Schedule for: 22w5073 - Predicting Pathways for Microplastic Transport in the Ocean (Online)

Feel free to explore Gathertown, using the login information that will be provided separately by BIRS. We will be using Gathertown for thematic discussions, and for informal chats with the speakers and other participants following the talks Monday to Thursday during the workshop. Gathertown will also be used as a place to meet during "office hours", should you choose to set this up.

Online Via Zoom

Online via Zoom

Understanding how particles interact in turbulent flow is an important and open question with many industrial and natural applications. Using laboratory experiments in a homogeneous, isotropic turbulent flow, I investigate how the kinematics of flat, non-spherical particles within the inertial subrange of turbulence respond to turbulent forcings. I used a particle tracking 3D reconstruction technique to determine the evolution of particle orientation over time. From these results, I found that the flat particles tended to tumble more than spin, and the rate of tumbling depended on the particle size. This is generally associated to a preferential alignment with the vorticity. These results are consistent for those of fibres and cuboids in the same size regime, where particle motion is dictated by coherent eddies on the same size scale as the particles. Online Via Zoom

Recently, a number of authors have used global particle tracking simulations to identify the effect that different surface currents have on marine litter accumulation, including the role of surface waves through their Stokes drift. However, in the upper-ocean boundary layer and in the presence of the Coriolis force, a wave-driven Eulerian flow forms that must be superimposed onto the Stokes drift in order to obtain the correct Lagrangian velocity. Taking into account both the Coriolis--Stokes force and the surface wave stress, Higgins et al. (2020, GRL) derived an expression for this unsteady wave-driven Eulerian-mean flow in the form of a convolution between the Stokes drift and the so-called Ekman--Stokes kernel. In this paper, we apply this Ekman--Stokes kernel to generate a 12-year global hindcast of the wave-driven Eulerian current and show that its inclusion in particle tracking simulations has a significant effect on the distribution of floating marine litter. Using Lagrangian simulations, we find that the wave-driven Eulerian current is sensitive to the value of viscosity but generally opposes the dispersive behaviour of the Stokes drift, reducing the amount of cross-Equator particle transport and transport to the polar regions, resulting in closer agreement between modelled and observed microplastic distributions. Online via Zoom

Online via GatherTown

Participants will have the opportunity to chat informally with the speakers and interact with each other through conversations and playing interactive games set up in the Lounge on GatherTown.

Microplastic transport by surface waves is subject to inertial effects due to particle size and density difference with sea water. We consider the effects of inertia for negatively buoyant microplastics settling in surface waves as described by linear wave theory in arbitrary depth. We consider particles that fall under both a linear drag regime (applicable for particles smaller than 0.5 mm in size) and in a non-linear drag regime in the transitional Reynolds number range (applicable for particles up to 5 mm in size). Expanding the equations of motion in the particle Stokes number allows us to find kinematic expressions for inertial particle motion and expanding these equations in the dimensionless wave amplitude with multiple timescales allows us to find expressions for the wave-averaged drift velocities. These drift velocities can be used in large-scale models that do not resolve surface waves. Due to a combination of dynamic and kinematic effects, the horizontal particle drift is smaller than the classical Stokes drift and the vertical particle drift is larger than the particle terminal settling velocity. We also demonstrate that a cloud of negatively buoyant particles disperses in the horizontal direction due to vertical shear in the horizontal drift. Finally, time permitting, we will also discuss recent work examining microplastic beaching via field and lab studies.

Despite the ubiquity of microplastic particles in the oceans, the lifetimes and fates of these plastics are largely unknown. One pathway of removal relevant to floating microplastics is that of chemical degradation by exposure to sunlight, which is dictated by both the depth and, for non-spherical microplastics, orientation of the particles in the water column. Positively buoyant particles exist near the ocean surface, but wind produces waves and turbulence which mix particles down in the water column. Here, experiments are performed in a laboratory wind-wave channel over a range of wind speeds relevant to the ocean surface. Positively buoyant rod-, disk-, and sphere-shaped HDPE particles are seeded in the flow, and their transport and dispersion are measured using a novel particle shadow tracking technique. A large field of view facilitates the tracking of particles over long times. The parameter space of non-spherical, non-tracer particles in wind-driven turbulence is systematically explored, and both Eulerian and Lagrangian statistics of particle depth and orientation are obtained. The results will provide insight into the physical processes governing microplastic particle transport and fate in the ocean.

