MEPS 179:285-290 (1999)  -  doi:10.3354/meps179285

Phytoplankton nutrient control in the oligotrophic South Pacific subtropical gyre (Tuamotu Archipelago)

Philippe Dufour*, Loic Charpy, Sylvain Bonnet**, Nicole Garcia

IRD and CNRS, COM, rue de la Batterie des Lions, F-13007 Marseille, France
**Deceased June 1996

ABSTRACT: The identification of the nutrients likely to limit phytoplankton in the Tuamotu Archipelago ocean (South Pacific subtropical gyre) is deduced from DIN (NO3- + NO2- + NH4+), PO4++, and SiO2 in situ concentrations and strengthened by the results of 4 other studies on the same area: (1) Ømax, the maximum quantum yield of photosynthesis, (2) phytoplankton cell size, (3) nutrient enrichment on natural populations, (4) bacterial biomass and nutrient limitation of bacteria. The low Ømax and the small size of phytoplankton support the idea of nutrient control. Low DIN concentrations (mean = 0.02 µM), low average DIN:P-PO4++ ratios = 0.3 at:at, low average DIN:SiO2 = 0.04 at:at, and significant increases of phytoplankton standing stock and bacterial production after N enrichments point to N as the main limiting nutrient. P is the second limiting nutrient as evidenced by the relative low phosphate concentrations (0.2 µM) and by the synergetic effect of DIN + PO4++ enrichments. Silicon concentrations appear to limit the proliferation of siliceous taxa as evidenced from its average concentration of 1 µM and from the scarcity of diatoms. Natural concentrations of vitamins, Mo, Mn, Fe and chelators are sufficient to support the increase of phytoplankton standing stock induced by N and P additions and therefore are not limiting.

KEY WORDS: Phytoplankton · Nutrient control · South Pacific · Central gyre · Oligotrophy

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