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Theses

Transport de carbone organique dissous dans un bassin versant agricole à nappe superficielle

Abstract : Increasing dissolved organic carbon (DOC) concentrations have been reported during the last four decades in streams from Northern Europe and North America. The aim of this thesis is to improve our knowledge on the main sources of DOC, the mechanisms that control the delivery of DOC to the stream and to quantify the role of hydro-climate and its variability on the production and transfer of DOC. This study takes place in a small agricultural experimental catchment, Kervidy-Naizin, located in Western France. We try to interpret mid-term daily time-series (7 years), and hourly time-series (8 floods) of DOC, by analysing the major climatic/hydrological factors considered as the principal regulators of DOC production and delivery to the stream. Finally a simple model have been developed to test some of the hypothesis suggested by the data analysis . We identified superficial layers of the valley bottom soils as the main source of DOC. From autumn to spring, this allochtonous component represents the major part of the flux of DOC in the river and is relayed by an autochtonous contribution in summer. A seasonal analysis identified two kinds of allochtonous sources: one source is only significant in autumn, and its size is limited,and appears to depend on the time of oxygenation of the superficial layers in the valley bottom during the previous summer; the second source is dominant from winter to spring, and seem to be non-limiting whatever the climatic conditions. We supposed that this second component correspond to the soil humic substances located in the valley bottom. The nature of the first one remains to be elucidated. While the Kervidy Naizin catchment is heavily affected by the intense agriculture activities as revealed by the average streamwater nitrate concentration of 80 mg.l-1, this study shows that the sources of DOC here is not essentially different from that observed in alpine or/and forested catchments. Hydro-climatic factors such as the water table variations or the temperature do not seem to have had any effects on the annual mean DOC concentration. Neither the deepening of the water table depth nor the extremely warm 2003 summer produced any increases of the mean annual DOC concentrations, which remained stable at 5.5 mg.l-1. As a whole, climatic factors may play a role in the observed DOC trends but this role likely occurs with interaction with others, yet unidentified, factors. The modelling approach confirmed the role of the superficial layers of the wetland soil as the principal source of DOC to the stream and the existence of an autochtonous production during summer. Complementary studies need to be conducted to determine the exact nature of DOC sources and to identify more precisely the production and decomposition dynamics of DOC.
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https://tel.archives-ouvertes.fr/tel-00440196
Contributor : Isabelle Dubigeon <>
Submitted on : Wednesday, December 9, 2009 - 4:57:40 PM
Last modification on : Monday, July 6, 2020 - 3:38:31 PM
Long-term archiving on: : Thursday, June 17, 2010 - 7:57:32 PM

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  • HAL Id : tel-00440196, version 1
  • PRODINRA : 248697

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B. Morel. Transport de carbone organique dissous dans un bassin versant agricole à nappe superficielle. Sciences de la Terre. Agrocampus - Ecole nationale supérieure d'agronomie de rennes, 2009. Français. ⟨tel-00440196⟩

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