The Pulse of the Amazon: Fluxes of Dissolved Organic Carbon, Nutrients, and Ions From the World's Largest River

Abstract

On a global scale, rivers are important processors and conduits of terrestrially derived materials, transporting significant quantities of dissolved and particulate elements to downstream aquatic ecosystems, the atmosphere, and the ocean (Aufdenkampe et al., 2011;Gaillardet et al., 1999;Meybeck, 1982). In particular, the riverine fluxes of carbon (C), nitrogen (N), and phosphorus (P) represent important components of terrestrial net ecosystem production (NEP) and the transfer of energy and nutrients to lotic and marine ecosystems. Major and trace elements in rivers, on the other hand, reflect watershed geology, chemical weathering rates, and hydrology. The concentration, proportions, and fluxes of these elements in rivers represent integrated signals of the various processes that occur throughout the drainage basin, including mobilization, dilution, transport, transient storage, and weathering (Wohl, 2020). Time series datasets of concentration and water discharge allow for further refinement of annual flux estimates and can indicate the degree to which a given constituent is mobilized, diluted, or responds chemostatically with respect to discharge (Godsey et al., 2009;House & Warwick, 1998). Moreover, the geochemistry of rivers can be used to estimate the rate of continental denudation and the related consumption of atmospheric CO 2 , one of the major goals of river geochemistry (Gaillardet et al., 1999).

Drivers of Organic Molecular Signatures in the Amazon River

“…These compositional differences can be interpreted in the context of bioavailability for heterotrophs because smaller, reduced, and aliphatic compounds are typically more biolabile, whereas larger, aromatic, and oxygenated compounds are usually more stable on the timescales considered here (D’Andrilli et al., 2015; Riedel et al., 2016; Textor et al., 2019). These trends are consistent with previous Amazonian DOM compositions from Óbidos, where aromatic compounds were relatively enriched at peak discharge (Drake et al., 2021; Seidel et al., 2016), as well as from other high discharge (Mann et al., 2014; Pang et al., 2021; Spencer et al., 2016) and arctic (Johnston et al., 2018; Spencer et al., 2008; Stedmon et al., 2011) rivers, with their diverse compositions suggesting heterogeneous sources. Riverine aromatic and oxygenated compounds typically originate from organic soil horizons and fresh plant litterfall that are mobilized during periods of increased hydrological connectivity, whereas microbial DOM is leached from deeper subsurface soil and groundwater under baseflow (McLaughlin & Kaplan, 2013; Wagner et al., 2019).…”

“…However, monthly concentrations during the low discharge period were attenuated during the La Niña year (Figure 3b, green shaded region) indicating greater dilution from the widespread precipitation that occurred in the Northern and Western sub‐basins at that time (Espinoza et al., 2013). Average SiO 2 concentrations were also significantly higher during the normal year (9.94 ± 1.96 vs. 8.02 ± 0.23 mg L −1 , p = 0.006; Figure 3c) reflecting the greater proportion of Andean water passing through Óbidos during the La Niña and diluting locally weathered Si from lower Amazon reaches (Devol et al., 1995; Drake et al., 2021; Moquet et al., 2016).…”

“…The distinction between the years was also supported using inorganic geochemical measurements from the Amazon mainstem at Óbidos. Major dissolved inorganic ions (Cl − , SO 4 2− , Ca 2+ , Mg 2+ , Na + , K + ) sourced primarily from the Andes (Moquet et al., 2016) displayed predictable seasonal variability with higher concentrations in the dry period and lower concentrations in the wet period due to dilution (Devol et al., 1995; Drake et al., 2021; Moquet et al., 2016) (Figure 3b). However, monthly concentrations during the low discharge period were attenuated during the La Niña year (Figure 3b, green shaded region) indicating greater dilution from the widespread precipitation that occurred in the Northern and Western sub‐basins at that time (Espinoza et al., 2013).…”

“…The Amazon is the largest river on Earth by discharge and drainage area, contributing almost 20% of the global riverine discharge to the ocean and draining a massive basin of over 6 × 10 6 km 2 (Raymond & Spencer, 2015). The Amazon River transports inorganic solutes, which are diluted with increasing discharge, and biogenic solutes, which are enriched with discharge; the so‐called “rhythm” of the Amazon (Devol et al., 1995; Drake et al., 2021; Moquet et al., 2016; Richey et al., 1990). Among the biogenic solutes, DOM is transported from the high Andes, grasslands, forests, and floodplains, and continually processed on the journey to the Atlantic Ocean (Aufdenkampe et al., 2007; Benner et al., 1995; Seidel et al., 2016; Ward et al., 2013).…”

Effects of hydrologic variability and remedial actions on first flush and metal loading from streams draining the Silverton caldera, 1992–2014

“…Load estimation techniques are frequently used to quantify nutrient loads and most LOADEST applications to date are focused on nitrate, phosphorus, dissolved organic carbon or sediment load (e.g. Drake et al, 2021; additional applications at https://water.usgs.gov/software/loadest/apps/). LOADEST applications involving metals and/or mining‐affected watersheds are virtually non‐existent, possibly because of the reactive nature of many metals.…”