Role of Marine Snows in Microplastic Fate and Bioavailability

Microplastics contaminate global oceans and are accumulating in sediments at levels thought sufficient to leave a permanent layer in the fossil record. Despite this, the processes that vertically transport buoyant polymers from surface waters to the benthos are poorly understood. Here we demonstrate that laboratory generated marine snows can transport microplastics of different shapes, sizes, and polymers away from the water surface and enhance their bioavailability to benthic organisms. Sinking rates of all tested microplastics increased when incorporated into snows, with large changes observed for the buoyant polymer polyethylene with an increase in sinking rate of 818 m day^-1 and for denser polyamide fragments of 916 m day^-1. Incorporation into snows increased microplastic bioavailability for mussels, where uptake increased from zero to 340 microplastics individual^-1 for free microplastics to up to 1.6 × 10⁵ microplastics individual^-1 when incorporated into snows. We therefore propose that marine snow formation and fate has the potential to play a key role in the biogeochemical processing of microplastic pollution.