Diurnal variation in CO₂, dissolved oxygen (DO), pH and RpH and their correlations in ponded paddy field water

Abstract

We determined that the levels of CO₂, dissolved oxygen (DO), pH and RpH in ponded paddy field water changed with solar radiation. In the ponded water, three types of relationships were recognized among these elements during diurnal variation. In phase I with increasing solar radiation, CO₂ decreased and DO and pH increased, with photosynthesis by photosynthetic bacteria and micro-algae. The relationship between CO₂ and DO was almost linear, while that between CO₂ and pH was nearly exponential. In phase II, solar radiation was present but there was essentially no CO₂ in the ponded water, despite changes in this radiation. After DO and pH peaked, they gradually decreased. In phase III, CO₂ increased with DO and pH. These physical factors were mutually related, and each relationship showed hysteresis. However, it was difficult to explain all these phenomena by only chemical changes in the ponded water. To clarify the hysteresis relationships, further studies should examine in detail the roles of cyanobacteria and micro-algae in these physical phenomena in the ponded water.