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Abstract
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.
Highlights
River systems represent the primary pathway for carbon transport from the terrestrial to the marine environment, and are critical in determining the quantity and composition of carbon reaching the coastal zone
Given the large altitudinal gradient covered in this survey, water temperatures ranged widely from as low as 11.7◦C in some upper headwater streams to ∼30◦C in the lower Tana River, and high temperatures in surface waters of Masinga Reservoir (Table 1)
The elevated water residence time in Masinga reservoir and associated biological processes were responsible for a major reduction in dissolved inoraganic nitrogen (DIN) (Tables 1 and 3)
Summary
River systems represent the primary pathway for carbon transport from the terrestrial to the marine environment, and are critical in determining the quantity and composition of carbon reaching the coastal zone. A recent data compilation suggests a substantial transfer of ∼2 Pg C y−1 from the terrestrial biome into freshwater systems, yet less than half of this is estimated to reach the ocean (Cole et al, 2007). Freshwater systems function as biogeochemical “hotspots” on the land-ocean interface (McClain et al, 2003). Due to the overwhelming evidence for a high degree of biogeochemical processing of organic matter in freshwater systems (e.g., Wollheim et al, 2006; Cole et al, 2007; Battin et al, 2008), the original view of rivers as mere inactive conduits for organic matter and nutrients has significantly evolved.
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