Limnol. Oceanogr., 44(5), 1999, 1268–1281

Groundwater flow in an idealized atoll platform is simulated with CASTEM 2000, a computer code developed at the CEA (Commissariat à l’Energie Atomique). Darcy’s law and a coupled solute transport equation (diffusion–dispersion–convection) is solved by the mixed-hybrid finite-element method. Inward and upward circulations of oceanic water are shown to occur. These steady-state circulations result from temperature and, to a lesser extent, salinity gradients. Inclusion of a karstified dolomitic horizon, observed at the base of the carbonate structure of certain atolls, is necessary to account for the negative vertical temperature gradient indicated by field data. The position of the platform in relation to the oceanic temperature and salinity distributions exerts a significant control on the interstitial hydraulic regime. The effect of a secondary karstic layer closer to sea surface is also modeled. In this case, calculations show that the near-surface carbonate edifice is deprived of a significant portion of the flow. Finally, an upper limit on the possible flow rate through the platform is calculated. This upper estimate is shown to be independent of the possible uncertainties on the hydraulic conductivity of the system. Corresponding upper limits on the nutrient fluxes of PO and NO are evaluated. A comparison with 32 2 4 3 current import and export data for the surface ecosystem indicates that in every case, nutrient fluxes hardly reach a few percent of the exports. Therefore, the deep oceanic nutrient pool brought to the surface by thermally driven interstitial circulation is only a minor nutrient source for the reef ecosystem. The composition and circulation of interstitial waters in the porous carbonate framework of coral reefs are central to our understanding of reef development and geological evolution. Atoll reef hydrology is also of prime importance to the management of fresh groundwater for communities on these islands. Hydrogeological studies of coral reefs were spurred by the American and French underground nuclear tests in Pacific atolls (Buddemeier and Holladay 1977; Wheatcraft and Buddemeier 1981). Cores drilled in atolls revealed that coral reefs are complex porous media with both horizontal and vertical heterogeneity over a wide range of spatial scales, including cracks, karsts, and strata of composite materials with different permeabilities. The occurrence of large dolomitized horizons also led geologists to suggest that reefs should be regarded as hydrologically active environments, as dolomitization requires the passage of large volumes of seawater in order to supply the necessary magnesium (Buddemeier and Oberdorfer 1986). 1 Corresponding author. 2 Present address: University of Reading, Department of Meteorology, Earley Gate, P.O. Box 243, Reading RG6 6BB, U.K.