1. As the water of a shallow pond, Sarusawa-ike, in the city of Nara has suddenly become brown in color in February, 1955, the author has studied this phenomenon in regard to the water color and phytoplankton, what changes took place in the water color and phytoplankton during the course of recovering of the color from brownish to normal.
2. The water colors were measured by the method used by YAMAZAKI and WATANABE (1955) and the results obtained were indicated by the C. I. E. color language (Table 1). The brightness (Y) and excitation purity (Pe) became greater in February than in the other months. It is presumed from this fact that the reflected light from the upper layer of water is greater in February than in the other months.
3. The curves of spectral transmittance to the pond water in February (Fig.1) have shown no similarity to the curves of that to individual cells of both
Anabaena subcylindrica and brown-colored
Tracherornonas Dybowskii (Fig. 2). However, from the fact that
A. subcylindrica was the dominant species more than 90 percent in composition of the plankton, its occurrence seemed to be restricted to the period of February. In that time the water color was markedly brown.
4. The change of phytoplankton in this pond from February to July is shown in Fig. 2. From this and a normal condition observed in 1949 suggest that the recovery of water color to normal condition will be in July. It will be clear in Table 2 that there are two types of change in the representative species. i. e. the one is that which is due to the seasonal succession of both Anabaena subcylindrica and
Lyngbya contorta, and the other seems to be caused by the change of
Microcystis aeruginosa,
Melosira italica and
Synedra berolinensis in the course of recovering from uncommon to the common condition.
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