Dynamic evolution of nutrient discharge under stormflow and baseflow conditions in a coastal agricultural watershed in Ishigaki Island, Okinawa, Japan

Abstract

Excessive terrestrial nutrient loadings adversely impact coral reefs by primarily enhancing growth of macroalgae, potentially leading to a phase-shift phenomenon. Hydrological processes and other spatial and temporal factors affecting nutrient discharge must be examined to be able to formulate effective measures for reducing nutrient export to adjacent reefs. During storm events and baseflow periods, water samples were obtained from the tropical Todoroki River, which drains an intensively agricultural watershed into Shiraho coral reef. In situ nutrient analyzers were deployed for 6 months to hourly measure dissolved nutrient (NO3--N and PO4 3--P) concentrations. Total phosphorus (TP) and suspended solid concentration (TSS) were increased by higher rainfall intensity (r = 0·94, p < 0·01) and river discharge Q (r = 0 88, p < 0 01). In contrast, NO3--N concentration tends to decrease drastically (e.g. from 3 to 1 mg l-1) during flood events. When base flow starts to dominate afterwards, NO3--N manifested an increasing trend, but decreases when baseflow discharge becomes low. This counter-clockwise hysteresis for NO3--N highlights the significant influence of groundwater discharge. N delivery can therefore be considered a persistent process compared to sediment and P discharge, which are highly episodic in nature. Based on GIS analysis, nutrient concentration along the Todoroki River was largely affected by the percentage of sugarcane/bare areas and bedrock type. The spatial distribution of N concentration in the river reflects the considerable influence of subsurface geology-higher N levels in limestone-dominated areas. P concentrations were directly related to the total length of artificial drainage, which enhances sediment transport. The use of high-resolution monitoring data coupled with GIS-based spatial analysis therefore enabled the clarification of control factors and the difference in the spatio-temporal discharge characteristics between N and P. Thus, although erosion-reduction schemes would reduce P discharge, other approaches (e.g. minimize fertilizer) are needed to reduce N discharge. Copyright © 2010 John Wiley & Sons, Ltd.