Microplastic abundance and distribution in the ocean
Most plastic debris collected in our surface-towing plankton nets consists of microplastics, or plastic particles smaller than 5 millimeters in size. Microplastics include easily identifiable industrial resin pellets, the “raw material” of plastic products sometimes referred to as “nurdles”, and fragments of plastic created when larger items break apart upon exposure to sunlight and weathering in the environment.
Microplastic measurements from more than 14,000 plankton net tows have been used to map their distribution in the ocean. Microplastics are transported by surface ocean currents and accumulate in the center of the ocean gyres of the North Atlantic, North Pacific, and other subtropical ocean basins. SEA’s decades-long time series in the western North Atlantic and eastern North Pacific Oceans also allow investigation of time trends in floating plastic abundance in the regions where they accumulate.
Law, K. L., S. Morét-Ferguson, N. A. Maximenko, G. Proskurowski, E. E. Peacock, J. Hafner and C. M. Reddy, 2010. Plastic accumulation in the North Atlantic subtropical gyre. Science, 329, 1185-1188, doi:10.1126/science.1192321.
Law, K. L., S. E. Morét-Ferguson, D. S. Goodwin, E. R. Zettler, E. DeForce, T. Kukulka and G. Proskurowski, 2014. Distribution of surface plastic debris in the eastern Pacific Ocean from an 11-year dataset. Environ. Sci. Technol., 48, 4732-4738, doi:10.1021/es4053076.
Van Sebille, E., C. Wilcox, L. Lebreton, N. Maximenko, B. D. Hardesty, J. A. van Franeker, M. Eriksen, D. Siegel, F. Galgani and K. L. Law, 2015. A global inventory of small floating plastic debris. Environ. Res. Lett., 10, 124006, doi:10.1088/1748-9326/10/12/124006. (open access)
Wilcox, C., B. D. Hardesty, K. L. Law, 2019. Abundance of floating plastic particles is increasing in the western North Atlantic Ocean. Environ. Sci. Technol. 54, 790-796, doi:10.1021/acs.est.9b04812
Plastic degradation in seawater
Plastics are synthetic polymers often designed for strength and durability, which makes them extremely resistant to biodegradation in the environment. Plastic items fragment into smaller and smaller pieces after exposure to sunlight weakens and embrittles the material. Although aspects of this chemical degradation are reasonably well understood, scientists still don’t know how small are the smallest microplastics in the environment, and whether the smallest plastics can be fully metabolized by microorganisms.
Using the decades-long sample archive of floating plastic debris collected by SEA Semester students in the Atlantic and Pacific Oceans, SEA researchers and collaborators investigate the physical and chemical signatures of degradation, with complementary laboratory and field exposure experiments designed to determine the time scales of these weathering signatures.
Morét-Ferguson, S., K. L. Law, G. Proskurowski, E. K. Murphy, E. E. Peacock and C. M. Reddy, 2010. The size, mass, and composition of plastic debris in the western North Atlantic Ocean. Mar. Poll. Bull., 60, 1873-1878, doi:10.1016/j.marpolbul.2010.07.020.
Andrady, A., K. L. Law, J. Donohue, B. Koongolla, 2021. Accelerated degradation of low-density polyethylene in air and in sea water. Sci.Tot. Environ. 811. doi:10.1016/j.scitotenv.2021.151368. (open access)
Vertical mixing of floating microplastics
Many (but not all) plastics float in the ocean because they are less dense than seawater. However, energy from the wind creates turbulence in seawater that can mix these buoyant pieces tens of meters deep, out of reach of the surface plankton nets used to collect them.
SEA scientists assess the physical properties of microplastics collected in their plankton nets, such as their size, surface area, mass and polymer type, then use an experimental lab setup to measure how fast particles rise back to the surface after they are submerged in seawater. Together with the physics of ocean turbulence, these data inform models that predict how much buoyant plastic is mixed below the sea surface under different wind and weather conditions.
Kukulka, T., G. Proskurowski, S. Morét-Ferguson, D. Meyer and K. L. Law, 2012. The effect of wind mixing on the vertical distribution of buoyant plastic debris: Observations and modeling. Geophys. Res. Lett., 39, L07601, doi:10.1029/2012GL051116.
Brunner, K., T. Kukulka, G. Proskurowski and K. L. Law, 2015. Passive buoyant tracers in the ocean surface boundary layer: 2. Observations and simulations of microplastic marine debris. J. Geophys. Res. Oceans, 120, 7559-7573, doi:10.1002/2015JC010840.
Kukulka, T. and K. L. Law, 2016. Upper ocean turbulence and its influence on microplastic particle transport. J. Ocean Technol., 11, 96-97.
Estimating the amount of plastic waste entering the ocean from land
The best way to address plastic pollution in the ocean is to prevent unwanted plastics from entering the marine environment in the first place. As prevention measures are designed – whether technological, behavioral, or policy-oriented in nature – it is important to have a baseline estimate of the input of plastics prior to implementation, to be able to assess the effect of the action taken.
SEA scientists have participated in collaborative studies estimating the amount of plastic waste generated on land that was not properly captured and contained in waste management infrastructure, globally and in the United States. These studies can inform policymakers about improvements in waste management systems that would reduce the amount of plastic waste lost to the environment.
Jambeck, J. R., R. Geyer, C. Wilcox, T. R. Siegler, M. Perryman, A. Andrady, R. Narayan and K. L. Law, 2015. Plastic waste inputs from land into the ocean. Science, 347, 768-771, doi:10.1126/science.1260352.
Borrelle, S. B., J. Ringma, K. L. Law et al. (20 authors), 2020. Predicted growth in plastic waste exceeds efforts to mitigate plastic pollution. Science 369, 1515-1518. doi:10.1126/science.aba3656.
Law, K. L., N. Starr, T. R. Siegler, J. R. Jambeck, N. J. Mallos, G. H. Leonard, 2020. The United States’ contribution of plastic waste to land and ocean. Sci. Adv. 6: eabd0288. doi: 10.1126/sciadv.abd0288 (open access)
Examples of SEA Student Work
Barr, K., Mewborne, N., and S. Kim. Sessile rafting communities and the characteristics of their host marine debris in the North Pacific Gyre. Unpublished student research paper, Class S-304, Sea Education Association.
Barron, E. and T. Felcher. Distribution of five microplastic types in the North Pacific Subtropical Gyre. Unpublished student research paper, Class S-304, Sea Education Association.
Croucher, T. Concentrations of microplastics in correlation to the proximity of coral reefs in the South Pacific. Unpublished student research paper, Class S-280, Sea Education Association.
Melvin, W. Investigation of plastic transporting E. coli in the western North Atlantic Ocean. Unpublished student research paper, Class C-240, Sea Education Association.
Najjar, O. and S. Schlegel. The effect of wind mixing on the concentration of microplastics at surface and subsurface levels in the eastern North Pacific Subtropical Gyre. Unpublished student research paper, Class S-304, Sea Education Association.