Seasonal variations in the community structure of aquatic organisms in a paddy field under a long-term fertilizer trial

To estimate the effect of fertilizers on aquatic organisms ranging in size from 30 mum to 2 cm in a Japanese paddy field under a long-term fertilizer trial (the plots without fertilizers, with chemical fertilizers only, and w ith chemical fertilizers as well as two levels (7.5 and 22.5 tons ha(-1)) o f rice straw compost), we collected the floodwater samples once every 10 d during the cropping season throughout 3 y and mainly classified aquatic org anisms in the floodwater at the order level. The frequency of the presence of Chlorococcales, Lemanaceae, Euglenida, Oligotrichida, Pharyngophorida, S essilida, Turbellaria, Podocopida, and Cyclopoida was significantly differe nt among the plots, while the maximum population density was significantly different for Cladocera and Cyclopoida among the plots. The effects of the sampling year on the frequency of presence and the maximum population densi ty were also observed. The floodwater samples after midseason drainage in t hree plots with fertilizer application differed from the samples in the plo t without fertilizer application in the community structure of aquatic orga nisms. Principal component analysis showed that Lemanaceae, Pharyngophorida , and Turbellaria were important components in the floodwater samples after midseason drainage in the three plots with fertilizer application. Althoug h Cladocera appeared more abundantly after midseason drainage in the plot w ithout fertilizers and the plot with chemical fertilizers only, it appeared more abundantly before midseason drainage in the two plots with composts. Shannon-Weaver's index of diversity (H') was higher in the three plots with fertilizers than in the plot without fertilizers. The effect of fertilizer application on the community structure and the diversity of aquatic organi sms was more conspicuous than that of the sampling year.