The Chlorophyll Amount in the Euphotic Zone in Some Japanese Lakes and its Significance in the Photosynthetic Production of Phytoplankton Community

The variation of chlorophyll-a amount in euphotic zone was studied in some lakes of central Japan with special reference to the effect of factors controlling it, and moreover the significance of the variation of chlorophyll amount in the photosynthetic production of community was examined.1. The large chlorophyll-a amount of euphotic zone could be found in shallower eutrophic lakes (maximum value was 106.0mg/m2 in Lake Suwa), but not in deeper oligotrophic lakes (minimum was 10.5mg/m2 in Lake Motosu). The depth of euphotic zone was shallower in the former and deeper in the latter. A definite relation could not be found between the depth of euphotic zone and chlorophyll-a amount of euphotic zone.2. The optical mass of pure water (γDe), of phytoplankton (εAe) and of the other substances (Se) throughout the whole euphotic zone was calculated and compared each other. εAe was the smallest and Se was the largest. It was concluded that the substances responsible for the variation of Se value had the largest influence on the attenuation of underwater light and therefore the depth of euphotic zone.3. The relations of the chlorophyll amount of euphotic zone (Ae) to Se values and to the mean chlorophyll concentration of euphotic zone were examined. A clear relation could not be found between Ae and Se. Between Ae and the mean chlorophyll concentration, an approximate proportionality existed in the waters of low chlorophyll concentration, but not in those of higher chlorophyll concentration. It was concluded that the difference in chlorophyll concentration had a great significance in the variation of Ae in oligotrophic lakes.4. The relation between the chlorophyll-a amount of euphotic zone and the daily net production rate measured in situ was examined. An approximately linear relation could be found between both variables.5. The production rate of community per unit chlorophyll-a amount varied with waters and seasons. The range of the variation in the production rate per unit chlorophyll-a amount was generally smaller than that in the chlorophyll-a amount of euphotic zone.