Multiple drivers of production and particle export in the western tropical North Atlantic

Catarina V Guerreiro,Laura F Korte,Stefan Schouten,Leandro Ponsoni,Jan‐Berend W Stuut,Furu Mienis,Chris I Munday,Geert‐Jan A Brummer,Michèlle Van Der Does

doi:10.1002/lno.11442

Abstract

AbstractTo assess the impacts of Amazon River discharge, Saharan dust deposition, N2‐fixation and mixed‐layer deepening on the biological carbon pump, sediment traps were moored from October 2012 to November 2013 at two sites in the western tropical North Atlantic (49°W,12°N/57°W,12°N). Particle exports interpreted along with satellite‐ and Argo‐float data show peak fluxes in biogenic silica (31 mg m−2 d−1) and organic carbon (25 mg m−2 d−1) during the fall of 2013 that were ten to five times higher than any time earlier during the year. These high export fluxes occurred in tandem with high surface chlorophyll a concentrations associated with the dispersal of the Amazon River plume, following retroflection into the North‐Atlantic‐Counter‐Current. High fucoxanthin fluxes (> 80 μg m−2 d−1) and low δ15N‐values (−0.6‰) suggest a large contribution by marine diatom‐diazotrophic‐associations, possibly enhanced by wet Saharan dust deposition. During summer, the Amazon River plume resulted in high mass fluxes at 57°W that were enriched in biogenic silica but weakly influenced by diazotrophic‐associations compared to the fall event at 49°W. High carbonate‐carbon fluxes (17 mg m−2 d−1) dominated a second single event at 49°W during spring that was likely triggered by mixed‐layer deepening. Rain‐ratios of BSi/Ccarb amounted to 1.7 when associated with high export fluxes linked to the Amazon River plume. Compared to an annual average of 0.3, this indicates a more efficient uptake of CO2 via the biological pump compared to when the plume was absent, hence supporting earlier observations that the Amazon River plume is important for ocean CO2 sequestration.

Highlights

The tropical oceans are a source for natural CO2 outgassing due to upwelling of CO2-rich deep waters (Lefèvre et al 1998; Takahashi et al 2002; Mikaloff Fletcher et al 2007)
While surface mixed layer temperature was similar at both sites (> 27C), salinity was significantly lower at site M4 (34.1) than at site M5 (35.6), indicating freshwater influence from the Amazon River in the open western tropical North Atlantic (WTNA)
Our observations suggest that primary production in the WTNA is stimulated by several processes including Amazon River discharge, nitrogen fixation by marine diazotrophs, seasonal mixed-layer deepening and Saharan dust deposition

Summary

Introduction

The tropical oceans are a source for natural CO2 outgassing due to upwelling of CO2-rich deep waters (Lefèvre et al 1998; Takahashi et al 2002; Mikaloff Fletcher et al 2007). The distribution of the plume is strongly seasonal (Muller-Karger et al 1988; Masson and Delecluse 2001), covering an area of around 2 million km following dispersal by ocean currents (Körtzinger 2003) As it spreads offshore, the plume water mixes with oceanic water into a shallow surface layer with low salinity and high dissolved silicon concentrations (Shipe et al 2006), likely to act as a source of labile iron that was not precipitated in the Amazon estuary (Subramaniam et al 2008). This is especially the case for the dust-born iron, an essential element for Chl a synthesis and N2 fixation by marine phytoplankton (Falkowski 1997; Carpenter et al 1999; Mahaffey et al 2003; Mills et al 2004)

Methods

Results

Discussion

Conclusion