PhD: Closing the marine plastic mass budget Using observational data to inform numerical models

Plastic waste is causing pollution in the ocean. A lot of plastic waste remains afloat, and accumulates in the middle of the ocean, in the so called `floating garbage patches’. Part of the plastic waste ends up on the coastlines, starts to sink down after some time, or fragments into smaller particles. In this doctoral research, we have calculated where most of the plastic pollution is residing in the ocean. To do so, we combined numerical simulations of the ocean currents with observational measurements of plastic concentrations. We made an overview for the oceans worldwide and made more detailed calculations for the Dutch North Sea coastline, and the highly polluted Mediterranean Sea.

One important conclusion is that the total amount of plastic pollution in the oceans worldwide is likely much higher than previously expected: roughly 3,000,000 tonnes (3 billion kilograms). Previous research, with estimated quantities of roughly 250,000 tonnes, mainly used net measurements to calibrate the numerical models. Net measurements are likely to miss larger objects, which contribute to most of the marine plastic mass. We estimate that a relatively large part of marine plastics sinks down over time (for example due to growth of algae). Fragmentation of plastics plays an important role: most of the plastic items found in the ocean are small fragments originating from larger objects. We estimate that beaches contain about 2 percent of the global marine plastic mass. On Dutch beaches we expect that the total mass of plastic items reaches values of up to 30.000 kilograms.