Long-term Monitoring of Vertical Water Quality in Northern Basin of Lake Biwa

The long-term monitoring of vertical water quality has been continued using the common name “Imazuokichuo” in the northern basin of Lake Biwa since 1979. The results of the main monitored parameters such as water temperature, dissolved oxygen (%), and COD, T-N and T-P concentrations from 1979 to 2004 were investigated. Annual changes in the temperature of water at depths of 0.5 and 10 m showed a tendency to increase with atmospheric temperature. The tendency of the temperature of water at a depth 90 m to increase was low in terms of the annual average, but high in terms of the annual minimum. Annual changes in the dissolved oxygen (%) of water at a depth of 90 m showed a tendency to be nearly constant in terms of the annual average but showed no clear tendency in terms of the annual minimum. Annual changes in COD concentration showed tendencies to decrease in the period from 1980 to 1985 and increase in 1985 at all depth intervals. Annual changes in T-N concentration showed tendencies to increase gradually in the period from 1979 to 1999, decrease in 2000, to be nearly constant in 2001, and markedly decrease in 2002 at all depth intervals. Annual changes in T-P concentration showde a tendency to be nearly constant at all depth intervals except 90 m.