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 δ 15 N and δ 18 O measurements of groundwater nitrate, we demonstrate the role of brackish groundwater located within a coral island's landmass as a major reservoir of...