Distribution, origin and cycling of carbon in the Tana River (Kenya): a dry season basin-scale survey from headwaters to the delta
Bouillon, S.; Abril, G.; Borges, A.V.; Dehairs, F.; Govers, G.; Hughes, H.J.; Merckx, R.; Meysman, F.J.R.; Nyunja, J.; Osburn, C.; Middelburg, J.J. (2009). Distribution, origin and cycling of carbon in the Tana River (Kenya): a dry season basin-scale survey from headwaters to the delta. Biogeosciences 6(11): 2475-2493. http://dx.doi.org/10.5194/bg-6-2475-2009
The Tana River basin (TRB) is the largest in Kenya (~120 000 km2). We conducted a survey during the dry season throughout the TRB, analyzing a broad suite of biogeochemical parameters. Biogeochemical signatures in headwater streams were highly variable. Along the middle and lower river course, total suspended matter (TSM) concentrations increased more than 30-fold despite the absence of tributary inputs, indicating important resuspension events of internally stored sediment. These resuspended sediment inputs were characterized by a lower and 14C-depleted OC content, suggesting selective degradation of more recent material during sediment retention. Masinga Dam (a large reservoir on the upper river) induced a strong nutrient retention (~50% for inorganic N, ~72% for inorganic phosphate, and ~40% for dissolved silicate). Moreover, while DOC pools and d13C signatures were similar above, in and below the reservoir, the POC pool in Masinga surface waters was dominated by 13C-depleted phytoplankton, which contributed to the riverine POC pool immediately below the dam, but rapidly disappeared further downstream, suggesting rapid remineralization of this labile C pool in the river system. Despite the generally high turbidity, the combination of relatively high oxygen saturation levels, low d18O signatures of dissolved O2 (all <+24.2‰), and the relatively low pCO2 values suggest that in-stream primary production was significant, even though pigment data suggest that phytoplankton makes only a minor contribution to the total POC pool in the Tana River.
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