Turbulent relationships on the rocks

Posted on December 9, 2010

The rocky surfaces of river beds are typically colonised by biofilms, which are complex mixtures of algae, bacteria and invertebrates.  Biofilms play an important role in stream ecosystems by generating most of the primary production, by providing a food source for consumers, and by breaking down biodegradable material.  As a result there is interest in understanding the factors that control biofilm biomass, which is determined by the balance between biofilm growth and the rate of detachment.  Growth and detachment are both affected by flow conditions.  Hydraulic factors influence the locations where biofilm can colonise, the transfer of nutrients and gases into the biofilm layer, and the competitive success of different biofilm species.  In turn, hydrodynamic conditions are themselves influenced by the thickness and form of the biofilm layer.  While researchers have used different hydraulic indicators in models of biofilm dynamics, new experimental results suggest that biofilm detachment and nutrient transport in the near-bed layer can’t be adequately represented by a single measure of turbulence.  An improved model of detachment incorporates two main variables, namely friction velocity (the drag of flow over the river bed) and the roughness of the bottom surface. 

Reference:  Graba, M., Moulin, F.Y., Boulêtreau, S., Garabétian, F., Kettab, A., Eiff, O., Sánchez-Perez, J.M. & Sauvage, S.  2010.  Effect of near-bed turbulence on chronic detachment of epilithic biofilm: experimental and modeling approaches.  Water Resources Research 46, W11531, doi:10.1029/2009WR008679.