REPORT C. Pinazo Æ S. Bujan Æ P. Douillet Æ R. Fichez C. Grenz Æ A. Maurin 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 Received: 1 August 2002 / Accepted: 15 April 2003 / Published online: 13 May 2004 Ó Springer-Verlag 2004 Abstract A coupled three-dimensional physical-biologi- cal 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 biogeochem- ical cycling of organic matter, taking into account advection and diffusion processes driven mainly by local wind fields and freshwater discharges. Modeled phyto- plankton dynamic were strongly correlated with both freshwater nutrient inputs and wind-driven hydrody- namic 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 disper- sion 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 demon- strated that eutrophication in the southwest lagoon of New Caledonia is confined to the most restricted coastal embayments, while most of the lagoon experiences sus- tained oligotrophic conditions. Keywords Coupled physical-biological three- dimensional modeling Æ Biogeochemistry Æ Ecosystem Æ Phytoplankton Æ Lagoon Æ New Caledonia Introduction The New Caledonia Reef is the second widest tropical reef system in the world after the Australian Great Barrier Reef. This single, continuous barrier reef sur- rounding New Caledonia’s main island is 1,100-km long and delimits a lagoon area of 23,400 km 2 . Terrigenous inputs of either natural or anthropogenic origin is per- ceived as one of the main structuring factors in this enclosed system (Labrosse et al. 2000). A coupled physical-biological, three-dimensional modeling ap- proach was used in order to gain better insights into the ecological functioning and potential impact of land- derived inputs in this coral reef environment. The use of models to simulate marine coastal envi- ronments has increased significantly during the past 25 yr in parallel with the availability of powerful com- puting tools. Most of the early models focused on the first levels of the pelagos (nutrients, phytoplankton) under steady-state conditions (Steele 1962; Billen 1978). Deterministic approaches were progressively used to model time-varying conditions related to abiotic forc- ings (hydrodynamics, meteorology, air-water exchanges, etc.), and to introduce more precise ecological processes in NPZD models (Billen and Lancelot 1988; Fasham et al. 1990). Two main types of models are used in marine bio- geochemistry: C. Pinazo (&) Æ S. Bujan Æ C. Grenz Æ A. Maurin Station Marine d’Endoume (Bat 4), Centre d’Oceanologie de Marseille, Chemin de la Batterie des Lions, 13007 Marseille, France E-mail: pinazo@com.univ-mrs.fr P. Douillet Æ R. Fichez Institut de Recherche pour le Developpement (IRD), BP A 5 Noumea, New Caledonia, 98848, Canada Coral Reefs (2004) 23: 281–296 DOI 10.1007/s00338-004-0378-x