Nutrient availability and the ultimate control of the biological carbon pump in the Western Tropical South Pacific Ocean
Surface waters (0&ndash;200&thinsp;m) of the western tropical South Pacific (WTSP) were sampled along a longitudinal 4000&thinsp;km transect (OUTPACE cruise, 18 Feb., 3 Apr. 2015) during the stratified period between the Melanesian Archipelago (MA) and the western part of the SP gyre (WGY). Two distinct areas were considered for the MA, the western MA (WMA) and the eastern MA (EMA). The main carbon (C), nitrogen (N), phosphorus (P) pools and fluxes allow for characterization of the
... cterization of the expected trend from oligotrophy to ultra-oligotrophy, and to build first-order budgets at the daily and seasonal scales (using climatology). Sea surface chlorophyll a reflected well the expected oligotrophic gradient with higher values obtained at WMA, lower values at WGY and intermediate values at EMA. As expected, the euphotic zone depth, the deep chlorophyll maximum and nitracline depth deepen from west to east. Nevertheless, phosphaclines and nitraclines did not match. The decoupling between phosphacline and nitracline depths in the MA allows excess P to be locally provided in the upper water by winter mixing. We found a significant biological <q>soft tissue</q> carbon pump in the MA sustained almost exclusively by N<sub>2</sub> fixation and essentially controlled by phosphate availability in this iron-replete environment. The MA appears to be a net sink for atmospheric CO<sub>2</sub> while the WGY is in quasi steady state. We suggest that the necessary excess P, allowing the success of nitrogen fixers and subsequent carbon production and export, is mainly brought to the upper surface by local deep winter convection at an annual scale rather than by surface circulation. We also suggest that mesozooplankton diel vertical migration plays a dominant role in the transfer of carbon from the upper surface to deeper water in the MA. While the origin of the decoupling between phosphacline and nitracline remains uncertain, the direct link between local P upper waters enrichment, N<sub>2</sub> fixation, organic carbon production and export, offers a possible shorter time scale than previously thought between N input by N<sub>2</sub> fixation and carbon export. The low iron availability in the SP gyre and P availability in the MA during the stratified period may appear as the ultimate control of N input by N<sub>2</sub> fixation. Because of the huge volume of water to consider and because the SP Ocean is the place of intense denitrification in the east (N sink) and N<sub>2</sub> fixation in the west (N source), precise seasonal C, N, P budgets would be of prime interest to understand the efficiency, at the present time, and in the future, of the oceanic biological carbon pump.