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Accueil du site → Master → Pays Bas → 2012 → Effect of organic matter quality on ecosystem metabolism in the upper catchment of the Mara River, Kenya

UNESCO-IHE Institute for Water Education, Delft (2012)

Effect of organic matter quality on ecosystem metabolism in the upper catchment of the Mara River, Kenya

Salcedo Borda, J.S.

Titre : Effect of organic matter quality on ecosystem metabolism in the upper catchment of the Mara River, Kenya

Auteur : Salcedo Borda, J.S.

Université de soutenance : UNESCO-IHE Institute for Water Education, Delft

Grade : Master of Science (MS) 2012

Ecosystem metabolism is an integrative measure of the processes controlling nutrient cycling and organic matter dynamics in streams. Dissolved organic matter (DOM) is a supply of organic carbon and nitrogen for the aquatic food webs, and therefore has a direct and indirect impact on biogeochemical processes in aquatic ecosystems. The extent to which these organic forms of nitrogen and carbon are metabolized in freshwaters depends on the biogeochemical composition of DOM. Despite its recognized importance in freshwater system, characterization of DOM quality has been a focus on marine ecosystem rather than in river waters. Also, the source and importance of different organic matter quality in the stream channel can be altered by the land use in a streams’s catchment. The aim of this study is to assess the link between the organic matter quality and ecosystem metabolism in the upstream of the Mara River with different land use/land cover. Whole-stream metabolism was measured in ten sampling sites with different land uses (4 forests, 2 mixed and 4 agricultural) were selected in the upper catchment of Mara River, using open system two stations method, in which concentrations of dissolved oxygen concentration was measured over diurnal cycle and atmospheric exchange was measured using a propane gas tracer. Measures of metabolism were related to DOM quality, as measured by optical characteristics. Additionally, fifty sites, along the Nyangores and Amala sub catchments with contrasting land use, were sampled using a synoptic approach in which water samples were analyzed for fluorescence and absorbance spectra and, DOC, DON and nutrient concentration. Excitation-emission matrices were obtained from the fluorescence spectra to characterize DOM quality and relate it to land use. Results have shown that these streams from the upper Mara catchment are heterotrophic with net ecosystem production range from -1.2 to -7.4 gO2 m-2d-1\. Higher concentration of nutrients and DOC were found in agricultural than in forested and mixed sites. High aromatic fraction and high molecular weight of DOC were found in all the streams, making them less labile DOM for community respiration. These results show that no significant correlations were found that relate ecosystem metabolism measurements and organic matter quality with land use in the upper Mara catchment. This study is one of the first to attempt the link between the measures of organic matter quantity and quality to ecosystem metabolism in tropical streams from Kenya. Therefore, for future studies we recommend that the measurement of change of dissolved oxygen should be done for longer periods in order to see the difference of ecosystem metabolism rate under base flow and storm events.

Sujets  : organic matter ecosystems metabolism rivers Kenya


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