Abstract
We proposed a method to estimate previous levels of dissolved oxygen (DO) in aquatic environments ranging from hours to an entire day, based on measured values of dissolved methane (DCH4) and nitrous oxide (DN2O). The most important aspect of our method is that it can be used to estimate previous DO levels without the need for continuous monitoring. We assumed the theoretical behavior of DCH4 and DN2O and how they would influence increases and decrease in DO, based on microbial metabolic reactions under various environmental conditions. The patterns we made corresponded reasonably well with the data obtained from an in situ wastewater treatment experiment: and we set the threshold at 0.025 mg C/L for DCH4 and 0.003 mg N/L for DN2O in order to determine the previous DO condition. From the verification of the threshold by continuous monitoring in actual ponds, at least the DCH4 threshold appeared to reflect previous anaerobic condition, and to be useful in order to determine and screen the required minute investigation site. We also made the test decision of lake environment on the data in previous reports based on the method with the threshold of DCH4, DN2O: the results were reasonable. Finally, we proposed how to examine and improve the new method form the viewpoint of such as inaccuracy of the thresholds.
