In Osaka Bay, water pollution phenomena such as red tide and blue tide still occur frequently, in spite of the efforts to improve water qualities for years. To make a decision about effective environmental measures in the bay, it is necessary to clarify the budget and cycle of nutrients in the waters. In this study, focusing on the water mass intruding from the outer ocean, a cluster analysis using salinity, nutrient concentrations, and stable isotope ratio δ18O and δD is conducted. The cluster analysis divides the coastal sea waters in and around Osaka Bay into six groups, including the clusters related to Kuroshio surface water and sub-surface ocean water, and the results show that the sub-surface ocean water had intruded into the Kii Channel due to off-shore migration of the Kuroshio Current in spring 2009.
