Résumé
We investigated the effects of changes in nutrient concentrations on phytoplankton biomass and community composition during 8 field trips performed during different seasons in the southwestern coral lagoon of New Caledonia. The lagoon is characterized by spatial variation in macronutrient concentrations, with locally elevated values in the bays bordering the city of Nouméa. Low DIN:DIP (dissolved inorganic nitrogen:dissolved inorganic phosphorus) and elevated Si:DIN ratios suggest that nitrogen is the macronutrient that likely drives phytoplankton community composition. Most of the microphytoplankton groups discriminated by inverted microscopy and the picoplankton groups distinguished by flow cytometry present significant and distinct relationships with inorganic nitrogen concentrations. Picophytoplankton-dominated assemblages are replaced by microphytoplankton-dominated assemblages with increasing DIN concentrations. Within the picophytoplankton,Prochlorococcusabundance dominates in the adjacent oceanic and southern lagoon shelf sites, and assemblages shift toSynechococcus-dominated populations in the bays, with an increasing proportion of picoeukaryotic phytoplankton. Within the microplankton, 142 species of microphytoplankton were identified, mainly represented by diatoms, dinoflagellates, and coccolithophorids. Nutrient enrichment in the bays favors large diatoms at the expense of coccolithophorids and dinoflagellates, which dominate in adjacent oceanic and southern shelf waters. Therefore, although moderate, the elevated nitrogen concentrations in the bays result in increased phytoplankton biomass, accompanied by important shifts in the phytoplankton community structure.