Changes in aggregation and fragmentation dynamics of marine aggregates throughout the course of a phytoplankton bloom*

Cohesive organic particulate matter in the ocean readily clusters and aggregates into large (>0.5 mm) marine snow particles. These relatively large aggregates have higher settling velocities than their microscopic component particles, which facilitates the transport of this organic material out of the surface and to the deep ocean. This process is particularly prevalent during blooms of phytoplankton, where high concentrations of particles and sticky extracellular biopolymers facilitate particle-particle contacts and cohesion. Here, we perform aggregation and disaggregation experiments on field-sampled phytoplankton during a spring bloom in the California Current. Samples of phytoplankton were collected, then aggregated in a cylindrical rolling tank prior to disaggregation through exposure to controlled laminar shear. We found that the type of phytoplankton, the bloom stage, and biopolymer concentrations all influenced the aggregation and fragmentation rates of the marine snow particles when exposed to these hydrodynamic environments relevant to the surface ocean. These results enhance our ability to predict the formation of large, fast-settling particles in the ocean and ultimately our ability to understand processes contributing to carbon sequestration in the marine environment.