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

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° 30'S; 167°E). While the soluble reactive phosphate (SRP) concentration was below the analytical chemical detec- tion 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 phos- phate (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 orthophos- phate 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 win- ter 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.