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

  • Blanco A
  • Nadaoka K
  • Yamamoto T
 et al. 
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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 (NO(3)(-)-N and PO(4)(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, NO(3)(-)-N concentration tends to
decrease drastically (e.g. from 3 to 1 mg 1(-1)) during flood events.
When base flow starts to dominate afterwards, NO(3)(-)-N manifested an
increasing trend, but decreases when baseflow discharge becomes low.
This counter-clockwise hysteresis for NO(3)(-)-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 (C) 2010 John Wiley & Sons, Ltd.

Author-supplied keywords

  • Agricultural Watershed
  • Hydrochemistry
  • Nitrate
  • Nitrogen
  • Phosphorus

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