The principal phosphorus cycle and budget in summer of Harima-Nada, which was divided into three sub-waters on the base of hydorographic conditions, were investigated. Phosphorus flux associated with each pathway of the cycle was estimated and schematized (Fig. 5). In the nothern sub-waters, relatively high production of phytoplankton phosphorus was supposed to be caused mainly by phosphorus inflow through rivers, waste water and its release from the sediment. However, the higher phytoplankton production measured in the productive zone of whole region was not able to be maintained only by phosphorus input from land or sediment. Vertical stability of the water column in summer resulted in the serious phosphorus depletion by phytoplankton uptake in the surface water. Therefore, the standing stock of the nutrient was not sufficient in the productive zone, but high turnover rate of phosphorus, approximately one cycle per day as phytoplankton phosphorus, was estimated. Possible pathways of the nutrient supply which enable the high turnover are supposed to be through excretion from phytoplankton cells and by zooplankton. In the bottom water, high concentration of inorganic phosphate was observed where the mineralization of organic phosphorus and/or liberation of phosphate from the sediment prevailed. In those processes, the bioturbation by benthic animals presuinably plays an important role in the nutrient supply especially in the shallow waters.