Flow-velocity-dependent effects of turbid water on periphyton structure and function in flowing water

Abstract

Land-use change, human activities, and dam management supply highly turbid water with high flow velocity; however, little is known about its effect on river ecosystems. Here, we studied the effects of flow velocity and suspended silt concentration on periphyton structure and function using experimental channels. The effects of flow velocity on algal biomass in the periphyton community depended on the suspended silt concentration; flow velocity decreased chlorophyll a in low concentrations of suspended silt (clear water), but not in moderate or high concentrations (turbid water). Filamentous cyanobacteria density was significantly influenced by flow velocity, whereas densities of non-filamentous cyanobacteria, green algae, and diatoms were not. Thus, flushing effects on attached algae, especially filamentous cyanobacteria, were exerted in clear but not in turbid waters, perhaps because algal assemblages were resistant to water current through the binding of large amounts of silt by extracellular polymeric substances. Inorganic matter and its ratio in the periphyton community decreased with flow velocity, but increased with suspended silt concentration, irrespective of flow velocity. Periphyton function, assessed by maximum photosynthesis rates and light-use efficiency, was influenced by flow velocity and suspended silt concentration through changes in periphyton structure. Flood events would be expected to refresh the periphyton community by removing senescent algae and deposited fine particles. However, we demonstrated that algae and silt in the periphyton community exposed to turbid waters with human-induced silt may accumulate, irrespective of flow velocity.