In order to reveal the quality and characteristics of lagoon waters in Niigata Prefecture in recent years, and to investigate the behavior of the pollutants in lagoon waters, dissolved organic carbon (DOC), complexing capacity, the concentration of major ions and oxygen stable isotopic ratios (i.e., delta O-18) in some lagoon water samples in Niigata Prefecture were measured. Moreover, pH and electric conductivity (EC), dissolved oxygen (DO) and water temperature were measured in the field. Lagoon water samples were usually monthly taken from 2004 to 2006 at the fixed sampling point from 2 lagoons (i. e. Sakata and Toyanogata) in Niigata Prefecture.
Consequently, the following matters have been mainly clarified. (1) 6180 values of lagoon water samples is generally larger than that of river water samples. In particular, delta 1(8)O values in Sakata lagoon increase remarkably in summer. It can be closely related to the activity of Crustacea plankton as well as due to the effect of the supply of groundwater. (2) The values of DO and DOC of water samples in Sakata increase in summer. It might be attributable to the photosynthesis of aquatic plants. For the same reason, it can be considered that the complexing capacity of the samples in Sakata also increases in summer. (3) The relationship between delta O-18 values and the concentration of HCO3- can be a good indicator for the degree of the inflow from groudwaters to lagoon under the same geological condition. (4) The relationship between delta O-18 values and the DOC was relatively good in the water samples in Sakata.