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

[摘要] We studied patterns in organic carbon pools and their origin in the TanaRiver Basin (Kenya), in February 2008 (dry season), September–November 2009(wet season), and June–July 2010 (end of wet season), covering the fullcontinuum from headwater streams to lowland mainstream sites. A consistentdownstream increase in total suspended matter (TSM, 0.6 to7058 mg l⁻¹ and particulate organic carbon (POC, 0.23 to119.8 mg l⁻¹ was observed during all three sampling campaigns,particularly pronounced below 1000 m above sea level, indicating that mostparticulate matter exported towards the coastal zone originated from the midand low altitude zones rather than from headwater regions. This indicatesthat the cascade of hydroelectrical reservoirs act as an extremely efficientparticle trap. Although ⁷Be / ²¹⁰Pb_xs ratios/age ofsuspended sediment do not show clear seasonal variation, the gradualdownstream increase of suspended matter during end of wet season suggests itsorigin is caused by inputs of older sediments from bank erosion and/or riversediment resuspension. During wet season, higher TSM concentrationscorrespond with relatively young suspended matter, suggesting a contributionfrom recently eroded material. With the exception of reservoir waters, POCwas predominantly of terrestrial origin as indicated by generally highPOC : chlorophyll a (POC : Chl a) ratios (up to ~41 000).Stable isotope signatures of POC (δ¹³C_POC rangedbetween −32 and −20‰ and increased downstream, reflecting anincreasing contribution of C4-derived carbon in combination with an expectedshift in δ¹³C for C3 vegetation towards the more semi-aridlowlands. δ¹³C values in sediments from the main reservoir(−19.5 to −15.7‰) were higher than those found in any of theriverine samples, indicating selective retention of particles associated withC4 fraction. Dissolved organic carbon (DOC) concentrations were highestduring the end of wet season (2.1 to 6.9 mg l⁻¹), with stableisotope signatures generally between −28 and −22‰. A consistentdownstream decrease in % organic carbon (%OC) was observed for soils,riverine sediments, and suspended matter. This was likely due to betterpreservation of the organic fraction in colder high altitude regions, withloss of carbon during downstream spiraling. δ¹³C values for soiland sediment did not exhibit clear altitudinal patterns, but values reflectthe full spectrum from C3-dominated to C4-dominated sites. Very low ratios oforganic carbon to mineral surface area (OC : SA) were found in reservoirsediments and suspended matter in the lower Tana River, indicating that theseare stable OC pools which have undergone extensive degradation. Overall, ourstudy demonstrates that substantial differences occur in both the quantitiesand origin of suspended sediments and organic carbon along the river profilein this tropical river basin, as well as seasonal differences in themechanisms causing such variations.

[学科分类] 地球化学与岩石