About This Project

Microplastics – small bits of plastic millimeters or smaller in size – that float in the ocean are carried by surface currents to oceanographic “dead-ends” known as subtropical ocean gyres. Yet how these particles move vertically in the water column remains poorly understood. We will carry out laboratory experiments designed to inform numerical ocean models that predict where in the water column these microplastics may be found, which may have implications for the marine animals that live there.

Ask the Scientists

What is the context of this research?

Floating microplastics have been documented in all five subtropical oceans, yet their abundance accounts for less than 1% of the estimated 8 million metric tons of plastic entering the oceans from land each year. Until we know where this ‘missing plastic’ resides, including the processes that transport it once it has entered the sea, we cannot accurately evaluate the risk to marine animals. Floating microplastics are measured by towing a plankton net at the sea surface, the amount collected varies depending on the wind speed. The energy from the wind mixes buoyant microplastics down to depths out of reach of the nets used to measure them. This vertical mixing must be taken into account in order to obtain robust estimates of the amount of microplastics in the near-surface ocean.

What is the significance of this project?

The results of this experiment will fundamentally advance our understanding of the depth distribution of microplastics in the upper ocean, leading to better predictions of microplastic concentrations.

Using wind speed and sea surface plastic concentrations, computer models can predict the amount of plastic that has been mixed to depth. A key parameter in these models is the “particle rise velocity” - the speed at which a submerged particle would rise back to the surface if released at depth. SEA has studied microplastics since 1986 with more than 11,000 plankton net samples from the Atlantic and Pacific Oceans, the largest dataset of its kind. The archived plastic samples give us the opportunity to measure the rise velocity of real-world microplastics of various forms, shapes and sizes.

What are the goals of the project?

In this experiment we will measure the rise velocity of individual microplastics and determine the particle characteristics that have the greatest influence on the velocity. We will utilize a ‘rise-velocity chamber’- a custom-built tube that is 2.5 meters tall, filled with artificial seawater. Individual pieces will be released at the bottom of the chamber and timed as they rise a defined distance, giving us the rise velocity.

We hypothesize that there will be distinct size, shape and/or mass characteristics that determine how deep a microplastic particle is mixed. We would like to publish this research in a scientific journal and the results will provide a critical parameter for computer models developed by Tobias Kukulka to predict the behavior of microplastics subjected to wind mixing.