Along-stream transport and transformation of dissolved organic matter in a large tropical river

Thibault Lambert,Alberto V Borges,Philippe Massicotte,François Darchambeau,Steven Bouillon,Cristian R Teodoru,Frank C Nyoni

doi:10.5194/bg-13-2727-2016

Thibault Lambert, Alberto V Borges + Show 5 more

Open Access

https://doi.org/10.5194/bg-13-2727-2016

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Abstract

Abstract. Large rivers transport considerable amounts of terrestrial dissolved organic matter (DOM) to the ocean. However, downstream gradients and temporal variability in DOM fluxes and characteristics are poorly studied at the scale of large river basins, especially in tropical areas. Here, we report longitudinal patterns in DOM content and composition based on absorbance and fluorescence measurements along the Zambezi River and its main tributary, the Kafue River, during two hydrological seasons. During high-flow periods, a greater proportion of aromatic and humic DOM was mobilized along rivers due to the hydrological connectivity with wetlands, while low-flow periods were characterized by lower DOM content of less aromaticity resulting from loss of connectivity with wetlands, more efficient degradation of terrestrial DOM and enhanced autochthonous productivity. Changes in water residence time due to contrasting water discharge were found to modulate the fate of DOM along the river continuum. Thus, high water discharge promotes the transport of terrestrial DOM downstream relative to its degradation, while low water discharge enhances the degradation of DOM during its transport. The longitudinal evolution of DOM was also strongly impacted by a hydrological buffering effect in large reservoirs in which the seasonal variability of DOM fluxes and composition was strongly reduced.

Highlights

dissolved organic carbon (DOC) concentrations in the Zambezi River ranged from 1.9 ± 0.1 to 4.9 ± 1.0 mg L−1 during the wet periods and from 1.2 to 2.9 mg L−1 during the dry period (Fig. 3a)
In the Kafue River, DOC was generally higher during the wet seasons compared to the dry season
DOC concentrations in the upper Zambezi and the Kafue River were closely correlated with the extent of wetlands (Fig. 4)

Summary

Introduction

The composition, transport and transformation of dissolved organic matter (DOM) in large rivers are key aspects for determining regional and global carbon (C) budgets (Schlesinger and Melack, 1981), the fate of terrigenous DOM flowing to the oceans (del Giorgio and Pace, 2008; Massicotte and Frenette, 2011), the influence of fluvial inputs on DOM biogeochemistry in coastal and oceanic environments (Holmes et al, 2008), and the functioning of inland waters as active pipes with regards to the global C cycle (Cole et al, 2007; Borges et al, 2015a). Riverine DOM is mainly derived from terrestrial soils (e.g., Weyhenmeyer et al, 2012), but it can be fueled by sources within the aquatic system (Lapierre and Frenette, 2009; Massicotte and Frenette, 2011). Once in the aquatic ecosystem, terrestrial DOM is exposed to in-stream processing such as photodegradation (Cory et al, 2007; Spencer et al, 2009), microbial respiration (Amon and Benner, 1996; Fasching et al, 2014), and flocculation (von Wachenfeldt and Tranvik, 2008) that usually operate simultaneously and lead to the removal and the transformation of DOM during its transport (Massicotte and Frenette, 2011; Cawley et al, 2012).

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