Nitrogen and oxygen isotopes in the aquatic realm: Applications from coral reefs to Arctic Ocean

Abstract

Nitrogen (N) and oxygen (O) stable isotope ratios (15N, 18O) have been widely used to identify sources of nitrogen in the aquatic system and to quantify the rate of N cycling. The dual isotope approach is based on the fact that nitrate from different origins have distinct isotopic signature ranges (see Kendall et al. 2008 for a review). For example, inorganic nitrate fertilizers show significantly higher 18O values compared to most other nitrate sources, whereas their 15N value is generally quite low. In contrast, nitrate derived from organic sources tend to exhibit elevated 15N values, but comparatively low 18O values. Nitrogen and oxygen isotopic compositions may also vary because of kinetic isotopic fractionation during biochemical reactions responsible for nitrogen transformation processes such as denitrification, N assimilation, and nitrification. Because each process has its characteristic mode and extent of isotopic fractionation, patterns in spatiotemporal changes in 15N and/or 18O values of nitrogenous compounds may help identify and quantify the key nitrogen transformation process (or processes) in a given system. Recently, the determination of 15N of dissolved organic nitrogen (15N-DON) in various oligotrophic regions of the open oceans has begun to improve our understanding of DON dynamics in oceanic ecosystems (Knapp et al. 2005). These studies have generally suggested that open ocean DON may be predominately refractory. In support of this notion, investigators have found that 15N-DON varied little over seasons in some oceanic regions where seasonal variation in primary productivity was large. However, the uniformity in 15N-DON as shown in the data collected over a monthly timescale does not preclude the possibility that there is a significant, pulsed flux of newly produced labile DON. Coral reefs are an excellent ecosystem to study 15N-DON because DON is the prevalent form of dissolved nitrogen and because of the potential significance of DON in reef N cycling. This study examined variations in time and space of DON concentrations and, for the first time, 15N of total dissolved organic nitrogen (15N-TDN) in the water column over Fukido Reef, a fringing coral reef located on Ishigaki Island, Japan.