Composition and structure of aquatic organism communities in various water conditions of a paddy field

The purpose of this study was to clarify the differences of the community structure and the diversity of aquatic organisms (i) among sampling sites that the distances from inlets or outlets were different each other, and (ii) between the floodwater and the irrigation water during the crop season in a paddy field. The irrigation water was sampled from one inlet. The taxonomical groups and the number of aquatic organisms ranging in size from 30 mum to 2 cm in the floodwater and the irrigation water were surveyed approximately every 10 days during the growth period of the rice plant. Aquatic organisms were classified mainly at the order level. Thirty-eight taxonomical groups of aquatic organisms were found in the floodwater, while 18 groups were found in the irrigation water. We were not able to find the differences of the community structure of aquatic organisms among the sites. In the floodwater, the number of taxonomical group increased and the community structure changed during the late flooding period (over 50 days after the onset of flooding) at any site, while those in the irrigation water hardly changed. Although the community structure of aquatic organisms differed between the floodwater and the irrigation water throughout the flooding period, the differences became especially bigger during the late flooding period. Principal component analysis showed that three groups (Pennales, Dinoflagellida, Choreotrichida) characterized the community structure in the irrigation water. Their population densities tended to be the highest at the site near inlets and the lowest at the site far from inlets.