Fate of the Amazon River dissolved organic matter in the tropical Atlantic Ocean

Abstract

Constraining the fate of dissolved organic matter (DOM) delivered by rivers is a key to understand the global carbon cycle, since DOM mineralization directly influences air-sea CO 2 exchange and multiple biogeochemical processes. The Amazon River exports large amounts of DOM, and yet the fate of this material in the ocean remains unclear. Here we investigate the molecular composition and transformations of DOM in the Amazon River-ocean continuum using ultrahigh resolution mass spectrometry and geochemical and biological tracers. We show that there is a strong gradient in source and composition of DOM along the continuum, and that dilution of riverine DOM in the ocean is the dominant pattern of variability in the system. Alterations in DOM composition are observed in the plume associated with the addition of new organic compounds by phytoplankton and with bacterial and photochemical transformations. The relative importance of each of these drivers varies spatially and is modulated by seasonal variations in river discharge and ocean circulation. We further show that a large fraction (50-76%) of the Amazon River DOM is surprisingly stable in the coastal ocean. This results in a globally significant river plume with a strong terrigenous signature and in substantial export of terrestrially derived organic carbon from the continental margin, where it can be entrained in the large-scale circulation and potentially contribute to the long-term storage of terrigenous production and to the recalcitrant carbon pool found in the deep ocean.

Microbially-Mediated Transformations of Estuarine Dissolved Organic Matter

“…As such, compounds that decreased or increased in relative abundance had distinct chemical characteristics. There was a relative enrichment of molecules that occupy the upper left corner of the van Krevelen diagram after the incubation (Figure 2B), i.e., a shift toward low O/C and high H/C ratios, which is consistent with molecular transformations observed in dark incubations of DOM from the Amazon River mouth (Medeiros et al, 2015c;Seidel et al, 2015). Additionally, while formulae whose relative abundances strongly decreased after the incubation (i.e., more negative loadings of PC1 DOB−long in Figure 2B) were characterized by high masses (553.5 ± 78.7 Da), formulae whose relative abundances were enriched after the incubation (i.e., more positive PC1 DOB−long loading in Figure 2B) were characterized by significantly lower masses (286.0 ± 93.9 Da) ( Figure 3A and Table 1; Wilcoxon rank-sum test, p < 0.01).…”

“…The correlation coefficient between PC1 and salinity for the pre-incubation samples was −0.87 (p < 0.01). The loading of PC1 ( Figure 1B) shows a pattern typical of river-to-ocean transects (Medeiros et al, 2015c), with high salinity samples enriched with compounds with high H/C ratios compared to lowsalinity samples, which is consistent with the higher abundance of aliphatic structures in marine DOM (Sleighter and Hatcher, 2008;Osterholz et al, 2016). This is also consistent with the strong gradient in DOM sources in the system associated with terrigenous vs. oceanic inputs (Medeiros et al, 2015a).…”

“…As riverine DOM is transported toward the ocean, its quantity and composition is often altered (Cole et al, 2007) by a variety of chemical, biological and physical processes (Tzortziou et al, 2008), making estuaries hot spots of DOM cycling. Significant gradients exist in the composition of DOM in estuaries (Sleighter and Hatcher, 2008;Medeiros et al, 2015c;Seidel et al, 2015). In general, DOC concentrations and the DOM aromaticity decrease toward the ocean (Abdulla et al, 2013) at the same time that DOM lability increases (D'Andrilli et al, 2015).…”

“…Despite these challenges, much has been learned by tracking changes in DOC concentration and DOM composition during incubation experiments, including the variation in biomineralization rates as a function of DOM source (e.g., Moran and Zepp, 1997;Obernosterer and Benner, 2004), the relationship of microbial activity to the production of labile (e.g., Kawasaki and Benner, 2006) and refractory (e.g., Ogawa et al, 2001;Jiao et al, 2010;Lechtenfeld et al, 2015) DOM, and how bacterial-derived refractory material compares structurally to refractory DOM in the ocean (e.g., Osterholz et al, 2015). Studies using ultrahigh resolution mass spectrometry have revealed additional molecular-level transformations in DOM as a result of microorganism activity (Kujawinski et al, 2004), including a significant decrease in the molecular diversity of terrigenous DOM (Seidel et al, 2015) and a preferential degradation of oxygen-rich molecules on a time scale of several days (Kim et al, 2006;Medeiros et al, 2015c;Seidel et al, 2015).…”

Deciphering associations between dissolved organic molecules and bacterial communities in a pelagic marine system

“…Most molecular formulae with positive correlations to the first canonical axes of salinity (Figures 5c and d) and DNA ( Figure 6a) were relatively saturated (H/C ratio of 41), which is typical for marine DOM (Koch et al, 2005;Sleighter and Hatcher, 2008;Kujawinski et al, 2009;Medeiros et al, 2015). Some combustionderived condensed aromatic compounds ('black carbon', very low H/C and O/C ratios) also partially correlated with the canonical axis of DNA.…”