INFLUENCE ON THE RIVERBED'S BIOFILM BY THE CHANGE OF MAINTENANCE FLOW DISCHARGE

In the Ohyama River located in the upper Chikugo River, the maintenance flow discharge has been regulated to be 4.5m3/s from mid-March to early October and 1.8m3/s in the remaining term since 2002. To assess the influence on the river ecosystem by the change of discharge, we sampled river water for analysis of nutrients and biofilm at an early development stage for identification and counting of periphyton under a microscope, at flat shallow before and after decreasing the discharge. The results show a difference of periphyton composition between not only flow conditions, but also positions, that is, the center line of stream and the side pool. Several species of Planktonic algae which are found in the upper dam was identified, and the most were dead or broken down. Thus, they would influence the ecosystem in the biofilm as an organic load. In the dead water zone, filamentous algae glowing greatly caused nutrients depletion for the short term