Abstract. Surface waters (0–200 m) of the western tropical South Pacific (WTSP) were
sampled along a longitudinal 4000 km transect (OUTPACE cruise, DOI: 10.17600/15000900)
during the austral summer (stratified) period (18 February to 3 April 2015)
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), and phosphorus (P) pools and fluxes provide a
basis for the characterization of the expected trend from oligotrophy to
ultra-oligotrophy, and the building of first-order budgets at the daily and
seasonal timescales (using climatology). Sea surface chlorophyll a well reflected
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 for excess
P to be locally provided in the upper water by winter mixing. We found a
significant biological “soft tissue” carbon pump in the MA sustained almost
exclusively by dinitrogen (N2) fixation and essentially controlled by phosphate
availability in this iron-rich environment. The MA appears to be a net
sink for atmospheric CO2, 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 timescale rather than by
surface circulation. While the origin of the decoupling between phosphacline
and nitracline remains uncertain, the direct link between local P upper
water enrichment, N2 fixation, and organic carbon production and
export, offers a possible shorter timescale than previously thought between
N input by N2 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 N2 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 N2
fixation in the west (N source), precise seasonal C, N, P, and iron (Fe) budgets would
be of prime interest to understand the efficiency, at the present time and
in the future, of the oceanic biological carbon pump.
