FIELD OBSERVATIONS OF DO CONCENTRATION VARIATION IN NAGATSURA-URA LAGOON ON SOTHERN SANRIKU COAST

Nagatsura-ura Lagoon is located at the southern end of Sanriku Coast, Japan with the area of 1.41km2 and the maximum water depth of 10m. The lagoon area has been used for cultivating oyster for these several decades. In the recent years, however, the appearance of anoxic water causes severe damage to the fishery industry in this area. In the present study, filed observations of water quality, such as water temperature, salinity, dissolved oxygen (DO) concentration, and phytoplankton were carried out every one week in Nagatsura-ura Lagoon to investigation detailed process of occurrence of anoxic water and its decline. During the course of continuous measurement for 6 months from May to November 2002, abrupt recovery of DO can be measured. Based on the analysis of phytoplankton, it is concluded that the recovery was induced by water exchange between the lagoon and the sea through a narrow channel connecting these two.