Why microplastics do not transfer persistent organic pollutants through aquatic food webs

Microplastic particles are ubiquitous in the world's oceans, may be ingested by marine organisms, and are enriched in hydrophobic organic chemicals relative to their surrounding aqueous environment. These three well documented observations often lead to speculation that marine microplastics serve as an important vector to pump persistent organic pollutants (POPs) such as PCBs and flame retardants into higher levels of aquatic food webs. Since field-collected microplastic particles contain relatively high concentrations of many POPs, even in remote oceanic environments, it is tempting to view them as POPs-enriched packets that efficiently deliver toxins to shellfish and finfish. In fact, one of the major justifications for legislative action against the use of microplastics in consumer products is the potential risk of increasing bioaccumulation in receiving waters. Despite this widespread conjecture, both theory and experiments demonstrate that microplastics play almost role either as a reservoir for pollutants or as a vector for pollutant transfer. In this paper, we will use field measurements of microplastic and POPs concentrations in the Salish Sea together with poly-parameter chemical partitioning, pharmacokinetic, and plankton grazing models to quantitatively explore potential microplastic-bioaccumulation relationships. Such models are driven by the thermodynamic distribution of hydrophobic chemicals between water, microplastics (both clean and biofilm-coated), and the digestive tract of marine organisms, as well as by the relative abundance of microplastic particles within the complex mixture of natural particles encountered in coastal waters.