A Numerical Model Approach Toward a Settling Process and Feedback Loop of Ocean Microplastics Absorbed Into Phytoplankton Aggregates

To quantify the settling velocity of buoyant oceanic microplastics absorbed into phytoplankton (algae) aggregates heavier than seawater, a vertical two-dimensional numerical particle tracking model (PTM) representing the motion of both particles and their interaction in oceanic turbulence was developed. The model incorporated "in silico phytoplankton," and could generate an aggregate particle with phytoplankton and microplastic particles positioned close to the "aggregation radius." The model included the dense phytoplankton concentrations typically observed in the spring/autumn boom or red tides, and reproduced sinking microplastics with settling velocities of ? (10(-5)-10(-4)) m/s, which were comparable to the vertical velocities ubiquitously observed in oceans, by the absorption into phytoplankton aggregates. The model reproduced the observed vertical distribution of microplastic abundance, showing subsurface maxima and a rapid decrease in particles smaller than 1 mm in the water column, while only an equilibrium between rise velocity of buoyant microplastic particles and vertical diffusion by oceanic turbulence could not reproduce the observed abundance. Buoyant microplastics, therefore, are likely to settle to the deep layer by the absorption into phytoplankton aggregates in the actual oceans. A feedback loop was suggested in which the settling velocities of aggregates were also dependent on the abundance of microplastics in the ambient water. However, a further in-depth examination is required to confirm the feedback by including ambient ocean circulations as well as oceanic turbulence, and by replacing the settling velocity with an alternative formula appropriate for aggregates absorbing lightweight microplastics, which might decrease the velocity more rapidly than expected.