A biogeochemical box model describing the south-west lagoon of New-Caledonia was developed in order to simulate the seasonal cycle of carbon and nitrogen. We used fluxes generated by a 3D hydrodynamic model to simulate horizontal exchanges between boxes and added freshwater influxes as nitrogen sources from the land. Average residence time proved to be less than 11 days for the lagoon as a whole. Standard simulations showed baseline values of chlorophyll a between 0.2 and 0.4 μgL-1. Influences of freshwater influxes proved to be significant (increases up to 1 μgL-1) only in shallow areas protected from wind exposure and during short periods of heavy rainfall (tropical depressions). Tropical climatic events have reduced impact in space and time and long-term simulations over decades with increased nutrient inputs did not show any significant process of eutrophication. Hydrodynamics seemed to be one of the major control factors with respect to organic matter cycling in the lagoon.
