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

Abstract

15Surface 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 20 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 25 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 30 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 35 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.

N<sub>2</sub> fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)

“…However, two other main processes can contribute to export: physical mixing resulting in export of dissolved organic matter (Carlson et al, 1994;Carlson and Ducklow, 1995;Copin-Montégut and Avril, 1993;Toggweiler, 1989) and zooplankton diel migrations that actively transport organic matter out of the photic layer (Longhurst et al, 1989(Longhurst et al, , 1990Longhurst and Glen Harrison, 1988;Vinogradov, 1970). In the present study, the DON export flux was limited to eddy diffusion as we performed our survey during the stratification period, and the low downward flux of DON estimated by Moutin et al (2018) was unable to explain the observed imbalance. However, zooplankton might play a significant role, although it is hard to quantify.…”

Dynamics and controls of heterotrophic prokaryotic production in the western tropical South Pacific Ocean: links with diazotrophic and photosynthetic activity

“…In addition to the measurements of chlorophyll a, BP, PP, T DIP and DOC described below, other data presented in this paper include hydrographic properties, nutrients and N 2 fix, for which detailed protocols of analysis and considerations for methodology are available in Moutin et al (2017Moutin et al ( , 2018 and Bonnet et al (2018). Briefly, DIP and nitrate concentrations were measured using standard colorimetric procedures on a AA3 AutoAnalyzer (Seal-Analytical).…”

“…The overall correlation between in vivo fluorescence (Chl iv) and Chl a was very patchy (Chl a = 1.582 × Chl iv + 0.0241, n = 169, r = 0.61). This is due to the heterogeneity at the time of sampling and the nature of the populations present, i.e., essentially different fluorescence yields over depth and species (Neveux et al, 2010). Thus in vivo fluorescence was used only to track high-frequency variability at the LD sites, the shape of a vertical profile's distributions and the location of the dcm, as well as longitudinal trends.…”

Carbonate system distribution, anthropogenic carbon and acidification in the Western Tropical South Pacific (OUTPACE 2015 transect)

“…Alvarez et al, 2009, Carter et al, 2017. For the OUTPACE area, Moutin et al (2018) shows that the mixed layer depth do not exceed 70 m in the area. Even if the depth of the deep chlorophyll maximum was encountered below 100 dbar along the transect, we will consider CANT values up to 100 dbar.…”

“…The difference between these sub-regions is evidenced by the difference in oligotrophy . Due to specific conditions in the transition area between the MA and WGY , SD11, SD12 and LDB were discarded from both groups in this study following the arguments in Moutin et al (2018). TrOCA is a source of significant errors but also that even with regionally "tunned" parameters a global bias in the method exists.…”