Distribution of plastic polymer types in the marine environment; A meta-analysis

Highlights: • Incidence of common polymers is similar in intertidal- and shallow subtidal sediments. • Open oceans favours segregation of microplastic polymer types according to their density. • There was no confirmation that buoyant plastic types reliably sink out of the water column. • Sediment trap and deep sea studies constitute an important knowledge gap. -- Abstract: Despite growing plastic discharge into the environment, researchers have struggled to detect expected increases of marine plastic debris in sea surfaces, sparking discussions about "missing plastics" and final sinks, which are hypothesized to be coastal and deep-sea sediments. While it holds true that the highest concentrations of plastic particles are found in these locations (10³-10⁴ particles m⁻³ in sediments vs. 0.1–1 particles m⁻³ in the water column), our meta-analysis also highlights that in open oceans, microplastic polymer types segregated in the water column according to their density. Lower density polymers, such as polypropylene and polyethylene, dominated sea surface samples (25% and 42%, respectively) but decreased in abundance through the water column (3% and 2% in the deep-sea, respectively), whereas only denser polymers (i.e. polyesters and acrylics) were enriched with depth (5% in surface seawater vs. 77% in deep-sea locations). Our meta-analysis demonstrates that some of the most abundant and recalcitrant manufactured plastics are more persistent in the sea surface than previously anticipated and that further research is required to determine the ultimate fate of these polymers as current knowledge does not support the deep sea as the final sink for all polymer types.