Nitrogen Source Tracking with δ15N Content of Coastal Wetland Plants in Hawaii

Interand intra-site comparisons of the nitrogen (N) stable isotope composition of wetland plant species have been used to identify sources of N in coastal areas. In this study, we compared δ15N values from diff erent herbaceous wetland plants across 34 diff erent coastal wetlands from the fi ve main Hawaiian Islands and investigated relationships of δ15N with land use, human population density, and surface water quality parameters (i.e., nitrate, ammonium, and total dissolved N). Th e highest δ15N values were observed in plants from wetlands on the islands of Oahu (8.7–14.6‰) and Maui (8.9–9.2‰), whereas plants from wetlands on the islands of Kauai, Hawaii, and Molokai had δ15N values usually <4‰. Th e enrichment in δ15N values in plant tissues from wetlands on Oahu and Maui was most likely a result of the more developed and densely populated watersheds on these two islands. Urban development within a 1000-m radius and population density were positively correlated to average δ15N vegetation values from each wetland site (r = 0.56 and 0.51, respectively; p < 0.001). Th is suggested that site mean δ15N values from mixed stands of wetland plants have potential as indices of N sources in coastal lowland wetlands in Hawaii and that certain sites on Oahu and Maui have experienced signifi cant anthropogenic N loading. Th is information can be used to monitor future changes in N inputs to coastal wetlands throughout Hawaii and the Pacifi c. Nitrogen Source Tracking with δ15N Content of Coastal Wetland Plants in Hawaii