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 (150 degrees W) between 1 degrees N and 16 degrees S. Three areas could be distinguished along the transect: (1) the equatorial area between 1 degrees N and 6 degrees S, where nitrate concentrations were typically eutrophic, reaching up to 3 mu g-at L-1 in surface waters; (2) an intermediate mesotrophic area between 6 degrees and 10 degrees S, where surface nitrate concentrations decreased from 1 mu g-at L-1 to zero; and (3) the oligotrophic area beyond 10 degrees S, 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 6 degrees S may in part result from the important ammonium supply (100 ng-at L-1 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-at N L-1 d(-1)) which can fuel 20%-100% of the nitrate uptake. Sinking particles represented <10% of total carbon fixation and similar to 10%-50% of new production in terms of carbon and nitrogen. From these discrepancies it was suggested that (1) new production rates were overestimated because of the high level of nitrification that provided "regenerated nitrate" and (2) advection of dissolved organic carbon and nitrogen played an important role in export. The specific hydrodynamical circulation, a conveyor belt generated by upwelling at the equator and downwelling some degrees south, associated with biological in situ remineralization of ammonium and nitrate as well, appeared to be a very efficient system for recycling inorganic nitrogen in the euphotic layer and thus for supporting the high regenerated production levels. On the other hand, the high nitrate/silicate ratios (>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.