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Dissolved organic matter composition and reactivity in Lake Victoria, the world’s largest tropical lake

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Abstract

We report a data set of dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) composition (stable carbon isotope signatures, absorption and fluorescence properties) obtained from samples collected in Lake Victoria, a large lake in East Africa. Samples were collected in 2018–2019 along a bathymetric gradient (bays to open waters), during three contrasting seasons: long rainy, short rainy and dry, which corresponded to distinctly water column mixing regimes, respectively, stratified, semi-stratified and mixed regimes. Eight DOM components from parallel factor analysis (PARAFAC) were identified based on three-dimensional excitation–emission matrices (EEMs), which were aggregated into three main groups of components (microbial humic-like, terrestrial humic-like, protein-like). Spatially, the more productive bays were characterized by higher DOM concentration than deeper more offshore waters (fluorescence intensity and DOC were ~ 80% and ~ 30% higher in bays, respectively). Seasonally, the DOM pool shifted from protein-like components during the mixed regime to microbial humic-like components during the semi-stratified regime and to terrestrial humic-like components during the stratified regime. This indicates that pulses of autochthonous DOM derived from phytoplankton occurred when the lake was mixing, which increased the availability of dissolved inorganic nutrients. Subsequently, this freshly produced autochthonous DOM was microbially processed during the following semi-stratified regime. In the open waters, during the stratified regime, only terrestrial refractory DOM components remained because the labile and fresh stock of DOM created during the preceding mixed season was consumed. In the bays, the high terrestrial refractory DOM during the stratified regime may be additionally due to the allochthonous DOM input from the runoff. At the scale of the whole lake, the background refractory DOM probably comes mainly from precipitation and followed by river inputs.

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The full data-set is publically available at zenodo.org (Deirmendjian et al. 2020).

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Acknowledgements

This work was funded by the Fonds National de la Recherche Scientifique (FNRS) (Lavigas project, contract T.0156.18). The Académie Royale de Belgique (Fonds Agathon de Potter), the Fonds Léopold III and the University of Liège provided additional funding. We thank Marc-Vincent Commarieu for help during sampling, Zita Kelemen (KU Leuven) for her contribution to the stable isotope analyses, Nathalie Gypens (ULB) for access to the Perkin-Elmer UV/Vis 650S spectrophotometer, Sarah Safieddine for rain DOC data, three anonymous reviewers for constructive comments on a previous version of the manuscript, and the crew of the MV Hammerkop for their helpful assistance during the cruises: AVB is a research director at the FNRS.

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Correspondence to Loris Deirmendjian.

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Deirmendjian, L., Lambert, T., Morana, C. et al. Dissolved organic matter composition and reactivity in Lake Victoria, the world’s largest tropical lake. Biogeochemistry 150, 61–83 (2020). https://doi.org/10.1007/s10533-020-00687-2

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  • DOI: https://doi.org/10.1007/s10533-020-00687-2

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