Abstract. The Oligotrophy to UlTra-oligotrophy PACific Experiment (OUTPACE) cruise took
place in the western tropical South Pacific (WTSP) during the austral summer
(March–April 2015). The aim of the OUTPACE project was to investigate a
longitudinal gradient of biological and biogeochemical features in the WTSP,
and especially the role of N2 fixation in the C, N, and P cycles. Two
contrasted regions were considered in this study: the Western Melanesian
Archipelago (WMA), characterized by high N2 fixation rates,
significant surface production and low dissolved inorganic phosphorus (DIP)
concentrations, and the South Pacific Gyre (WGY), characterized by very low
N2 fixation rates, surface production and high DIP concentrations.
Since physical forcings and mixed layer dynamics in both regions were
similar, it was considered that the gradient of oligotrophy observed in
situ between the WMA and WGY was not explained by differences in physical
processes, but rather by differences in biogeochemical processes. A
one-dimensional physical–biogeochemical coupled model was used to
investigate the role of N2 fixation in the WTSP by running two
identical simulations, only differing by the presence (simWMA) or
absence (simWGY) of diazotrophs. We showed that the nitracline
and the phosphacline had to be, respectively, deeper and shallower than the
mixed layer depth (MLD) to bring N-depleted and P-repleted waters to the
surface during winter mixing, thereby creating favorable conditions for the
development of diazotrophs. We also concluded that a preferential
regeneration of the detrital phosphorus (P) matter was necessary to obtain
this gap between the nitracline and phosphacline depths, as the nutricline
depths significantly depend on the regeneration of organic matter in the
water column. Moreover, the model enabled us to highlight the presence of
seasonal variations in primary production and P availability in the upper
surface waters in simWMA, where diazotrophs provided a new source of nitrogen
(N) to the ecosystem, whereas no seasonal variations were obtained in simWGY,
in the absence of diazotrophs. These main results emphasized the fact that
surface production dynamics in the WTSP is based on a complex and sensitive
system which depends on the one hand on physical processes (vertical mixing,
sinking of detrital particles), and on the other hand on biogeochemical
processes (N2 fixation, remineralization).
The Role of the Southern Ocean in Global Processes
