Density-induced water circulations in atoll coral reefs: A numerical study

Groundwater flow in an idealized atoll platform is simulated with CASTEM 20 00, a computer code developed at the CEA (Commissariat a l'Energie Atomique ). Darcy's law and a coupled solute transport equation (diffusion-dispersio n-convection) is solved by the mixed-hybrid finite-element method. Inward a nd upward circulations of oceanic water are shown to occur. These steady-st are circulations result from temperature and, to a lesser extent, salinity gradients, Inclusion of a karstified dolomitic horizon, observed at the bas e of the carbonate structure of certain atolls, is necessary to account for the negative vertical temperature gradient indicated by field data. The po sition of the platform in relation to the oceanic temperature and salinity distributions exerts a significant control on the interstitial hydraulic re gime. The effect of a secondary karstic layer closer to sea surface is also modeled. In this case, calculations show that the near-surface carbonate e difice is deprived of a significant portion of the flow. Finally, an upper limit on the possible how rare through the platform is calculated. This upp er estimate is shown to be independent of the possible uncertainties on the hydraulic conductivity of the system. Corresponding upper limits on the nu trient fluxes of PO43- and NO3- are evaluated. A comparison with current im port 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 inte rstitial circulation is only a minor nutrient source for the reef ecosystem .