Characterization of the water quality of dam lakes on Shikoku Island, Japan

This study clarifies the distribution and variation of bioelements in dam lakes of the Shikoku region, Japan. The survey was carried out from 2002 to 2003 on three dam lakes in the Shikoku Mountains with high precipitation levels and on three dam lakes in the Sanuki mountain range with low precipitation levels. By the conventional classification for eutrophication, the dam lakes in the Shikoku Mountains were classified as oligotrophic to mesotrophic, whereas those in the Sanuki mountain range were classified as mesotrophic to eutrophic. Furthermore, for the dam lakes in the Sanuki mountain range, it was indicated that the amount of precipitation directly and indirectly affected lake water quality, such as high nitrogen concentration, low dissolved oxygen concentration, and increase in primary production. Especially, in a hypertrophic dam lake, during the period of low precipitation the surface water flowed out and the water under the thermocline, with no dissolved oxygen, appeared on the lake surface. As a result, the anaerobic environment extended across all the layers. Because a compulsory water-level operation was carried out in the dam lake, it was considered that the effect of the hydrological environment in the catchment area on the dam lake was relatively large in comparison with that in natural lakes.