Abstract
In eutropic lakes or coas t a l waters, a marked diurnal variation of oxygen content is found. There are two types of v a riations. In one of them, a minimum occurs mostly at about 6h and a maximum at about 17h. This type of variation is often seen in stable and homogeneous water masses. In the other type, minimum and maximum occur at different hours from the above-mentioned. In this case, the influence of diffusion and advection or convection upon the diurnal variation of oxygen content must be considered. In this paper, the former case is chiefly treated. In this case, the amplitude of the diurn a l variation depends upon the intensity of radiation, the d u r ation of insolation, water temperature, number and size o f phytoplanktons, etc. Indeed, the amplitude is larger on a clear day than on, a cloudy day and it is also larger in summer than in winter. It was found by calculation that the diurnal variation should have a minimum and a maximum at a certain time distance respectively before and after the noon. By using the data of the diurnal variations of dissolved oxygen, the amoun t of the oxygen production or consumption during a whol e day is computed. Assuming a certain size and number of phytoplanktons, computed values agreed well with the observation.
