Organic Molecular Signatures of the Congo River and Comparison to the Amazon. Issue 6 (18th June 2022)

• Record Type: Journal Article
• Authors: Kurek, Martin R.; Stubbins, Aron; Drake, Travis W.; Dittmar, Thorsten; M. S. Moura, José; Holmes, R. Max; Osterholz, Helena; Six, Johan; Wabakanghanzi, José N.; Dinga, Bienvenu; Mitsuya, Miyuki; Spencer, Robert G. M.
• Is Part Of: Global biogeochemical cycles. Volume 36:Issue 6(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-06-18
• Subjects: DOM -- DOC -- Amazon -- Congo -- FT‐ICR MS; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1029/2022GB007301
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: The Congo and Amazon are the two largest rivers on Earth and serve as major sources of dissolved organic carbon to the ocean. We compared the dissolved organic matter (DOM) composition of both rivers using Fourier‐transform ion cyclotron resonance mass spectrometry to investigate seasonal and regional differences in DOM composition exported to the ocean. We found that over a 15‐month observational period in the Congo River, molecular aromaticity and oxygenation between the wet and dry periods varied slightly, but most of the relative abundance of DOM formulae (∼90%) were present in all samples, suggesting that Congo River DOM quality is stable across different hydrological conditions. In contrast, the multi‐year DOM composition in the Amazon River was highly susceptible to changes in hydrology, with clear differences in molecular aromaticity, oxygenation, and heteroatom (N, S, P) content between the wet and dry seasons. Overall, the DOM composition of the Congo River was more terrestrial than Amazon River DOM, which was more characteristic of aquatic DOM. Finally, we compared the relative contribution of island of stability (IOS) formulae between the rivers and found that both rivers export similar amounts of these formulae annually, more than several major rivers combined, and that the Congo is more than twice as efficient in exporting these IOS formulae. With changing precipitation and land use, the quantity and composition of exported DOM will likely reflect the mobilization of additional terrestrial and anthropogenic sources that will also be subjected to downstream land‐to‐ocean cycling. Plain Language Summary: Almost 16% of the global riverine dissolved organic carbon (DOC) export is delivered by just two rivers: Amazon and Congo. Although both rivers drain immense tropical rainforests, differences in landscape, precipitation, and riverine discharge all contribute to the distinct molecular composition of their dissolved organic matter (DOM). We compared DOM composition of both rivers over several years and found that hydrology is an important driver for determining the DOM composition of both rivers; however, it is more important for the Amazon than Congo, which had DOM that was more similar across the wet and dry seasons. Overall, both rivers had more aromatic and oxygen‐rich DOM than temperate or arctic rivers, but DOM from the Congo had a more terrestrial signature than the Amazon River DOM. Finally, we found that DOM molecular formulae thought to be stable over long periods of time in marine systems were exported in similar amounts by both rivers, even though the Amazon exports about twice as much DOC annually compared to the Congo, suggesting that the Congo is more than twice as efficient per unit volume at exporting molecular formulae that is thought to be stable in the ocean for long periods of time. Key Points: Congo River dissolved organic matter (DOM) composition remains mostly stable across the year, while Amazon River DOM varies seasonally with discharge Amazon River DOM is less aromatic, less oxygenated, and more heteroatom‐rich than Congo River DOM Both rivers export similar amounts of island of stability molecular formulae but the Congo is over twice as efficient