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.

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It is generally accepted that the bioavailability and toxicity of trace metals in surface water to aquatic life depend on the speciation of metal, not on total or dissolved metal concentration. Metal speciation is a function of water chemistry including temperature, pH, organic content, and the compositions and concentrations of ions and solid phases in solution. To develop efficient and effective methods of assessing and managing the risk posed by metals to aquatic life, it is important to determine the effects of water chemistry on the bioavailability of metals in surface water . For this reason, we have employed the technique called diffusive gradients in thin-films (DGT) in the study of several Japanese rivers, including meta-contaminated rivers adjacent to abandoned mines, to gain information on the bioavailability of metals in Japanese water systems. The measurements of labile metals as determined by the DGT technique suggest that the biologically available fractions of metals in urban rivers are relatively low compared with those in rivers adjacent to abandoned mines. The fractions of Zn and Cd appear to be higher than that of Cu. An inverse relationship was found between labile- Cu fraction and dissolved organic carbon (DOC). Our result suggested that large amount of Zn and Cd occurred as hydrated ions or small inorganic complexes in water of the rivers studied, whereas most of the Cu occurred as Cu-organic complexes. This study demonstrates the importance of obtaining metal speciation data in the pragmatic risk assessment and management of metals in water environments, especially in urban rivers with high natural and anthropogenic organic contents.

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