Nutrient availability and the ultimate control of the biological carbon pump in the western tropical South Pacific Ocean Thierry Moutin, Thibaut Wagener, Mathieu Caffin, Alain Fumenia, Audrey Gimenez, Melika Baklouti, Pascale Bouruet-Aubertot, Mireille Pujo-Pay, Karine Leblanc, Dominique Lefèvre, et al. To cite this version: Thierry Moutin, Thibaut Wagener, Mathieu Caffin, Alain Fumenia, Audrey Gimenez, et al.. Nutrient availability and the ultimate control of the biological carbon pump in the western tropical South Pacific Ocean. Biogeosciences, European Geosciences Union, 2018, 15 (9), pp.2961-2989. 10.5194/bg- 15-2961-2018. hal-01801747 HAL Id: hal-01801747 https://hal.archives-ouvertes.fr/hal-01801747 Submitted on 5 Jul 2018 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Biogeosciences, 15, 2961–2989, 2018 https://doi.org/10.5194/bg-15-2961-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Nutrient availability and the ultimate control of the biological carbon pump in the western tropical South Pacific Ocean Thierry Moutin1, Thibaut Wagener1, Mathieu Caffin1, Alain Fumenia1, Audrey Gimenez1, Melika Baklouti1, Pascale Bouruet-Aubertot2, Mireille Pujo-Pay3, Karine Leblanc1, Dominique Lefevre1, Sandra Helias Nunige1, Nathalie Leblond4, Olivier Grosso1, and Alain de Verneil1,5 1Aix Marseille Univ., CNRS, Université de Toulon, IRD, OSU Pythéas, Mediterranean Institute of Oceanography (MIO), UM 110, 13288, Marseille, France 2Sorbonne Universités – UPMC Univ. Paris 06 – LOCEAN, BP100, 4 place Jussieu, 75252 Paris CEDEX 05, France 3Laboratoire d’Océanographie Microbienne – UMR 7321, CNRS – Sorbonne Universités, UPMC Univ Paris 06, Observatoire Océanologique, 66650 Banyuls-sur-mer, France 4Observatoire Océanologique de Villefranche, Laboratoire d’Océanographie de Villefranche, UMR 7093, Villefranche-sur-mer, France 5The Center for Prototype Climate Modeling, New York University in Abu Dhabi, Abu Dhabi, UAE Correspondence: Thierry Moutin ([email protected]) Received: 29 December 2017 – Discussion started: 10 January 2018 Revised: 23 March 2018 – Accepted: 24 April 2018 – Published: 16 May 2018 Abstract. Surface waters (0–200 m) of the western in this iron-rich environment. The MA appears to be a net tropical South Pacific (WTSP) were sampled along a sink for atmospheric CO2, while the WGY is in quasi-steady longitudinal 4000 km transect (OUTPACE cruise, DOI: state. We suggest that the necessary excess P, allowing the 10.17600/15000900) during the austral summer (stratified) success of nitrogen fixers and subsequent carbon production period (18 February to 3 April 2015) between the Melane- and export, is mainly brought to the upper surface by local sian Archipelago (MA) and the western part of the SP gyre deep winter convection at an annual timescale rather than (WGY). Two distinct areas were considered for the MA, by surface circulation. While the origin of the decoupling the western MA (WMA), and the eastern MA (EMA). The between phosphacline and nitracline remains uncertain, the main carbon (C), nitrogen (N), and phosphorus (P) pools direct link between local P upper water enrichment, N2 fixa- and fluxes provide a basis for the characterization of the tion, and organic carbon production and export, offers a pos- expected trend from oligotrophy to ultra-oligotrophy, and sible shorter timescale than previously thought between N the building of first-order budgets at the daily and seasonal input by N2 fixation and carbon export. The low iron avail- timescales (using climatology). Sea surface chlorophyll a ability in the SP gyre and P availability in the MA during the well reflected the expected oligotrophic gradient with higher stratified period may appear as the ultimate control of N in- values obtained at WMA, lower values at WGY, and in- put by N2 fixation. Because of the huge volume of water to termediate values at EMA. As expected, the euphotic zone consider, and because the SP Ocean is the place of intense depth, the deep chlorophyll maximum, and nitracline depth denitrification in the east (N sink) and N2 fixation in the west deepen from west to east. Nevertheless, phosphaclines and (N source), precise seasonal C, N, P, and iron (Fe) budgets nitraclines did not match. The decoupling between phospha- would be of prime interest to understand the efficiency, at cline and nitracline depths in the MA allows for excess P the present time and in the future, of the oceanic biological to be locally provided in the upper water by winter mixing. carbon pump. 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 Published by Copernicus Publications on behalf of the European Geosciences Union. 