A study of the evolution of the particle boundary layer in a reservoir, using laser particle sizing.

The dynamics of the particle boundary layer in the Boadella reservoir was studied using an in situ laser optical particle-sizing instrument. This layer was found at the bottom of the reservoir from summer until the end of the year, when the reservoir was fully mixed. Most of the particles in this layer are remnants of the summer algae bloom and are trapped in the boundary layer due to the thermal stratification. The phytoplankton bloom is mainly composed of diatoms, with diameter d approximately 5 microm and green algae with diameter d approximately 15 microm. Inorganic particles and decomposed organic particles with d < 3 microm are also encountered in the boundary layer. On the other hand, particles with diameter between 30 and 100 microm are mostly found in the epilimnion of the reservoir. These are a mixture of aggregates of inorganic particles, colonies of phytoplankton, zooplankton, detritus, etc. Different mixing events occurring during autumn resuspended the small particles in the boundary layer, while the greater particles settled down. The extent of the resuspension has been parameterized with a non-dimensional number that balances the stress across the interface and the strength of the stratification.