On the Oxygen and Chlorophyll Maxima Found in the Metalimnion of a Mesotrophic Lake

Abstract

In Lake Haruna (mesotrophic lake) the oxygen and chlorophyll maxima were found in the metalimnion. Dominant species of phytoplankton in the low layer of the euphotic zone wasCryptomonassp. and it showed a characteristic shade type in the photosynthesis-light curve. The photosynthetic activity ofCryptomonassp. at low light intensities and low temperature was remarkably higher than that of other phytoplankters.From the photosynthesis-light curve ofCryptomonassp. obtained in the laboratory and depth-light profile observed in the lake, the depth-photosynthesis relation of the alga was calculated and it was found to be in good accordance with the relation measuredin situ.It was thus concluded that the oxygen maximum in the metalimnion of Lake Haruna is due to the special photosynthetic characteristics ofCryptomonassp. living in that lake which is adapted to the environment of low light intensity and low temperature prevailing at the foot of the euphotic zone.The lake was found to show the chlorophyll maximum in the deeper layer. This was attributed to the intensive growth ofCryptomonasand not to the accumulation of the deteriorated phytoplankton sedimented from the surface layer.