Nitrogen source tracking with 15N content of coastal wetland plants in hawaii

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Received for publication January 7, 2009. 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 15N values from different herbaceous wetland plants across 34 different coastal wetlands from the five main Hawaiian Islands and investigated relationships of 15N with land use, human population density, and surface water quality parameters (i.e., nitrate, ammonium, and total dissolved N). The highest 15N 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 15N values usually <4. The enrichment in 15N 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 15N vegetation values from each wetland site (r = 0.56 and 0.51, respectively; p < 0.001). This suggested that site mean 15N 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.

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