Spatial distribution of aquatic organisms in a paddy field during the off-crop season under drained conditions

Survival sites of aquatic organisms ranging in size from 30 micro m to 2cm were surveyed in a paddy field under drained conditions during the off-crop season. Seven sites were selected as potential survival locations in the field: (A) dry rice straws on the ground, (B) moistened rice straws on the soil surface, (C) rice stubble, (D) 0 - 1.5cm surface soil among rice stubble, (E) 1.5 - 3.0cm surface soil among rice stubble, (F) 0 - 1.5cm surface soil beneath rice stubble, and (G) 1.5 - 3.0cm surface soil beneath rice stubble. The samples that were periodically collected from the respective sites were incubated for 50 days under flooded conditions, and the emergence of aquatic organisms during the incubation was recorded. Aquatic organisms were classified mainly on the order level. Thirty six varieties of aquatic organisms were found in the present survey. The number of the varieties of aquatic organisms that appeared was consistently larger at sites D to G than that at sites A to C. In particular, the number of the varieties at site F was the largest, while the smallest number was observed at site A. It was known that the soil was the main site of survival for aquatic organisms in the paddy field. The number of recorded samples increased with time after drainage for six varieties of aquatic organisms (Charales, Dicotyledonopsida, Lemanaceae, Oligotrichida, Podocopida, and Cyclopoida), while it decreased for four varieties (Zygnematales, Euglenida, Nematoda, and Diptera). Stichotrichida, Ploima, Chaetonotida, and Nematoda survived at all locations in the paddy field throughout the sampling time. Principal component analysis showed that the main habitats of four varieties (Dicotyledonopsida, Monocotyledonopsida, Lemanaceae, and Podocopida), two varieties (Colpodida and Bdelloida), and seven varieties (Pennales, Zygnematales, Volvocida, Actinophryda, Heterotrichida, Oligotorichida, and Haplotaxida) were soil among rice stubble, rice straws, and soil beneath rice stubble, respectively. Cluster analysis confirmed that the composition of aquatic organisms was different among the sites.