Observing aggregate formation using high-resolution stereoscopic imaging

The aggregation of individual phytoplankton into marine snow allows particles to sink more quickly, thus resulting in the transport of particulate organic carbon from surface waters to the deep ocean. The rate of aggregate formation has previously been measured in experiments indirectly by quantifying how particle size or particle concentration changes over time. Here, we will be using high-resolution imaging to directly observe aggregate formation for the first time.

We conducted experiments to investigate aggregate formation using stereoscopic imaging, tracking individual particles directly in a 3D volume. Phytoplankton cultures were rolled in cylindrical tanks and imaged by two cameras illuminated by an infrared laser sheet. Using particle tracking velocimetry (PTV) we were able to directly track particles and view collisions and coagulation events, thus quantifying the coagulation coefficient, α, which represents the stickiness of particles. We measured α for the first 200 minutes of aggregate formation to observe how this quantity changes over time for the initial formation period of marine snow, how it varies for different initial phytoplankton concentrations, and how it varies between particles of different sizes. Observing aggregate formation on the individual scale for the first time can further our understanding of how different biological and physical factors may affect the carbon cycle on much larger scales.