Short-term variations of microbiological and physicochemical parameters in submersion water over a rice field.

Diurnal variations of the number of colony-forming bacteria and total bacteria and of some physicochemical parameters (temperature, pH, redox potential, oxygen concentration, carbonates and total alkalinity) have been studied in the flood layer over the rice field. Confidence limits (95%) around means were evaluated according to accuracy of plate and microscopic counts and to patchiness within the rice field. Bacterial clumping both at the meter and the millimeter scale has been evidenced, and transformation of raw data was necessary to normalize counts. Seven per cent of total bacteria enumerated by epifluorescence microscopy could develop colonies on agar plates. Significant variations of numbers of colony-forming bacteria and bacterial biomass (evaluated to 0.016 mg/l dry-weight) were not correlated with physicochemical parameters within the flood layer, indicating additional influence of grazing and exchange with sediment interface.