Characterized by oligotrophic to mesotrophic waters, the SW lagoon of New Caledonia receives an important part of urban effluents from the city of Nouméa. The aim of this work was to assess the response of phytoplankton and bacterioplankton communities to nutrient inputs in this coastal tropical lagoon. An annual survey showed a weak seasonality of phytoplankton and bacterioplankton biomass and production with maximal values in December-January (austral summer) and April-May, and minimum values in August (austral winter). This seasonal trend is overlaid by important short term variations of these variables that most likely originate from wind-induced renewal of lagoon waters. As gradients in nutrient concentrations in the vicinity of Nouméa City are observed during these 3 characteristic periods, these periods were chosen in order to assess the response of bacterioplankton and phytoplankton to nutrient enrichment. Nitrogen was the element limiting phytoplankton production. Phytoplankton biomass and production increase with increasing nutrient concentration as does phytoplankton average size. Bacterioplankton production increases proportionally more than primary production, with bacterioplankton carbon demand largely exceeding particulate primary production at the most eutrophic stations. Within the picophytoplankton, Prochlorococccus dominates in the surrounding oceanic and oligotrophic lagoon waters. However, in the most eutrophicated sites Synechococcus and picoeukaryotic phytoplankton are the dominant species. Among the nano- and microphytoplankton, the dominance of the coccolithophorids and dinoflagellates in oligotrophic waters is replaced by diatom-dominated populations at the most eutrophicated stations. Bacterioplankton community structure assessed using T-RFLP does not exhibit important spatial variations for both free-living and particle-attached bacteria, except in some nutrient-rich head of the bay stations. Free-living bacterioplankton community structure appeared to be stable and independent of bacterial production variations throughout a year long survey at a station considered representative of average lagoon conditions. Conversely, particle-attached bacteria display important temporal variations in community structure.
