The Nagase-gawa, a remarkable acidtrophic river, is the largest one among the inflows of Lake Inawashiro-Ko. The attached algal flora of this river and its tributaries were studied in July, 1972 and May, 1973. Values of pH of water at stations 1,2 and 3 were very low. At these stations, the standing crops were poor and the floras were different from those of the other stations. At these three staions, diatoms, green algae and a protozoan were found abundantly. It seems to be interesting that blue-green algae can not be found at all at the stations where pH values of water are low, though at the stations (Sts. 4,5,7 and 8) where the pH values are nearly neutral, Homoeothrix janthina and the other blue-green algae were found in abundance.
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