Carbon and nitrogen uptake and export in the equatorial Pacific at 150°W: Evidence of an efficient regenerated production cycle

Biomass, inorganic carbon and nitrogen uptake, ammonium regeneration, nitrification, and vertical flux of particulate matter were measured in the equatorial Pacific at 21 daily productivity stations occupied on a meridional transect (150oW) between ioN and 16oS. Three areas could be distinguished along the transect: (1) the equatorial area between IoN and 6oS, where nitrate concentrations were typically eutrophic, reaching up to 3/g-at L - in surface waters; (2) an intermediate mesotrophic area between 6 o and 10oS, where surface nitrate concentrations decreased from 1 /g-at L - to zero; and (3) the oligotrophic area beyond 10oS, characterized by warm and nitrate poor waters. Although nitrate was the main form of inorganic nitrogen available for phytoplankton growth (70%- 100% of total), its uptake was severely retarded in the equatorial sector. This lack of nitrate depletion in the equatorial sector between 0 and 6oS may in part result from the important ammonium supply (100 ng-at L - d --1) which could sustain up to 85% of total inorganic nitrogen (nitrate + ammonium) utilization by phytoplankton. In addition, regenerated production also resulted from in situ nitrification (20-80 ng-atN L - d -) which can fuel 20%-100% of the nitrate uptake. Sinking particles represented 1.5) observed in the upwelling waters seemed to indicate that silicate is not efficiently recycled in this specific circulation system because of its low regeneration rate as well as its sink via rapidly sedimenting diatoms cell walls; this may be also true for iron. This reinforces the idea of Si and/or Fe limitation which was put forward earlier to explain low new production levels in the equatorial Pacific.