Diffusional nutrient fluxes at the sediment-water interface and organic matter mineralization in an atoll lagoon (Tikehau, Tuamotu Archipelago, French Polynesia)

Fluxes of dissolved inorganic N, P and Si from the sediments were calculated using pore water gradient concentration measured using the peeper technique at 8 stations in the lagoon of Tikehau Atoll, French Polynesia. Nutrient concentrations of pore water reached maximum values of 130 mu M NH4, 7 mu M PO4 and 30 mu M SiO2. Fluxes calculated from concentration gradients were positive at all stations. N and P fluxes represented 6 and 4 % of the N and P deposition rates and between 0.1 and 6.8% of the N requirements and between 0.1 and 1.7 % of the P requirements of lagoonal primary production. Study of geochemical processes and stoichiometry of the organic matter shows that a great part of deposited organic matter is oxidized in the water column. The amount of organic matter oxidized inside the sediment is estimated to 2.5 mg kg(-1) in the upper centimeter and 5.2 mg kg(-1) below. Pore water was supersaturated with respect to aragonite and calcite, and the kinetics of carbonate dissolution were faster than the reverse reaction. The dissolution/precipitation of carbonate plays an important role in the composition of the pore water.