Effects of Boundary Layer near the Substratum on the Periphyton Growth: an Experimental Study

Sufficient nutrient supply is one of the key factors for the periphyton growth. The formation of the viscous sublayer near the substratum, however, extremely hampers the nutrient transport through the layer with its low diffusivity. Under two different velocities in a laboratory flume, the transition of Periphyton biomass and species were measured, together with light and nutrient conditions. A higher amount of biomass, mostly Navicula in both currents, occurred in the fast current than in the slow current, although nutrient concentrations in an upper layer and light conditions were almost the same. The estimation of the nutrient diffusion rate indicates that the absorbed amount by the periphyton mat is almost equal to the quantity transported by diffusion. Thus, with its higher diffusion rate, the higher biomass of periphyton occurred in the fast current. This result implies that, under the nutrient limited condition, the periphyton growth may be regulated by the nutrient diffusion rate in the viscous sublayer rather than the absolute concentration in the upper layer.