Turbulence controls size distribution of aggregates: in-situ
observations by a microstructure profiler
Marine aggregates are ubiquitous particles formed from the accretion of
smaller biogenic and non-biogenic components. Visible aggregates, known
as marine snow, are typically in the 0.5 to few mm size range.
Aggregates are well recognised as hotspot of microbial and planktonic
activities. Aggregates formation is an important pathway for
transferring materials and carbon flux from surface to the deep ocean.
Because aggregate sinking velocity and carbon mass content is size
dependent, understanding the physical mechanism controlling aggregate
size distribution is fundamental to determining the biological carbon
pump efficiency. Turbulence is a physical mechanism in the aggregates
formation and destruction. However, the relative roles of turbulence in
aggregates formation and destruction have not been fully tested in
observational studies. In this study, we analysed simultaneous in-situ
observations of turbulence and aggregate in the various aquatic systems.
A microstructure profiler, TurboMAP-L, was used to collect shear data
and a digital still logger camera was used to collect images of
aggregates. Digital images were subsequently used to determine
aggregates abundances and size distributions. Direct comparison of
turbulence intensity and aggregate size distributions show that
turbulence below 𝜀=10-6[W/kg] enhances aggregation, increasing
average particle size; greater turbulence causes particle breakup,
limiting the average maximum aggregate size and decreasing the slopes of
size distributions. This indicates the role of turbulence controlling
aggregate size distributions. We also present fluorescence data
collected by TurboMAP-L and focus on difference of aggregates size
distributions among different aquatic systems.