Stable nitrogen isotope composition in sedimentary organic matter as a potential proxy of nitrogen sources for primary producers at a fringing coral reef

The stable nitrogen isotope ratio (δ 15N) in macroalgae is effectively used as a time-integrated bioindicator to record nitrogen sources for primary producers during their growing periods in aquatic ecosystems. However, the utility of this tool is limited because the occurrence of these organisms is often restricted in space and time. To investigate the potential of chemical composition in sedimentary organic matter (SOM) as a proxy for time-integrated environmental conditions, nitrogen (N) and carbon (C) contents and their stable isotope ratios (δ 15N and δ 13C) were determined, and systematically cross-checked against corresponding values in macroalgae at the Shiraho fringing reef in Okinawa, Japan. Preliminary trials showed that δ 15N in SOM processed by the “wash-out method” for δ 13C analysis yielded similar δ 15N values to the bulk sediment, despite the loss of some SOM during the process. The amounts of organic matter and the ratio of the HCl-insoluble portion were variable within the reef, probably reflecting local vegetation and subsequent decomposition. The distribution of δ 15N and δ 13C in SOM showed similar trends to those of macroalgae, with mostly constant differences of 1.4‰ and −6.7‰, respectively. These differences throughout the reef appeared to be explained in terms of mixed contributions from macrophyte and epibenthic microalgae growing in different seasons and years, with their debris undergoing diagenetic alteration. Therefore, macroalgae and SOM δ-values can be used in a complementary manner, over various time scales, as indicators of the integrated effect of dissolved inorganic nitrogen (DIN) sources on coral reef ecosystems.