Nutrient budget of the lagoonal waters in an open central South Pacific atoll (Tikehau,Tuamotu, French Polynesia)

Nutrient concentrations were measured in the lagoon and surrounding oceanic waters of Tikehau Atoll (Tuamotu Archipelago, French Polynesia) from 1984 to 1987. The atoll-mass effect alters the nutrient profiles: turbulent vertical mixing of the waters along the deeper slopes of the atoll induces nitrogen and phosphorus enrichment of the surface layer. Nutrient concentrations varied with year and month of sampling; except for ammonium, inorganic nutrient levels were lower inside the lagoon than in the surrounding oceanic waters. Nitrogen, phosphorus and silica budgets were calculated by mean differences in nutrient concentrations recorded between lagoon and oceanic surface waters and by the waterexchange rate through the passage linking the lagoon and oceanic waters and the reef-flat spillways. Particulate and dissolved organic nitrogen and ammonium are exported from the lagoon to the open ocean through the westward passage. The nitrogen budget is not balanced by the nitrate input from oceanic waters and the organic nitrogen and ammonium output from lagoonal waters. Nitrogen fixation would appear to constitute another source of nitrogen for lagoonal waters. The phosphorus budget is largely balanced by phosphate input from the oceanic waters and organic phosphorus output from the lagoon waters. The oceanic waters became impoverished in silicate during their crossing of the atoll reef edge and their residence in the lagoon. The atoll constitutes a source of nitrogen for the surrounding oceanic waters.