Abstract
An attempt is made to calculate the absolute amount of daily photosynthesis in submerged plants at various depths and under different weather conditions on the basis of photosynthesis-light intesity curve and surface and subsurface light intensity records, with reference to transparency and light extinction coefficient of water. Theoretical daily gross photosynthesis-depth curves well correspond to the vertical distribution of daily photosynthesis in natural communities of submerged macrophyte and that of phytoplankton in stagnation period of lakes and ocean.A logical interpretation is given to the empirically known correlation between compensation depth for aquatic green plants and transparency of water. Daily compensation depth for different seasons and weather conditions are in inverse proportion to the corresponding light extinction coefficient of water, or in proportion to the value of transparency. The monthly mean of daily compensation depth/transparency ratio for Elodea occidentalis in Lake Biwa is 2.0, 2.4 and 1.4 in January, April and July-August respectively. The ratio for Chlorella ellipsoidea ranges from 0.9 (rainy day, January) to 2.5 (fine day, July-August). It is noted that compensation depth is usually about twice as large as transparency. Considering that physiological activities of shoots, especially photosynthesis, are less active in basal parts of submerged macrophyte community than in upper parts, it is concluded that the daily compensation depth for the whole community might be considerably smaller than the values here calculated for apical parts of shoots.
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