11 - The Stimulation and Drag of Current

Abstract

This chapter explores the stimulation and drag of current. Currents in streams are significant forces in the ecology of algae in riffles and pools. Currents produce complex spatial and temporal patterns in benthic algae. Currents have stimulatory effects on algal metabolism and make large growths of filamentous green algae Cladophora possible in riffles of streams and along shorelines of lakes. However, there is not a simple positive relationship between current velocity and benthic-algal metabolism, accumulation processes, and biomass. Algal biomass is greater in fast than in slow currents in some habitats, while in other habitats, the biomass is greater in slow than in fast currents. This study indicates that the highest algal biomasses occur in intermediate current velocities in most habitats. The optimum current for benthic-algal assemblage depends on the dominant species, the nutrient concentrations in the habitat, and the density of algae in substrata. The optimum current velocity is probably higher for filamentous algae that are firmly attached to substrata and that can withstand faster currents than for benthic-diatom assemblages. The positive effects of current cannot be manifested unless sufficient light and nutrients are present.