Nitrogen transformations within a tropical subterranean estuary

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Publication details

Erler, DV, Santos, IR, Zhang, Y, Douglas, TR, Befus, KM, Hidden, A, Li, L & Eyer, BD 2014, 'Nitrogen transformations within a tropical subterranean estuary', Marine Chemistry, vol. 164, pp. 38-47.

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A detailed geochemical groundwater survey was conducted within a carbonate sand subterranean estuary (STE) system on the tropical island of Rarotonga, Cook Islands, to identify N sources and transformation along the groundwater flow paths. There were two distinct sources of N to the STE: (1) local organic material which produced NH4+ in the anoxic deeper groundwater (3.5–4 m below the aquifer surface), and (2) an NO3 rich upper saline plume located in the shallow groundwater near the high tide mark of the STE. Ammonium concentrations decreased away from the organic source and the δ15N-NH4+ signature became increasingly enriched. The calculated kinetic fractionation factor for NH4+ loss was much less than that reported for nitrification in other systems, implying that mixing was important in reducing concentration. A simple fractionation/mixing model showed that NH4+ loss via nitrification increased towards the surface where it contributed up to 80% of the observed decrease in NH4+ concentration. A fast moving (~ 2 m d− 1) upper saline plume of shallow NO3 rich groundwater was a major feature of the studied STE. Denitrification was estimated to account for up to 88% of the decrease in NO3 concentration in this plume. The potential denitrification rate based on an isotope tracer addition experiment was 270 μmol L− 1 d− 1. Assuming that similar NO3 rich plumes occur all along the foreshore of the studied lagoon, we estimate that groundwater can contribute 4–15% of the daily lagoon N standing stock. We conclude that submarine groundwater discharge of shallow N rich porewater into the studied lagoon system is strongly regulated by denitrification within the STE.

Grant ID

ARC/LP100200732, ARC/DP0876683

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