Abstract
To see dynamics of organic matter supplied from a dam reservoir to a stream, we studied changes in phytoplankton abundance, composition and physiological activity in Kanogawa Dam Reservoir and Hiji Stream between July 2004 and June 2005. For physical variables, water temperatures from the upper to the lower stream were less variable, whereas current velocities at upper stream stations were higher than those at lower stream stations. This suggests higher mixing turbulence in stream water at the upper stream stations. Chl.a concentration remained almost constant or slightly increased from the reservoir to the nearest station in the stream, and showed no marked differences at five stations within the stream. Algal physiological activity was also almost constant from the upper to the lower streams. By contrast, remarkable changes in phytoplankton composition were detected from the upper to lower stream. Cell densities of the cyanobacterium Microcystis and the diatom Fragilaria decreased from the upper to lower streams. By contrast, those of the diatoms Cymbella and Cyclotella increased, and the attached diatom Cocconeis showed almost constant cell densities from the upper to lower streams. The results suggested that water velocity and mixing turbulence are important for changes in phytoplankton composition in the stream.
