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Distribution and origin of suspended sediments and organic carbon pools in the Tana River Basin, Kenya

Tamooh F., Van den Meersche K., Meysman F., Marwick T.R., Borges A.V., Merckx R., Dehairs F., Schmidt S., Nyunja J., Bouillon S.. 2012. Biogeosciences Discussions, 9 (3) : p. 2523-2566.

DOI: 10.5194/bgd-9-2523-2012

We studied patterns in organic carbon pools and their origin in the Tana River Basin (Kenya), in February 2008 (dry season), September-November 2009 (wet season), and June-July 2010 (end of wet season), and covering the full continuum from headwater streams to lowland mainstream sites. A consistent downstream increase in total suspended matter (TSM, 0.6 to 7058 mg l?1) and particulate organic carbon (POC, 0.23 to 119.8 mg l?1) was observed during all three sampling campaigns, particularly pronounced below 1000 m above sea level, indicating that most particulate matter exported towards the coastal zone originated from the mid and low altitude zones rather than from headwater regions. This indicates that the cascade of hydroelectrical reservoirs act as an extremely efficient particle trap. The decrease in 7Be/210Pbxs ratios of TSM downstream (range: 0.43 to 1.93) during the wet season indicated that the increasing sediment load in the lower Tana was largely due to recent surface erosion. During lower flow conditions, however, the gradual longitudinal increase in TSM coincided was more variable 7Be/210Pbxs ratios (0 to 4.5), suggesting that bank erosion and/or remobilisation of older sediments are the sources of the increasing TSM concentrations downstream. With the exception of reservoir waters, POC was predominantly of terrestrial origin as indicated by generally high POC/Chl-a ratios (up to ? 41 000). Stable isotope signatures of POC (?13CPOC) ranged between -32 and -20 ¿ and increased downstream, reflecting an increasing contribution of C4-derived carbon in combination with an expected shift in ?13C for C3 vegetation towards the more semi-arid lowlands. Sediments from the main reservoir (Masinga) showed ?13C values higher (-19.5 to -15.7 ¿ than found in any of the riverine samples, indicating selective retention of particles associated with C4 fraction. Dissolved organic carbon (DOC) concentrations were highest during the end of wet season (2.1 to 6.9 mg l?1), with a stable isotope signatures generally between -28 and -22 ¿. A consistent downstream decrease in % organic carbon (% OC) was observed for both soils, riverine sediments, and suspended matter; likely due to better preservation of the organic fraction in colder high altitude regions, with loss of carbon during downstream spiraling. Both ?13C values for soil and sediment did not exhibit clear altitudinal patterns, but values reflect the full spectrum from C3-dominated to C4-dominated sites. Very low ratios of organic carbon to mineral surface area (OC:SA) were found in reservoir sediments and suspended matter in the lower Tana River, indicating that these are stable OC pools which have undergone extensive degradation.

Mots-clés : bassin versant; carbone; propriété physicochimique du sol; Énergie hydraulique; Échantillonnage; réservoir d'eau; sédiment; kenya

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