Nutrient availability and the ultimate control of the biological carbon pump in the Western Tropical South Pacific Ocean

Type Article
Date 2018-05
Language English
Author(s) Moutin Thierry1, Wagener Thibaut1, Caffin Mathieu1, Fumenia Alain1, Gimenez Audrey1, Baklouti Melika1, Bouruet-Aubertot Pascale2, Pujo-Pay Mireille3, Leblanc Karine1, Lefevre Dominique1, Nunige Sandra Helias1, Leblond Nathalie4, Grosso Olivier1, de Verneil Alain1, 5
Affiliation(s) 1 : Univ Toulon & Var, Aix Marseille Univ, CNRS,UM 110, IRD,OSU,Pytheas Mediterranean Inst Oceanog MIO, F-13288 Marseille, France.
2 : UPMC Univ Paris 06, Sorbonne Univ, LOCEAN, BP100,4 Pl Jussieu, F-75252 Paris 05, France.
3 : UPMC Univ Paris 06, Sorbonne Univ, CNRS, Lab Oceanographie Microbienne,UMR 7321, F-66650 Banyuls Sur Mer, France.
4 : Observ Oceanolog Villefranche, Lab Oceanographie Villefranche, UMR 7093, Villefranche Sur Mer, France.
5 : New York Univ Abu Dhabi, Ctr Prototype Climate Modeling, Abu Dhabi, U Arab Emirates.
Source Biogeosciences (1726-4170) (Copernicus Gesellschaft Mbh), 2018-05 , Vol. 15 , N. 9 , P. 2961-2989
DOI 10.5194/bg-15-2961-2018
WOS© Times Cited 20
Note Special issue Interactions between planktonic organisms and biogeochemical cycles across trophic and N2 fixation gradients in the western tropical South Pacific Ocean: a multidisciplinary approach (OUTPACE experiment) Editor(s): T. Moutin, S. Bonnet, K. Richards, D. G. Capone, E. Marañón, and L. Mémery
Abstract

Surface waters (0–200 m) of the western tropical South Pacific (WTSP) were sampled along a longitudinal 4000 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 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 soft tissue carbon pump in the MA sustained almost exclusively by N2 fixation and essentially controlled by phosphate availability in this iron-replete 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 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, N2 fixation, organic carbon production and export, offers a possible shorter time scale 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 budgets would be of prime interest to understand the efficiency, at the present time, and in the future, of the oceanic biological carbon pump.

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Moutin Thierry, Wagener Thibaut, Caffin Mathieu, Fumenia Alain, Gimenez Audrey, Baklouti Melika, Bouruet-Aubertot Pascale, Pujo-Pay Mireille, Leblanc Karine, Lefevre Dominique, Nunige Sandra Helias, Leblond Nathalie, Grosso Olivier, de Verneil Alain (2018). Nutrient availability and the ultimate control of the biological carbon pump in the Western Tropical South Pacific Ocean. Biogeosciences, 15(9), 2961-2989. Publisher's official version : https://doi.org/10.5194/bg-15-2961-2018 , Open Access version : https://archimer.ifremer.fr/doc/00420/53113/