive, limiting our ability to fully explain the sinking processes of MPs. One of the gaps in our understanding relates to the elevated content of exopolymers (EPSs) secreted by algae and bacteria, which locally transform seawater into a non-Newtonian liquid, altering the hydrodynamics of particle transport. In this study, we present a series of lab-scale experiments on the dynamics of isometric (spheres and irregular particles) and anisometric (disks, rods, and blades) MPs settling in artificial seawater with the addition of polysaccharides. We find that upon the appearance of EPSs in seawater, the sinking velocity of MPs diminishes and may fluctuate, the orientation pattern changes in a non-intuitive way, and MPs may tumble. As measured in rheological tests, these consequences result from seawater gaining viscoelastic and shear-thinning properties. Our findings raise concerns that mucus-rich seawater may favor the aggregation of MPs with organic matter, interaction with biota, and biofouling, which can affect the biogeochemistry of the marine ecosystem. Based on these findings, we recommend that seawater rheology, modified by excessive amounts of EPSs during algal blooms, should be considered in biogeochemical and microplastic transport models.
Keywords: Exopolymers; Microplastics; Particle tracking velocimetry; Settling experiments; Settling velocity.
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