Preprints
https://doi.org/10.5194/bg-2021-92
https://doi.org/10.5194/bg-2021-92
23 Apr 2021
 | 23 Apr 2021
Status: this discussion paper is a preprint. It has been under review for the journal Biogeosciences (BG). The manuscript was not accepted for further review after discussion.

Unravelling Light and Microbial Activity as Drivers of Organic Matter Transformations in Tropical Headwater Rivers

James F. Spray, Thomas Wagner, Juliane Bischoff, Sara Trojahn, Sevda Norouzi, Walter Hill, Julian Brasche, Leroy James, and Ryan Pereira

Abstract. Connecting tropical rainforests to larger rivers, tropical headwaters export large quantities of carbon and nutrients as dissolved organic matter (DOM), and are thus a key component of the global carbon cycle. This DOM transport is not passive, however; sunlight and microbial activity alter DOM concentrations and compositions, affecting riverine greenhouse gas emissions and downstream ecosystems. The effects of sunlight and microbial turnover/activity on DOM concentrations and compositions in tropical headwaters are currently poorly understood, but novel Size Exclusion Chromatography (SEC) techniques coupled to suitable detectors can for the first time quantify their influences. Here, we present in-situ incubation experiments from from headwaters of the Essequibo River, in the Iwokrama Rainforest, Guyana, where sunlight oxidised up to 9% of dissolved organic carbon (DOC) over 12 hours, at higher rates than in larger tropical rivers. DOM transformations occurred in both photo-sensitive and supposedly photo-resistant pools. Microbial activity had varying, less clear influences on DOC concentrations over the same time span; compositionally, this appeared to extend beyond known bio-labile components. Biopolymers were particularly reactive to both processes. We show sunlight has the greater potential to mineralise headwater DOM and thus potentially influence degassing. Our approach provides a future template to constrain DOM transformations along river networks, identify biogeochemical activity hotspots, and improve greenhouse gas emissions estimations under changing environmental conditions.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
James F. Spray, Thomas Wagner, Juliane Bischoff, Sara Trojahn, Sevda Norouzi, Walter Hill, Julian Brasche, Leroy James, and Ryan Pereira

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-92', Anonymous Referee #1, 18 May 2021
    • AC1: 'Reply on RC1', James Spray, 16 Jul 2021
  • RC2: 'Comment on bg-2021-92', Anonymous Referee #2, 14 Jun 2021
    • AC2: 'Reply on RC2', James Spray, 16 Jul 2021

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-92', Anonymous Referee #1, 18 May 2021
    • AC1: 'Reply on RC1', James Spray, 16 Jul 2021
  • RC2: 'Comment on bg-2021-92', Anonymous Referee #2, 14 Jun 2021
    • AC2: 'Reply on RC2', James Spray, 16 Jul 2021
James F. Spray, Thomas Wagner, Juliane Bischoff, Sara Trojahn, Sevda Norouzi, Walter Hill, Julian Brasche, Leroy James, and Ryan Pereira
James F. Spray, Thomas Wagner, Juliane Bischoff, Sara Trojahn, Sevda Norouzi, Walter Hill, Julian Brasche, Leroy James, and Ryan Pereira

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Short summary
Sunlight and microbial activity break down river dissolved organic matter (DOM), releasing greenhouse gases, but are poorly understood in tropical headwaters. We incubated water in light and darkness, using novel methods to quantify changes in DOM concentration and character. Light removed up to 9 % of DOM, but microbial activity had a varied response. Importantly, light affected DOM compounds considered photo-resistant; likewise microbial activity degraded compounds thought to be bio-resistant.
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