Analysis of Nitrogen and Phosphorous Load Dynamics in Kako and Ibo River Basins During Flood Events

Abstract

　In the Seto Inland Sea, eutrophic and oligotrophic conditions attributable to excess or deficiency of nutrients (nitrogen and phosphorus) have been observed. The Harima-Nada, located in the eastern part of the Seto Inland Sea, is strongly affected by land-derived nutrients. Therefore, accurate estimation of rivers' nutrients load during rain events is desirable for appropriate nutrient management. For this study, changes in nutrients load in the Kako and the Ibo rivers, which flow into the Harima Nada, were investigated for rain events during 2015 and 2017. Total nitrogen (TN), dissolved total nitrogen (DTN), ammonium nitrogen (NH4-N), nitrite nitrogen (NO2-N), nitrate nitrogen (NO3-N), total phosphorus (TP), dissolved total phosphorus (DTP), and phosphate phosphorus (PO4-P) concentrations in river water samples were measured. The maximum observed increments in comparison to the baseflow conditions for TN and TP loads during a single rainfall event were, respectively, 28 and 76 times. Although TN and TP concentrations showed an increment when the river discharge increased, some other studied species showed no similar behavior. Differences between the rivers were also found. The relation between the river flow rates and the load of NO3-N and PO4-P in the Kako River showed a counterclockwise hysteresis effect for each of these nutrient forms. Results of this study suggest that a numerical hydrological model that incorporates effects of rain events is an effective tool for nutrients management in the Seto Inland Sea.