Impact of wind and freshwater inputs on phytoplankton biomass in the coral reef lagoon of New Caledonia during the summer cyclonic period: a coupled three-dimensional biogeochemical modeling approach

International audienceA coupled three-dimensional physical-biological model was developed in order to simulate the ecological functioning and potential impacts of land-derived inputs in the southwest lagoon of New Caledonia. This model considered pelagic biogeochemical cycling of organic matter, taking into account advection and diffusion processes driven mainly by local wind fields and freshwater discharges. Modeled phytoplankton dynamics were strongly correlated with both freshwater nutrient inputs and wind-driven hydrodynamic processes, the latter resulting in a large input of oceanic water from the southeast part of the lagoon under trade wind conditions. In situ data obtained during the summer (January 1998) under trade wind conditions supported predicted concentration gradients along several coast to reef transects and provided a validation of the coupled physical-biogeochemical model. An additional sensitivity analysis showed that the alteration of the biogeochemical parameters did not strongly affect the results of the model. Freshwater inputs of nutrients were simulated using a realistic scenario corresponding to the summer rainy season of 1997-1998 in New Caledonia. Despite occasional flooding events from the main rivers considered in these simulations, no significant meso-scale phytoplankton bloom was identified. Hydrodynamically driven dispersion and rapid uptake of nutrients by phytoplankton were sufficient to spatially constrain the impact of river inputs and maintain oligotrophic conditions. The fine spatial grid of our three-dimensional model demonstrated that eutrophication in the southwest lagoon of New Caledonia is confined to the most restricted coastal embayments, while most of the lagoon experiences sustained oligotrophic conditions