Increased abundance and ecological implications of plastic microdebris in the North Pacific Subtropical Gyre

Background/Question/Methods

Plastic pollution in the form of small particles (diameter < 5 mm), termed “microplastic,” has been observed in many parts of the world ocean. Microplastic was first detected in the North Pacific Subtropical Gyre (NPSG) in the early 1970s, and has since become a matter of scientific and public concern.  Relatively little is known about the extent of microplastic in the NPSG, but since most microplastic particles are on the ocean’s surface, surface-dwelling biota are most likely to be impacted. We asked 1) How do the abundance, distribution, and size of plastic particles the northeast Pacific vary over regional and temporal scales? 2) How are surface zooplankton impacted by plastic microdebris?

To quantify decadal-scale changes in Pacific microplastic, we compiled all available georeferenced data on ocean surface microplastic abundance between 1972-1984 and 1999-2010, including both publically available data and new data. We explored ecological impacts by a) evaluating abundance of the oceanic insect Halobates sericeus, which requires hard substrate for oviposition; and b) estimating zooplankton ingestion of microplastic. H. sericeus abundances were enumerated from archived surface plankton tows from 1972-3 and 2009-10. Zooplankton ingestion was evaluated by incubating NPSG zooplankton with 25-45 μm polyethylene beads for 20-22 hours at sea.

Results/Conclusions

Microplastic concentrations increased markedly in the NSPG between 1972-84 and 1999-2010. Numerical concentration of microplastic off Alaska significantly increased, but mass concentration did not. No increase was detected in the California Current or Eastern Tropical Pacific. H. sericeus was not associated with plastic density in 1972-3, but elevated levels of plastic in 2009-10 resulted in a significant positive relationship between H. sericeus and microplastic, and an overall increase in H. sericeus egg densities. When exposed to ecologically realistic concentrations of microplastic, surface-dwelling zooplankton consumed relatively few plastic particles. Overall zooplankton grazing, as estimated by fecal pellet production, was not correlated to microplastic density. These results suggest that the substantial rise in NPSG microplastic concentration is linked to increased oviposition in H. sericeus. However, surface-dwelling zooplankton in the NPSG may not be impacted by microplastic pollution through direct ingestion of plastic particles. These results are a first step to understanding the ecological implications of plastic microdebris in the NPSG.