Seasonal Variations of Planktons at Lake Ikeda

At Lake Ikeda, a typical oligotrophic lake in southern Japan, the transparency of waters and the aspects of plankton organisms have shown a remarkable change for past forty years. In 1929, the transparency showed 26.8m by a Secchi disk and the appearances of plankton were also very rare. In 1963-1966 obtervations, the transparency was between 11m and 5.5m and it was 7.5m in average. On the other hand, plankton volume increased remarkably, showing vivid seasonal variations every year. Some dominant species are as follows. Melosira italica, Melosira granurata, Suriella, Fragillaria, Staurastrum asterias, Staurastrum spp. Pediastrum duplex, Bosmina longirostris, Bosmina coregoni, Cyclops strennus, Thermocyclops hyalinus, Acanthodiaptomus pacificus, and many kinds of Rotatoria such as Asplanchna, Brachionus, Synchaeta. This abundance of plankton in recent years may be used as an indicator showing that Lake Ikeda is changing gradually from oligotrophic to eutrophic lake in these forty years