Nitrogen Source Tracking with δ 15 N Content of Coastal Wetland Plants in Hawaii

Inter‐ and 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 δ 15 N values from different herbaceous wetland plants across 34 different coastal wetlands from the five main Hawaiian Islands and investigated relationships of δ 15 N with land use, human population density, and surface water quality parameters (i.e., nitrate, ammonium, and total dissolved N). The highest δ 15 N 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 δ 15 N values usually <4‰. The enrichment in δ 15 N 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 δ 15 N vegetation values from each wetland site ( r = 0.56 and 0.51, respectively; p < 0.001). This suggested that site mean δ 15 N 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 significant anthropogenic N loading. This information can be used to monitor future changes in N inputs to coastal wetlands throughout Hawaii and the Pacific.