DON isotopes over a coral reef (Thibodeau et al., Coral Reefs)

Heterogeneous dissolved organic nitrogen supply over a coral reef: first evidence from nitrogen stable isotope ratios

B. Thibodeau, T. Miyajima, I. Tayasu, A. S. J. Wyatt, A. Watanabe, N. Morimoto, C. Yoshimizu, T. Nagata

Dissolved organic nitrogen (DON) potentially plays a major role in sustaining the high productivity and biological diversity of coral reefs. However, data are scarce regarding sources and sinks of DON. This study, for the first time, determined the 15N isotopic composition of total dissolved nitrogen (δ15NTDN), reflecting the isotopic signature of DON, in the water column over a coral reef. The uniformity in δ15NTDN during high tide (3.2 ± 0.3 ‰) indicated that the DON was mainly derived from offshore waters. In contrast, higher spatial heterogeneity of δ15NTDN (3.1 ± 0.9 ‰) and DON concentrations during low tide indicated the existence of local DON sources patchily distributed over the reef. Low δ15NTDN values located mid-reef were indicative of DON release from organisms that obtained their N via N2 fixation, whereas high δ15NTDN appeared to reflect localized release of DON by organisms exposed to dissolved inorganic nitrogen with elevated 15N, such as from terrestrial and offshore inputs. Collectively, the results highlight the importance of spatial patterns in DON release from reef communities in the N cycling of coral reefs.