Amazon River enhances diazotrophy and carbon sequestration in the tropical North Atlantic Ocean

Abstract

The fresh water discharged by large rivers such as the Amazon is transported hundreds to thousands of kilometers away from the coast by surface plumes. The nutrients delivered by these river plumes contribute to enhanced primary production in the ocean, and the sinking flux of this new production results in carbon sequestration. Here, we report that the Amazon River plume supports N(2) fixation far from the mouth and provides important pathways for sequestration of atmospheric CO(2) in the western tropical North Atlantic (WTNA). We calculate that the sinking of carbon fixed by diazotrophs in the plume sequesters 1.7 Tmol of C annually, in addition to the sequestration of 0.6 Tmol of C yr(-1) of the new production supported by NO(3) delivered by the river. These processes revise our current understanding that the tropical North Atlantic is a source of 2.5 Tmol of C to the atmosphere [Mikaloff-Fletcher SE, et al. (2007) Inverse estimates of the oceanic sources and sinks of natural CO(2) and the implied oceanic carbon transport. Global Biogeochem Cycles 21, doi:10.1029/2006GB002751]. The enhancement of N(2) fixation and consequent C sequestration by tropical rivers appears to be a global phenomenon that is likely to be influenced by anthropogenic activity and climate change.

Fig. 1.

Maps showing locations for N₂ fixation and DIC drawdown measurements. (Left) Total N₂-fixation rates for Trichodesmium and Richelia (μmol of N m⁻² d⁻¹) by cruise. The Amazon plume (salinity <35) is shaded in gray. The areas of the circles are proportional to the rates. (Right) Surface dissolved inorganic carbon drawdown (μmol of C kg⁻¹); the location of the shallow traps are indicated with station number. The white-filled circle indicates negative values or carbon excess.

Fig. 2.

Vertical profiles of mean light depths, chlorophyll a concentration, C and N fixation rates, and NO₃ concentrations binned by station type, details shown in Table S2. The error bars represent standard error.

Fig. 3.

Changes along the river plume as it moves offshore. (Top) Changes in surface nutrient concentrations as a function of salinity for each of the station types; the values and statistics are presented in Table 1. Error bars denote standard error; the thick horizontal line on the x axis indicates the mean salinity ±1 S.E. for each group of stations. (Middle) Changes in biological response and mass flux from floating sediment traps at 200 m presented as in A. (Bottom) A schematic of changes along the plume; the arrows showing the mean mass flux for the mesohaline, and oceanic stations. The brown particles represent coastal phytoplankton species; the dark green represents DDA; the red represents Trichodesmium; and the blue represents particles typical of oligotrophic oceanic phytoplankton. Phytoplankton chlorophyll, Trichodesmium, and Richelia concentrations are given in Table 1. Water below the euphotic zone is depicted in solid dark blue, and the 1% light depths are given in Table 1.