Diurnal stratification and its effects on wind-induced currents and water qualities in Lake Kasumigaura, Japan

Diurnal stratification and its effects on wind-induced current and water qualities in a shallow eutrophicated lake are discussed based on the field data obtained in Lake Kasumigaura (36° N, 140° E, Japan). Diurnal stratification makes a weak thermocline with the temperature difference of about 1°C. Because a thermocline prevents the vertical momentum transport, it inevitably creates the two-layered flow pattern. The temperature profile has double-thermocline when the thermocline formed one day remains in the following morning. Then, the old thermocline tends to stay near the bottom for several days because the wind mixing works only on the new thermocline developed at upper elevation. DO concentration near the bottom markedly decreases under such condition, because the vertical DO transport is suppressed by the thermocline. P04–P is released at high rate from the bed sediments after DO concentration falls below 4 ppm. The entrainment rate is discussed with reference to the basic models which are the turbulent erosion model (TEM) and the dynamic instability model (DIM). It is concluded that DIM, rather than TEM, is suitable to explain the mixing of diurnal stratification observed in this study. The entrainment rate E[Um] is found to vary as R i[U *]-1, where E[U m]= W e/U m, W z is the entrainment velocity, Um is the vertically averaged velocity in the mixed layer, R i[U *] = B/U * 2, B is the buoyancy of the mixed layer and U * is the friction velocity at the water surface.