Distribution of Cladophora glomerata in the riffle with reference to the stability of streambed substrata

The relationship of the patchy distribution of an attached green alga, Cladophora glomerata, with stability of streambed substrata under flood conditions was investigated in riffles of the Tama River, Japan. The effect of the size distribution of streambed stones on the development of this alga was particularly examined. Several new stones from each of six diameter classes were marked with colored paint and submerged in a limited area. We recorded the number of stones moved out of the original area after a flood with reference to the difference in substrate size and the presence of C. glomerata. The distribution of this alga was markedly restricted to the large stones in riffles. The proportion of stones moved out by the current increased in correspondence with the increase in flow discharge. Small stones were more easily moved than large stones. On the 71st day, all stones less than 10 cm in diameter had been moved out by moderately high flood three times. On all remaining stones greater than 20 cm in diameter, C. glomerata was observed. Furthermore, the proportion of stones with C. glomerata to the remaining stones with diameters of 20 cm were 0%, 66%, and 100%, respectively. The stable duration of streambed substrata, which is strongly correlated with stone size, is important for development and distribution of C. glomerata.