Size distributions and fractal properties of particles during a simulated phytoplankton bloom in a mesocosm

Marine snow aggregates (>500μm) composed primarily of phytoplankton are known to have fractal dimensions of 1.52 to 1.72 and are assumed to form by the physical coagulation of smaller particles. In order to study the bulk fractal properties of these smaller particles during a coagulation event, concentrations of small particles (2 to 300 μm) were measured during a simulated phytoplankton bloom in a laboratory mesocosm. Particle concentrations were presented as size distributions in terms of either solid volume or average length. Both distributions indicated that particles were continuously coagulating throughout the bloom as evidenced by a greater increase in the concentration of larger particles (50 to 300 μm in average length) than smaller particles (2 to 50 μm), and decreases in fractal dimensions. Average fractal dimensions of all particles in the size range 20–200 μm were calculated using a new method called the particle concentration technique (PCT). The PCT required both solid volume and length size distributions on the same population of particles as input. As coagulation of phytoplankton and other particles in the tank proceeded during the phytoplankton bloom (7 to 11 days after inoculation), the average fractal dimensions of these particles decreased from D = 2.49 ± 0.41, a value close to the Euclidean value of 3 for a sphere, to D = 1.68 ± 0.08, a value typical of larger marine snow aggregates. This suggests that although marine snow-sized aggregates can appear to be dominated by non-fractal particles such as phytoplankton, they are primarily formed from many types of smaller aggregates present in the water column that, on average, can have low fractal dimensions.