Scales and processes in the aggregation of diatom blooms: high time resolution and wide size range records in a mesocosm study

Diatoms and the large, fast-sinking aggregates they form during blooms play an important role in downward flux of particles in the ocean. To study how the aggregation process operates on particle dynamics, diatom blooms were generated and followed under controlled conditions in nutrient-enriched laboratory mesocosm where a homogeneous mixed surface layer was emulated. The size spectrum of particles (from 12 μm to several mm) was recorded each hour during the 1 month span of the experiment by a non-intrusive image analysis system with two CCD cameras. Beam attenuation was continuously recorded as an additional estimator for particle abundance. The high time resolution and wide size range of the records obtained with this design were able to resolve the time scale for coagulation as well as to determine the lowest time resolution needed to sample any experiment aimed to study aggregation of diatoms. Our results narrow previous theoretical time scales to the order of hours to days for the process of mass transfer from small particles to marine snow. Also, daily analyses of a broad range of biological and chemical variables permitted to link phytoplankton succession to the aggregation process. Finally, the evaluated role of different copious exopolymers suggested a lower implication of Coomassie stained particles (CSP) than transparent exopolymeric particles (TEP) in the formation of marine aggregates.