To assess the contribution of organic matter produced in the flood water to the fertility of a rice soil, the primary productivity and the algal biomass were examined throughout one crop. Primary productivity was estimated from the diurnal curve of dissolved oxygen. Just after transplanting, an algal bloom developed due to fertilizing or ploughing or both. Subsequently, when submerged weeds occupied the whole paddy, no distinct algal growth was found. At the ripening stage, the rice plant canopy suppressed the growth of aquatic plants. Benthic algal biomass did not change much throughout the crop period. The standing crop of algae ranged from 2 to 114 kg/ha fresh weight, while the maximum standing crop of submerged weeds (Najas sp., Chara sp.) was 400 kg/ha dry weight. The primary productivity of the flood water community was high (0.6-3.3 g O2m-2day-1) and equivalent to productivity values in eutrophic lakes. The total gross primary production of the flood water community during the cropping period corresponded to 10% and 15% of that of rice plants in the fertilized and non-fertilized plots respectively. Considering the movement of CO2 in the flood water, it is suggested that photosynthetic activity in the flood water prevents surplus CO2 from being lost.
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