δ15N of seagrass leaves for monitoring anthropogenic nutrient increases in coral reef ecosystems

In a coral reef environment, a slight increase in dissolved inorganic nitrogen (DIN; 1.0 μM) can alter the ecosystem via macroalgal blooms. We collected seagrass leaves from the tropical and subtropical Pacific Ocean in five countries and examined the interactions between nutrient concentrations (C, N, P), molar ratios of nutrients, and δ15N to find a possible indicator of the DIN conditions. Within most sites, the concentrations of nutrients and their molar ratios showed large variations owing to species-specific values. On the other hand, almost identical δ15N values were found in seagrass leaves of several species at each site. The correlations between δ15N and nutrient concentrations and between δ15N and molar ratios of nutrients suggested that nutrient availability did not affect the δ15N value of seagrass leaves by altering the physiological condition of the plants. Increases in δ15N of seagrass leaves mostly matched increases in DIN concentrations in the bottom water. We suggest that δ15N in seagrass leaves can be a good tool to monitor time-integrated decrease/increase of DIN concentrations at a site, both in the water column and the interstitial water.