MEPS 320:65-78 (2006)  -  doi:10.3354/meps320065

Response of phytoplankton communities to increased anthropogenic influences (southwestern lagoon, New Caledonia)

Séverine Jacquet1,*, Bruno Delesalle2, Jean-Pascal Torréton1, Jean Blanchot3

1IRD, UR CAMELIA, BP A5, 98848 Nouméa Cedex, New Caledonia
2Laboratoire des Ecosystèmes Aquatiques Tropicaux et Méditerranéens, E.P.H.E., UMR CNRS 8046, 66860 Perpignan Cedex, France
3IRD, UR CYROCO, BP 172, 97492 Sainte-Clotilde Cedex, La Réunion, France

ABSTRACT: 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, Prochlorococcus abundance dominates in the adjacent oceanic and southern lagoon shelf sites, and assemblages shift to Synechococcus-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.

KEY WORDS: Size-fractionated chlorophyll · Picoplankton · Phytoplankton composition · Flow cytometry · Nutrients · Eutrophication · Coral reef lagoon

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