Abstract
Lake Ogawara is one of large brackish water lake upon which sea water ascends and trespasses, located in the Pacific Ocean shore of the Aomori southeast. We investigated the water qualities in the lake from August 2002 to December 2003 in order to evaluate the effect of the seawater intrusion and the trophic level in the lake. The vertical profiles of salinity at the center indicated a halocline was always found near the depth of 18 meters throughout the year. Dissolved Oxygen (DO) concentration in the salt layer below the halocline ranged from 0.5 to 2.0 mg/L throughout the observation period. The salt waters near the bottom were almost under hypoxic or anoxic environments. The horizontal profiles of the concentration of chemical oxygen demand (COD) and chlorophyll-a (chl-a) showed the seasonal variation in the surface water near the lake entrance and the center. In the southwest part of the lake near the entrance, COD and chl-a concentrations were highest in summer, while they maximized in April in the center of the lake. All the observed data in the bottom were compared with three standard values which divided the trophic level of lake into three categories. The water quality in the lake is about to approach to eutrophic level.
