This dataset presents data collected along a transect sampled on two dates (6th and 9th of July 2019) on the reef flat of Surprise Island in the D'entrecasteaux Reefs, from land to ocean during the IGUANE cruise. Samples were also collected in the groundwater lens of the island and in the water column offshore of the island. As a comparison, the grounwater lens from two other tropical coral islands from the Western Indian Ocean (Tromelin and Grande Glorieuse) was also sampled, in April 2019 on the occasion of the Climate Eparses and CLIM-EPARSE programs. Urea, ammonium and nitrate concentrations were analyzed. Nitrate plus nitrite and nitrate δ15N (vs. AIR) and δ18O (vs. VSMOW) measurements have been performed using the denitrifier method (Weigand et al., 2016; doi:10.1002/rcm.7570) on seawater and groundwater samples from the IGUANE expedition (2019; doi:10.17600/18000897). Samples for nitrate δ15N (vs. AIR) and δ18O were treated with sulfamic acid before isotope analysis (Granger and Sigman, 2009; doi:10.1002/rcm.4307). This dataset also includes values of nitrogen isotopic composition of coral (Porites) skeleton-bound organic matter. The fraction of groundwater and oceanic water explaining the nitrogen isotopic composition of nitrate along the transects is also given, in percent.

Further funding information: Guano's influence on the functioning of coral ecosystems (IGUANE, https://www-iuem.univ-brest.fr/lemar/projets-scientifiques/iguane/?lang=en) Guano influence on coral reef functioning (GUANACO)