ON THE INFLUENCE OF DIURNAL STRATIFICATION ON WATER QUALITY IN A SHALLOW LAKE

Influences of diurnal stratification on water qualities in a shallow eutrophicated lake is discussed with the reference of field observation conducted in Lake Kasumigaura in 1988. The following are pointed out.Diurnal stratification makes a weak thermocline with the temperature difference of about 1°C, which prevents the vertical DO transport until the wind mixing erodes it out.The temperature profile has double-thermocline occasionally when an old thermocline formed on the day before remains in the morning. Then, the old thermocline tends to stay near the bottom for several days because the wind mixing works on only the new upper thermocline. This means that the DO supply to the bottom is cut off for several days.DO concentration near the bottom reaches zero when the old thermocline stays for three days. PO4-P is released at a high rate from the bottom after DO concentration falls below 4ppm.