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

TLDR

Growing plastic discharge has not led to expected increases in surface marine debris, raising concerns about missing plastics and hypothesized sinks in coastal and deep‑sea sediments. The meta‑analysis shows that while plastic concentrations are highest in sediments, microplastic polymers separate by density, with low‑density polyethylene and polypropylene dominating surface waters but declining with depth, whereas denser polymers such as polyesters and acrylics increase with depth, indicating that persistent, abundant plastics remain in surface waters and that the deep sea is not the final sink for all polymer types.

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.