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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 was Cry ptomonas sp. and it showed a characteristic shade type in the photosynthesis-light curve.The photosynthetic activity of Cry ptomonas sp. at low light intensities and low temperature was remarkably higher than that of other phytoplankters.From the photosynthesis-light curve of Cry ptomonas sp. 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 measured in situ.It was thus concluded that the oxygen maximum in the metalimnion of Lake Haruna is due to the special photosynthetic characteristics of Cry ptomonas sp. 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 of Cryptomonas and not to the accumulation of the deteriorated phytoplankton sedimented from the surface layer.The majority of oligotrophic or mesotrophic lakes in Japan have a marked oxygen maximum in their middle layers'.In some situations, especially in mesotrophic lakes,, a supersaturation of oxygen appears occasionally in the upper layer of the metalimnion. These lakes are usually small and deep ones in the mountain districts and many of them are protected from the disturbance by strong wind. Yoshimura'> has propounded the opinion that the full development of metalimnetic oxygen maximum is due partly to warming of water in the epilimnion by solar radiation and partly to the photosynthetic activity of phytoplankton in the stable layer in question. Recently, a more detailed study was made by Eberly2'3> in plusheterograde lakes of northern Indiana.He measured the metalimnetic oxygen maximum greater than 12.7 ppm and also found an immense bloom of Oscillatoria agardhii in the metalimnion. The layer of oxygen maximum, however, did not always coincide with that of photosynthetic maximum found by the in situ experiments.Wetzel4> reported that in aa lake of Indiana the major portion of the primary productivity occurred in the metalimnion where oxygen maximum developed and the dominant phytoplankton of the metalimnion were Ceratium hirundinella and Oscillatoria agardhii.It has generally been said that in the stagnant layer of lakes the supersaturation

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