Agricultural area is one of the major non-point sources of phosphorus pollutants in Japan, as in many countries. In this study the runoff loading of phosphorus from agricultural lands located in the Mikawa Bay watershed in central Japan was evaluated to clarify its impact on coastal areas. Water quality and the discharge of rivers in the area were investigated in 2002. Phosphorus was mainly discharged as dissolved form in dry periods while particulate phosphorus (PP) was discharged in great

more »

... tities in storm periods in the areas. The runoff characteristics of phosphorus were the washout type, which means that the concentrations increase with the increase of discharge. There was a positive correlation between the runoff characteristics of PP and the proportion of agricultural land use. Results of experimental desorption tests with soil of the agricultural fields suggested that desorption of phosphorus from the soil can quickly occur in coastal areas, depending on the pH and salinity of the water body. It was estimated that about 10% of the loading of phosphorus from the areas could be quickly changed from particulate form to dissolved form in seawater.