Dynamics of oxygen consumption during the formation of the anoxic zone in aquatic environment

Anoxic environments and communities of anaerobic organisms are encountered in aquatic environments and biotechnological reactors. Because of their importance, they are continuously studied. In this study, the dynamics of oxygen removal were observed during experiments reproducing the formation of the anoxic zone. Seven experiments were performed in an aquarium (volume: 60 l) with bottom sediments and water collected from different aquatic environments (river, pond, eutrophic lake, sea). To exclude reaeration, the water was isolated from the air by a layer of liquid paraffin. Below the paraffin layer the water was periodically mixed with a stirrer and sampled for oxygen concentration. Initially, a high rate of oxygen consumption was observed. Later, at low oxygen concentrations, the oxygen removal rate switched to a much lower one. Anoxic conditions were observed after 4-20 days of incubation, depending on the experiment. The point at which the microbial community converted from aerobic respiration to anaerobic metabolism was distinct and was observed at an oxygen concentration of 0.26-1.41 mg/l, depending on the experiment. The experiments were accompanied by bacterial counts and analyses of ciliate communities. The study indicates how the disappearance of oxygen during anoxic zone formation should be modeled, and provides data on the oxygen removal rates associated with aerobic and anaerobic communities of microorganisms.