Sediment and carbon fluxes along a longitudinal gradient in the lower Tana River (Kenya)

Abstract

We estimated annual fluxes of suspended matter and different carbon (C) pools at three sites along the lower Tana River (Kenya), based on monthly sampling between January 2009 and December 2011. Concentrations of total suspended matter (TSM), particulate organic carbon (POC), dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC) were monitored, as was the stable isotope composition of the carbon pools. Both TSM and POC concentrations showed strong seasonality, varying over several orders of magnitude, while DOC and DIC concentrations showed no seasonal variations. A strong shift in the origin of POC was observed, which was dominated by C3-derived C during dry conditions (low δ 13 C POC between À28‰ and À25‰), but had significant C4 contributions during high-flow events (δ 13 C POC up to À19.5‰). Between Garissa and the most downstream sampling point, a clear decrease in suspended matter and organic C fluxes was observed, being most pronounced during high-discharge conditions: on an annual basis, fluxes of TSM, POC, and DIC decreased by 34% to 65% for the 3 year study period. Our results suggest that floodplains along the lower Tana River could play an important role in regulating the transport of suspended matter and organic C. A comparison of current flux estimates with data collected prior to the construction of several hydropower dams reveals that the sediment loading is reduced during low discharge conditions.

Deposition and fate of organic carbon in floodplains along a tropical semiarid lowland river (Tana River, Kenya)

“…However, the amount of floodplain OC deposition is smaller than the total OC loss from the river system between Garissa and Tana Primate Reserve as estimated by Tamooh et al [2014], (53-68 Gg C yr À1 ), despite the fact that Tana Primate Reserve is located~80 km upstream of Garsen.…”

“…Here we present a study on C deposition and burial in the Tana River, a tropical river system located in Kenya. The lower Tana River has extensive floodplains and it has been suggested that substantial quantities of sediments and OC are retained within the system [Tamooh et al, 2014].…”

Seasonal and inter-annual variations in carbon fluxes in a tropical river system (Tana River, Kenya)

“…The historical paucity of datasets representing tropical river systems has been a strong impetus for the burgeoning research on sediment and C fluxes within these systems over the past decade, and has also witnessed a broadening geographical coverage: while the Amazon has traditionally functioned as the archetype for tropical rivers (Abril et al, 2014;Mayorga et al, 2005;Moreira-Turcq et al, 2003Richey et al, 2002), there has been an increase in data availability from other systems in Latin-America (Depetris and Kempe, 1993;Laraque et al, 2013;Mora et al, 2014), as well as a range of systems 15 in Asia (Aldrian et al, 2008;Bird et al, 2008;Sarin et al, 2002;Zhou et al, 2013), small islands (Lloret et al, 2011;Wiegner et al, 2009), and Africa (Borges et al, 2015;Bouillon et al, 2012;Brunet et al, 2009;Coynel et al, 2005;Tamooh et al, 2014;Wang et al, 2013;Zurbrügg et al, 2013).…”

“…Monitoring 20 campaigns most often achieve only monthly measurements or, in a few cases, 2-weekly or weekly measurements (e.g. Bouillon et al, 2012;Brunet et al, 2009;Tamooh et al 2014). The monitoring period in most cases lasted one or two years.…”

“…A newly collected dataset was merged with available data on sediment and C for the lower Tana River. The previously available data originate from longitudinal surveys (Bouillon et al, 2009;Tamooh et al, 2012Tamooh et al, , 2013 and monthly sampling efforts at Garissa and Tana River Primate Reserve (Tamooh et al, 2014). Based on those datasets, a strong retention of sediment, POC and DOC was inferred, which was then related to the exchange of C between the river and 30 its floodplains.…”