Shoichi Mori

Mori Institute of Ecology Co., Ltd.
(3-2-7 Uchikyuhoji-cho, Chuo-ku, Osaka 540 Japan)

Abstract

Planktons of the aerated lagoon were studied and their ecological structure assessed. Only a few genera of these plankton were dominant. Phytoplankton (especially Bacillariophyceae), mostly abundant in summer, decreased with drop of temperature. The number of Rotatoria and Cillata varied with phytoplankton and bacteria biomasses. In order to elucidate the process of wastewater eutrophication, water quality and phytoplankton at the stabilization ponds of a dredged solid waste disposal site were investigated within a period of five years (from 1983 to 1987). The ponds were characterized by an eutrophication which made it possible for the phytoplanktons to always be present in a number varying from 103 to 105 (low in winter and summer, high in spring and autumn).Vertical distribution, as measured 8 times from 1984 to 1987, showed that the amounts of chlorophyll-a and that of pheophytin and p-COD were uniformly distributed along the depth investigated. However, more concentrated zones were observed at a depth of above 1.5m. These data indicate that dead cells of phytoplankton are more decomposed along the depth (especially in the upper layer). Also, Nitrogen-Phosphorus (N-P) is generated along the depth where phytoplanktons are decomposed and produced. Zooplankton numbered from one inds./l to 104inds./l and were more numerous from spring to summer. Brachionus plicatilis was dominant and Apocyclops japonensis, copepodid of Apocyclops, nauplius of Copepoda were abundant.

Key words: plankton, aerated lagoon, stabilization pond, seasonal change, vertical distribution