Because surface gravity waves are one of the fundamental types of flows that transport microplastics in the ocean and because microplastics are typically non-spherical, we experimentally investigated the settling and dispersion of non-spherical particles in wavy flow. We find that the dispersion of particles is significantly increased by the presence of waves. The magnitude of this increase is a function of particle shape and volume. Particle vertical settling velocities can both increase and decrease in waves as a function of the particle shape and of the inertia of the local flow at the length scale of the particle. This variation in settling velocity comes from the particles preferentially sampling the flow while maintaining a constant average relative velocity to the flow. These results indicate that models of microplastics in the ocean must account for waves, inertia, particle shape, and particle volume.

Foundational experimental and numerical investigations of particles in turbulence are somewhat weighted towards the consideration of small spheres. However, the focus of recent work has shifted towards particle characteristics much more relevant to the transport of microplastics: irregular shapes, a broader range of densities, and sizes much larger than the smallest flow scales. It is not clear, however, how these characteristics might interact to influence important properties like settling velocities, turbulent slip velocity, and bulk transport. For example, small heavy particles and large light particles are both inertial and may even have the same Stokes number, but behave very differently in turbulence. Microplastics, with their wide range of shapes, sizes, and densities, present challenging questions to those seeking to simplify the physics of transport. What parameters matter, and to what degree? A fragment and a fiber have different settling velocities, even if they have the same density; is this difference significant enough to feed into larger-scale transport models? How does settling velocity change in turbulence, and how does this alteration vary across different environments, each with their own flow characteristics? The issue becomes even more complex when we consider that microplastics are not static, but dynamic participants in the ecosystem around them. They deform, degrade, and are colonized by the surrounding biota, leaving behind particles and aggregates which may be highly non-uniform in shape and mass distribution. How does this change their overall transport? While we will not attempt to answer all of these questions, we will discuss how size and shape may amplify the effects of very small density changes, such as those which may result from marginal degradation or colonization of microplastics. We will also present recent work on how non-uniform mass distributions can affect the behavior of large nonspherical particles. Lastly, we will outline our current efforts to more deeply quantify the role of irregular mass/density distributions on the overall transport of microplastics in environmental flows.

Plastic pollution in the ocean has been recognized as one of the largest environmental tragedies of our time. Ecosystem responses to plastic waste in the sea are often focused on megafauna. Comparatively less is known about interactions between plastic waste and marine microbes, including bacteria, microalgae, and fungi. Microbes have been found to utilizing fractions of microplastics (MPs) as a carbon source and surfaces to grow on. MPs may also get incorporated into microbial aggregates, acting as precursors for rapidly sinking, macroscopic aggregates (marine snow) that accelerate the vertical downward carbon flux in the ocean. We conducted laboratory experiments to investigate how growth and aggregation of planktonic microalgae are affected by the presence of MPs. Cell growth and aggregation efficiencies were measured in two separate incubations with a phytoplankton monoculture (Isochrysis sp.) and a mixed culture (Isochrysis sp. + Skeletonema sp.) in the presence of postconsumer high-density polyethylene MPs. Most notable effects were found for the mixed culture that showed higher cell growth and aggregation efficiencies in the presence of MPs compared to the non-MPs controls. The monoculture and mixed culture did not incorporate MPs into marine snow, while cells from another phytoplankton monoculture (Emiliana huxleyi) formed marine snow with MPs. Sinking rates of marine plastic snow (MaPS) in an unstratified water column were similar those of marine snow and about double those of MPs alone. Transition times of marine snow and MaPS through a stratified water column mimicking stratification in estuarine and open ocean environments were also very similar, and almost 50 times slower than denser nylon particles of the same diameter. Thus, MPs that made up about 50% of the total area of the aggregates did not measurably affect the settling behavior of marine snow under our controlled laboratory conditions. Our results suggest that both aggregate types possess similar (elevated) residence times in surface waters where they may be subject to grazing particularly at sharp density layers, representing a pathway of MPs into higher trophic levels in the ocean interior.

5 minute overviews of each of the four themed discussions will be given: Michelle DiBenedetto (particle shape) Ton van den Bremer (open ocean) Alexis Kaminski (estuaries) Bruce Sutherland (particle transformation)