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

Abstract

The identification of the nutrients likely to limit phytoplankton in the Tuamotu Archipelago ocean (South Pacific subtropical gyre) is deduced from DIN (NO3-+ NOz-+ NH,'), PO,", and SiOz in situ concentrations and strengthened by the results of 4 other studies on the same area:(1) 0,,,, 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 D,,,, and the small s~z e of phytoplankton support the idea of nutrient control. Low DIN concentrations (mean = 0.02 PM), low average D1N.P-PO,*^ ratios = 0.3 at:at, low average DIN:Si02 = 0.04 at:at, and significant increases of phytoplankton standing stock and bacterial production after N enrichments point to N a s the main limiting nutrient. P is the second limiting nutrient as evidenced by the relative low phosphate concentrations (0.2 PM) and by the synergetic effect of DIN + PO," enrichments. Silicon concentrations appear to limit the proliferation of siliceous taxa as evidenced from its average concentration of 1 pM and from the scarcity of diatoms. Natural collcentrations 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.

Basin-scale variability of phytoplankton biomass, production and growth in the Atlantic Ocean

“…It has been suggested that open ocean algae may have adapted to low nutrient conditions by losing their ability to grow fast (Jackson, 1980). However, Brand and Guillard (1981) demonstrated that several species of microalgae isolated from oligotrophic waters have the same growth potential as those cultured in the laboratory, and numerous studies in the central gyres have found growth responses of phytoplankton to experimental nutrient additions (Eppley et al, 1973;DiTullio et al, 1993;Graziano et al, 1996;Dufour et al, 1999). During the AMT cruises, we found dramatic increases in productivity and growth in some areas (e.g.…”

Physical and chemical control of the phytoplankton of Ahe lagoon, French Polynesia

“…On the other hand, nutrient budget analysis between lagoon and open ocean evidenced that oceanic waters are not an important N source for the lagoon. Surface oceanic water around Tuamotu atolls typically has average nutrient concentrations of 0.02 lM DIN and 0.21 lM SRP (Dufour et al, 1999). However, higher DIN values at the OCE station in October 2010, especially at 50 m (0.23 lM), indicates a possible surface N-enrichment due to turbulent vertical mixing of the waters along the shelf break, as observed in Tikehau atoll (Charpy-Roubaud et al, 1990).…”

“…Average silicate concentration in Ahe (1.55 ± 0.05 lM) was relatively low but slightly higher than average silicate concentration in other Tuamotu atolls (1.0 ± 0.1 lM; Dufour et al, 1999).…”

“…In atoll lagoons, planktonic processes are partially controlled by physical and chemical factors (Charpy-Roubaud et al, 1990;Charpy, 2001;Charpy and Blanchot, 1998;Charpy et al, 1997;Dufour et al, 2001Dufour et al, , 1999. Shallow lagoons have widely been assumed to present environmental homogeneity as they are routinely considered well-mixed (Buestel and Pouvreau, 2000;Charpy, 1996;Gonzalez et al, 1998;Torreton and Dufour, 1996).…”

Bacterial and archaeal biogeography of the deep chlorophyll maximum in the South Pacific Gyre

“…1). The small size of prokaryotes has been inferred to provide a competitive advantage under these nutrientlimited conditions (Dufour et al 1999). …”