Impact of heteroaggregation between microplastics and algae on particle vertical transport

Understanding the impacts of microplastics (MPs) on aqueous environments requires understanding their transport dynamics and how their presence affects other natural processes and cycles. In this context, one aspect to consider is how MPs interact with freshwater snow (FWS), a mixture of algae and natural particles. FWS is one of the primary drivers of the flux of organic matter from the water surface to the bottom sediment, where zooplankton, diurnal migration, fish faecal pellets settling and turbulent mixing can also play prominent roles. Understanding how MPs and FWS heteroaggregation affects their respective settling velocities is important to assess not only MPs fate and transport but also their ecological impacts by altering FWS deposition and thereby nutrient cycling. In this present study, we obtained a mechanistic understanding of the processes controlling MPs settling dynamics and heteroaggregation with FWS and the subsequent impacts on the settling rates of both MPs and ballasted FWS. Here we used a plexiglass column equipped with a stereoscopic camera system to track the settling velocities of (1) MPs of various compositions, densities and morphologies, (2) FWS flocs and (3) MP-FWS agglomerates. For each experimental set, thousands of particles were tracked over a series of image sequences. We found that agglomerates with high-density MPs settled at least twofold faster than FWS alone, implying a much smaller residence time in the water column, except for cases with MP fibres or low-density plastics. These findings will help to refine MP fate models and, while contingent on MPs number, may impact biogeochemical cycles by changing the flux of nutrients contained in FWS to the sediment.