Abstract
To elucidate the mechanism underlying the development of hypoxia, we observed the in situ consumption of oxygen at the bottom of a lake and carried out an oxygen consumption experiment using lake sediments and bottom water at an oligohaline brackish lagoon, Lake Shinji. We also measured the decreases in the indices of organic materials (COD (Mn), COD (Cr) and TOC) on the basis of the decrease in dissolved oxygen concentration. During the in situ observation of the bottom of the lagoon, the polyhaline water supplied by a neighboring polyhaline lagoon, Lake Nakaumi, showed an increase in oxygen concentration due to the supply of oxygen from surface water. Because the intensity of hypoxia at the observation point was correlated with the wind velocity, we assumed that the hypoxia occurs only when the circulation within the stratification becomes weak. The decrease in oxygen concentration in the sealed bottom water did not correspond to the indices of organic materials. It took more than two days for the dissolved oxygen concentration to decrease to a value near zero during the oxygen consumption experiment with bottom water and sediments. These results suggest that preventing the long-term stratification is more effective for preventing hypoxia than reducing the organic loads expressed in COD (Mn) for Lake Shinji.
