Title: Carbon cycling from a catchment perspective: an integrated approach to terrestrial-aquatic linkages in the Tana River basin, Kenya.
Other Titles: Koolstof cycling vanuit een stroombekken perspectief: een geïntegreerde benadering van terrestrische en aquatische links in het Tana River basin, Kenia.
Authors: Tamooh, Fredrick
Issue Date: 17-Jun-2013
Abstract: Rivers play an important role in the global carbon (C) cycle. They do not merely transport C from the terrestrial biome to the oceanic environment, but also bury and process organic matter, and thereby generally act as a source of CO2 to the atmosphere. This study focuses on riverine C cycling in Tana River Basin, Kenya, the longest river in Kenya (~1100 km), with a total catchment area of ~96,000 km². Data were collected during three basin-wide surveys along an altitudinal gradient, from 3600 m in headwaters to 8 m in lower Tana River in February 2008 (dry season), September-November 2009 (wet season), and June-July 2010 (end of wet season) as well as monthly sampling at three sites in lower Tana River between January 2009 and December 2011. A consistent downstream increase in total suspended matter (TSM, 0.6 to 7058 mg L-1) and particulate organic carbon (POC) was observed during all three sampling campaigns. With the exception of reservoir waters, POC was predominantly of terrestrial origin as indicated by generally high POC: Chlorophyll a (POC: Chl-a) ratios. Stable isotope signatures of POC (d13CPOC) ranged between -32‰ and -20‰ and increased downstream, reflecting an increasing contribution of C4-derived carbon. Dissolved organic carbon (DOC) concentrations were highest during the end of wet season (2.1 to 6.9 mg L-1), with stable isotope signatures generally between -28‰ and -22‰. A consistent downstream decrease in % organic carbon (%OC) was observed for soils, riverine sediments, and suspended matter. This was likely due to better preservation of the organic fraction in colder high altitude regions, with loss of carbon during downstream spiraling. d13C values for soil and sediment did not exhibit clear altitudinal patterns, but values reflect the full spectrum from C3-dominated to C4-dominated sites. With few exceptions, the entire riverine network was supersaturated in CO2. pCO2 values were generally higher in the lower main Tana River compared to headwater tributaries, opposite to the pattern typically observed in other river networks. First-order estimates show in-stream community respiration was responsible for the bulk of total CO2 evasion (59% to 89%) in the main Tana River while in tributaries respiration accounted for 4% to 52% of total CO2 evasion, suggesting CO2 evasion in tributaries was sustained by lateral CO2 inputs. While sediment loads increase downstream, both chlorophyll a (0.2 µg L-1 to 9.6 µg L-1) and primary production (0.004 µmol L-1 h-1 to 7.38 µmol L-1 h-1) increased consistently downstream. Diurnal fluctuations of biogeochemical parameters were found to be substantial only in the headwater stream, moderate in the reservoir and not detectable at main Tana River. The C sources supporting heterotrophic bacteria in both terrestrial soils and riverine sediments (bottom and suspended) were examined using d13C signatures of bacterial phospholipid-derived fatty acids (PLFA). Generally, bacterial communities appeared to use C3 and C4-derived carbon in the same proportions as they occur in the bulk organic matter pool across different environments. However, a positive correlation between d13C signatures of bacterial and green algal PLFA in the tributaries provides an indication for preferential use of this more labile C source when available. Annual fluxes of suspended sediments and different carbon species were quantified at different stations in the lower Tana River. Both TSM (24 to 9386 mg L-1) and POC (0.8 to 141.9 mg C L-1) concentrations were highly variable. DOC and DIC concentrations ranged from 0.8 to 5.2 mg C L-1 and 7.1 to 23.5 mg C L-1, respectively, but their seasonal variations were not systematic. A strong shift in the origin of POC was observed, being dominated by C3-derived C during dry conditions (low d13CPOC signatures between -28‰ and -25‰), but with significant C4 contributions during higher flow events (d13CPOC up to -19.5 ‰). In contrast, d13CDOC did not show any clear pattern with discharge despite the high range (-27.2‰ to -17.4‰). Over 80% of the sediment and organic carbon was transported during the high flow seasons (March to May and October to December combined). The floodplains along the main Tana River appear to play a critical role in regulating the transport of sediments and organic carbon as reflected by a significant reduction (35-65%) in the sediment, POC and DOC delivery along the stretch between Garissa and Tana River Primate Reserve (~350 km). Considering the additional trapping of sediments and carbon in the upstream reservoirs, and in floodplains downstream of the studied river section, it becomes evident that a major fraction of the sediment and C inputs into the Tana River network are retained and/or processed prior to entering the Indian Ocean.
ISBN: 978-90-8826-312-5
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Division Soil and Water Management

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