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 calc ulated using pore water gradient concentration measured using the peep er technique at 8 stations in the lagoon of Tikehau Atoll, French Poly nesia. 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 con centration gradients were positive at all stations. N and P fluxes rep resented 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 require ments of lagoonal primary production. Study of geochemical processes a nd stoichiometry of the organic matter shows that a great part of depo sited organic matter is oxidized in the water column. The amount of or ganic 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 supe rsaturated with respect to aragonite and calcite, and the kinetics of carbonate dissolution were faster than the reverse reaction. The disso lution/precipitation of carbonate plays an important role in the compo sition of the pore water.