Coupled effects of small-scale turbulence and phytoplankton biomass in a small stratified lake

Recent laboratory studies demonstrated that small-scale fluid motion mediates phytoplankton physiological responses. We have investigated to what extent the laboratory studies are consistent with field measurements in a small stratified lake. We propose the rate of energy dissipation and corresponding Kolmogorov velocity are important scaling variables that describe the enhanced algal growth and the uptake of nutrients in a moving fluid under laboratory and field conditions. The ratio of nutrient flux to an alga in a moving fluid versus the nutrient flux in a stagnant fluid (Sherwood number) is quantified by the ratio of advective nutrient transport to molecular diffusion of a nutrient (Ṕclet number, PeK). The advective transport of nutrients is described by the layer-averaged Kolmogorov velocity (ũ K). An enhanced algal growth due to fluid motion is proposed over the Ṕclet number range 6.7> PeK >1.3, with the maximal growth at PeK =2.9. Field measurements recorded by a microstructure profiler demonstrated encouraging agreement between laboratory and field findings. The current mechanistic models of phytoplankton population dynamics could consider the proposed Ṕclet number with redefined characteristic velocity scale (ũ K) in the formulation of subgrid scale closure fluxes on nutrient uptake and growth rate. Furthermore, the laboratory and field results presented in this study are intended to motivate researchers to question the validity of standard laboratory biotoxicity protocols and to modify existing procedures in the examination of effluent toxicity in the environment by including the fluid motion.