Simulated microplastics lurk below the ocean surface

It’s hard to figure out where plastic trash goes once it’s in the ocean. Various currents often transport macroscopic debris hundreds or thousands of miles. (See “It takes a global village to track plastic waste,” Physics Today online, 7 May 2021 , and the Quick Study by Erik van Sebille, February 2015, page 60 .) Once sunlight, salt water, and marine organisms degrade the material, its smaller size makes debris even harder to track.

Microplastic particles are no more than 5 mm in size, and researchers suspect that they’re more prevalent than larger pieces of trash. But the observations to date have found fewer microplastics than expected floating at the ocean surface. The discrepancy may be because of biofouling: When algae and other organisms grow on plastic and other materials, the extra weight may make the particles less buoyant.

To test for that possibility, Hannah Kreczak and her colleagues at Newcastle University in the UK built on a 2017 model that combined microplastic hydrodynamics and algae growth. They found some agreement with previous observations. In their simulation, the plastic microparticles didn’t float on the water’s surface because the algae biofilm that grew on them confined their vertical motion within a subsurface ocean layer.

The researchers’ model relates an algae population’s growth on a particle to its depth. The farther from the ocean surface the particle is, the less sunlight is available for the photosynthesizing algae. Among other parameters, the model depends critically on the size of the particle that the algae can grow on, the density of the seawater—which affects the microplastic’s relative buoyancy—and the rate at which algae attach to the particle and eventually die.

The plot shows the results of the simulation. A microplastic particle follows an oscillatory vertical motion (blue line) as the algae population expands and contracts (orange line). In a single oscillation, the particle starts sinking over time as the algae population grows under favorable light conditions. But once a particle falls below the euphotic zone depth (blue dashed line), algal growth slows, the population dies, and the plastic rises toward the water’s surface. Once it crosses into the euphotic zone again, the algae growth restarts, and the cycle repeats. The researchers aim to make their model more realistic by adding turbulent motions and wind-driven ocean effects. (H. Kreczak, A. J. Willmott, A. W. Baggaley, Limnol. Oceanogr., 2021, doi:10.1002/lno.11879 .)