2962 T. Moutin et al.: Nutrient availability and the ultimate control of the biological carbon pump 1 Introduction ability or climate change (Karl et al., 1997; Karl, 2014), the most probable changes for the near future in both N2 fixation The oceanic biological carbon pump corresponds to the and denitrification processes following climate forcing are transfer of carbon from the upper surface to the ocean inte- predicted to be a strengthening control of the carbon cycle rior by biological processes, greatly influencing atmospheric by P availability (Moutin et al., 2008). CO2 concentration and therefore the earth’s climate. It is a The western tropical South Pacific (WTSP) is a poorly highly ranked priority in current research in oceanography studied area where large blooms of diazotrophs were previ- (Burd et al., 2016). Two biological pumps have been de- ously observed by satellite (Dupouy et al., 2000, 2011), and fined (Volk and Hoffert, 1985), the “soft tissue” and “car- which has been recently qualified as a hotspot of N2 fixa- bonate” pumps associated with organic matter or calcium tion (Bonnet et al., 2017). It is hypothesized that following carbonate processes (e.g., production, export, remineraliza- the South Equatorial Current (SEC), the N-depleted and P- tion or dissolution). The “soft tissue” pump (see Moutin et enriched waters from areas of denitrification located in the al., 2017a; their Fig. 1), considering both its intensity and east Pacific reach waters with sufficient iron in the west to shorter timescales, is by far the larger contributor to the dis- allow N2 fixation to occur (Moutin et al., 2008; Bonnet et solved inorganic carbon gradient between the upper surface al., 2017). While horizontal advection of waters from the and the deep sea. Following climate alteration, the biolog- east through the SEC probably supports an active biological ical “soft tissue” pump begins to deviate from its equilib- pump in the WTSP, local vertical convection may also play a rium condition, meaning that its influence on atmospheric central role. CO2 change may occur at timescales shorter than previously In addition to the main objective of following the same wa- thought (Sarmiento and Gruber, 2006). Because the strength ter mass for several days (de Verneil et al., 2018) by a quasi- of the biological carbon pump depends on nutrient availabil- Lagrangian experiment (Moutin et al., 2017a) in order to pro- ity in the upper ocean, and more particularly on N avail- pose daily budgets (Caffin et al., 2018; Knapp et al., 2018), ability (Falkowski et al., 1998; Tyrell, 1999; Moore et al., or short term biological trends (Van Wambeke et al., 2018), 2013), which is in the long term regulated by external in- here we propose to work at larger space and time scales, in put by N2 fixation and internal denitrification (Grüber and complement to the work by Fumenia et al. (2018) showing Sarmiento, 1997; Codispoti et al., 2001; Deutsch et al., 2001, that N2 fixation in the WTSP may influence the whole South 2007; Brandes and Devol, 2002; Grüber, 2004; Mahaffey et Pacific (SP) Ocean. While many recent works focus on small al., 2005; Codispoti, 2007; Capone and Knapp, 2007; Moutin spatial scales influencing the biological carbon pump (Lévy et al., 2008; Deutsch and Weber, 2012; Landolfi et al., 2013; et al., 2012; Stukel et al., 2017), we found it important to Jickells et al., 2017), quantitative evaluation of the regula- also show results from a larger-scale study in the OUTPACE tion, interdependence, and patterns of change of these two (Oligotrophy to the UlTra-oligotrophy PACific Experiment) processes require attention at the present time. It has been special issue (Moutin et al., 2017b), showing that they are suggested earlier that N2 fixation may play a large part in complementary rather than mutually exclusive. Another in- changing atmospheric CO2 inventories (McElroy, 1983), but terest of this study is that we are far from resolving seasonal at long timescales and taking into account major differences variations in the main biogeochemical variables in the WTSP, in Aeolian iron input (Falkowski, 1997; Broecker and Hen- still largely under-sampled compared to the North Pacific and derson, 1998). Because N2 fixation may ultimately be con- Atlantic. Therefore, the aim of this study is to provide a large trolled by iron availability, and because dust delivery to the spatial (hundreds of kilometers) and temporal (annual) scale ocean is climate sensitive, there may be inextricably linked study of the main biogeochemical C, N, and P stocks and feedback mechanisms that regulate N2 fixation, atmospheric fluxes in the upper 200 m of the WTSP Ocean from measure- CO2 concentrations, and dust deposition over relatively long ments gathered during the stratified period, and to evaluate periods (Michaels et al., 2001; Karl, 2014).