MEPS 268:1-12 (2004)  -  doi:10.3354/meps268001

Seasonal variations of phosphate availability in the SW Pacific Ocean near New Caledonia

Nathalie Van Den Broeck1, Thierry Moutin1,*, Martine Rodier2, Aubert Le Bouteiller2

1Laboratoire d¹Océanographie et de Biogéochimie (UMR 6535), Centre d¹Océanologie de Marseille, Campus de Luminy, 13288 Marseille Cedex 09
2Institut de Recherche pour le Développement, BP A5, 98848 Nouméa, Nouvelle Calédonie, France
*Corresponding author. Email:

ABSTRACT: Seasonal availability of phosphate was assessed by the examination of P pools and turnover times of orthophosphate on 6 cruises between October 2001 and August 2002 in the upper water column (0 to 120 m) of the oligotrophic SW Pacific Ocean near New Caledonia (21°30¹S; 167°E). While the soluble reactive phosphate (SRP) concentration was below the analytical chemical detection limit (30 nM) in summer and fall, it reached detectable values in winter and spring. The annual mean depth-integrated (0 to 100 m) SRP pool was 6.8 mmol m-2 (SD = 3.9, n = 6). Particulate phosphate (PP) and soluble non-reactive phosphate (SNP) pools were, respectively, 1.7 (SD = 0.3, n = 6) and 23.0 mmol m-2 (SD = 4.8, n = 6). A strong seasonal variation of turnover times (T) of orthophosphate was observed (0.17 to 19 d) in the upper water column, with shorter values in summer and fall. Indirectly estimated orthophosphate concentration ([PO4]), based on turnover time measurements, ranged from 56 to 118 nM in winter to 0.3 to 0.8 nM in fall. A seasonal P sufficiency in spring and winter T > 100 h; [PO4] > 20 nM) and a strong deficiency in summer (T < 15 h; [PO4] < 2 nM) and the beginning of fall (T < 5 h; [PO4] < 1 nM) occurred. This P deficiency was larger than at the ALOHA station in the North Pacific Ocean. Primary production and phosphate availability followed the same seasonal pattern. In this iron-rich area known to sustain high N2 fixation rates, phosphate may appear as a key factor controlling carbon production.

KEY WORDS: Phosphate · Turnover · Pacific Ocean · Primary production

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