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

N Van Den Broeck,A Le Bouteiller,M Rodier,T Moutin

doi:10.3354/meps268001

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.

Highlights

In oceanic waters, nitrogen has traditionally been viewed as the nutrient-limiting productivity (Eppley et al 1973, Codispoti 1989), and the biological pump of carbon (Bishop 1989)
The other most crucial environmental control on N2 fixation is the bioavailability of the other nutrients which can constrain the extent of nitrogen fixation in a system, such as phosphate (Karl et al 1997, 2002, Sañudo-Wilhelmy et al 2001), or trace metals such as iron (Falkowski 1997, Kutska et al 2002, Fu & Bell 2003) and molybdenum (Karl et al 2002), which are required for the activity of nitrogenase
We studied phosphate availability for phytoplankton near New Caledonia from October 2001 to August 2002 in an area where blooms of the N2-fixing cyanobacterium Trichodesmium are frequently observed (Dupouy 1992, Dupouy et al 2000) and where the variability of the nutrient environment and the associated biological responses are still insufficiently described in the literature (Radenac & Rodier 1996)

Summary

Introduction

Nitrogen has traditionally been viewed as the nutrient-limiting productivity (Eppley et al 1973, Codispoti 1989), and the biological pump of carbon (Bishop 1989). The mean instead of 1.1 nM in the surface-water sample in orthophosphate turnover time in the mixed layer showed a strong seasonal variability (Table 2), ranging Depth-integrated primary production of the (0 to 100 m) water column (Table 3) showed a seasonal variability with higher values in spring (463.5 mgC m–2 d–1) and winter (442.5 mgC m–2 d–1) and lower values in fall (246.6 to 210.1 mgC m–2 d–1).

Results

Conclusion