The paper attempts to give an account of the periodicity of algae and its possible causes, in two freshwater ponds of Hyderabad . The physico-chemical complexes of these waters and the relative abundance of various unicellular and colonial forms have been described earlier (ZAFAR, 1964 a & b) .
Chloroccocales
This order is represented mainly by species of Characium, Tetraedron, Oocystis, Kirchneriella, Ankistrodesmus, Pediactrum, Sorastrum, Cruciginia, Scenedesmus and Coelastrum. As far as qualitative distribution is concerned, all these forms are present in both the ponds under investigation, but quantitatively pond B appears to be a better habitat for their development .
Higher atmospheric or water-temperature is considered to be one of the important factors in the periodicity of Chlorococcales . GRIF-FITHS (1912), FRITSCH & RICH (1913), WIEBE (1930), HOWLAND (1931), TRESSLER & DOMOGALLA (1931), HODGETTS (1922) and RAO (1955) have observed maximum growth of one or the other species of Chlorococcales during warmer months of the year . In the present ponds as well, water-temperature appears to play a somewhat important role in the periodicity of this order . In pond A Chlorococ cales start increasing in number in early summer (February), reach their optimum in the middle of April, and become rare in the second week of May . The second maximum of Chlorococcales commences after the break of monsoon (August) when the water-temperature falls to about 25Β°C, but this does not last longer than two to three weeks . They again tend to increase at the end of the rainy season