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

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) Ø_max, 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 Ø_max 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