Distribution, origin and cycling of carbon in the Tana River (Kenya): a dry season basin-scale survey from headwaters to the delta

Abstract. 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 δ¹³C 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 δ¹⁸O 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.