Quantified small-scale turbulence inhibits the growth of a green alga

1. Laboratory experiments were conducted to elucidate the effect of small‐scale turbulent fluid motion on the growth of laboratory cultures of the freshwater algae Scenedesmus quadricauda. Turbulent flow was generated using an oscillating‐grid apparatus. The experiments were performed under the range of fluid flow conditions similar to those occurring in nature. The only growth limiting factor was the effect of small‐scale fluid motion; all other environmental factors, such as light, temperature and nutrients, were kept constant. 2. Growth of Scenedesmus quadricauda, measured in terms of chlorophyll a concentration, was inhibited when the level of turbulence in the water column was increased. Algal growth was maximum in a quiescent fluid. The inhibitory effect of fluid motion was observed independently of flow regime (laminar, transitional, turbulent) in the water column. 3. Cell destruction and aggregation of dead and living cells of algae were observed in a turbulent flow. High shear rates, estimated from the dissipation of turbulent kinetic energy, caused the cell destruction, algal collision and agglomeration of algae. Data on Scenedesmus responses to small‐scale fluid motion will enhance and broaden our ability to develop predictive multispecies models for freshwater phytoplankton communities.