Tracing the fate of seabird-derived nitrogen in a coral reef using nitrate and coral skeleton nitrogen isotopes

Seabirds transfer nutrients from the ocean to their nesting island, potentially altering nitrogen (N) cycling within adjacent terrestrial and marine ecosystems. Yet, the processes involved in seabird-N transfer along the land–sea continuum remain elusive. Using δ¹⁵N and δ¹⁸O measurements of groundwater nitrate, we demonstrate the role of brackish groundwater located within a coral island's landmass as a major reservoir of nitrate (at millimolar levels). Nearly all of the total dissolved seabird-derived N leaching into the groundwater (mostly ammonium and uric acid) is converted to nitrate by nitrification, as supported by the relatively low δ¹⁸O of the groundwater nitrate (3.97‰ ± 0.30‰). Comparison of nitrate δ¹⁵N and δ¹⁸O suggests that little denitrification takes place within the groundwater lens, implying that the high δ¹⁵N of groundwater nitrate (13.73‰ ± 0.05‰) derives from the high trophic position of seabirds and postdepositional processes that increase the δ¹⁵N of seabird excreta. Seawater and coral skeleton samples from a reef flat exposed to groundwater had higher δ¹⁵N values than at sites devoid of groundwater influence, indicating that the main source of N at the latter site was the Subtropical Upper Water, while the groundwater nitrate dominated the exposed reef flat N pool up to 200 m from shore. In addition, these results indicate that coral-bound δ¹⁵N can detect seabird-derived nitrate δ¹⁵N, raising opportunities to reconstruct historical seabird-N inputs to coral reefs in relation to climatic and other changes, such as the introduction of invasive species.