Deformation and fragmentation behaviors of phytoplankton and plastic particle aggregates*

The formation and disruption of aggregates in natural and synthetic aqueous environments depend heavily on both the interfacial conditions of the particles and the hydrodynamic forces they are exposed to. The interfacial conditions relevant to the aggregation of plastic particles are relatively well understood but knowledge of the bonding properties of aggregates formed from biological aquatic particles is still limited. To better understand the biological bonding forces, we implemented aggregate breakup experiments using both phytoplankton and plastic particles. In this study, we cultured two species of diatom phytoplankton and made simulated marine aggregates in the laboratory using a cylindrical roller tank filled with seawater. We also prepared polyethylene and polystyrene aggregates using different aqueous salt solutions but in the same facility. The size of the formed aggregates ranged from hundreds of microns to a few millimeters for all particle types. To deform and disrupt the aggregates with calibrated hydrodynamic shear, we superimposed a harmonic oscillation to the rolling motion of the tank, which created a laminar oscillating boundary layer near the tank wall. Using high speed imaging and a particle tracking and breakup detection algorithm, we performed dynamic and morphological analyses of individual fragmentation events as well as population-level statistical analyses to compare the behavior of the different particle types. Here, we summarize the deformation and breakup behavior and compare the breakup strength of the four different types of aggregates investigated. We find that polystyrene aggregates were weaker than the phytoplankton aggregates but that polyethylene aggregates were too strong to be disrupted in shear relevant to the ocean. The bounds of two plastic particle aggregates provide us with more insights into the strength of phytoplankton aggregates.