Distribution and origin of suspended matter and organic carbon pools in the Tana River Basin, Kenya

Abstract

We studied patterns in organic carbon pools and their origin in the Tana River Basin (Kenya), in February 2008 (dry season), September–November 2009 (wet season), and June–July 2010 (end of wet season), covering the full continuum from headwater streams to lowland mainstream sites. A consistent downstream increase in total suspended matter (TSM, 0.6 to 7058 mg l<sup>−1</sup> and particulate organic carbon (POC, 0.23 to 119.8 mg l<sup>−1</sup> was observed during all three sampling campaigns, particularly pronounced below 1000 m above sea level, indicating that most particulate matter exported towards the coastal zone originated from the mid and low altitude zones rather than from headwater regions. This indicates that the cascade of hydroelectrical reservoirs act as an extremely efficient particle trap. Although <sup>7</sup>Be / <sup>210</sup>Pb<sub>xs</sub> ratios/age of suspended sediment do not show clear seasonal variation, the gradual downstream increase of suspended matter during end of wet season suggests its origin is caused by inputs of older sediments from bank erosion and/or river sediment resuspension. During wet season, higher TSM concentrations correspond with relatively young suspended matter, suggesting a contribution from recently eroded material. With the exception of reservoir waters, POC was predominantly of terrestrial origin as indicated by generally high POC : chlorophyll <i>a</i> (POC : Chl <i>a</i>) ratios (up to ~41 000). Stable isotope signatures of POC (δ<sup>13</sup>C<sub>POC</sub> ranged between −32 and −20‰ and increased downstream, reflecting an increasing contribution of C4-derived carbon in combination with an expected shift in δ<sup>13</sup>C for C3 vegetation towards the more semi-arid lowlands. δ<sup>13</sup>C values in sediments from the main reservoir (−19.5 to −15.7‰) were higher than those found in any of the riverine samples, indicating selective retention of particles associated with C4 fraction. Dissolved organic carbon (DOC) concentrations were highest during the end of wet season (2.1 to 6.9 mg l<sup>−1</sup>), 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. δ<sup>13</sup>C values for soil and sediment did not exhibit clear altitudinal patterns, but values reflect the full spectrum from C3-dominated to C4-dominated sites. Very low ratios of organic carbon to mineral surface area (OC : SA) were found in reservoir sediments and suspended matter in the lower Tana River, indi...

Baseline review and ecosystem services assessment of the Tana River Basin, Kenya

“…Tamooh et al (2012) report that analyses of suspended sediment loads in the Tana River reveal that lower basin sediments are now derived predominantly from riverbank erosion and occasional pulses from the lagas, confirming concerns that other researchers have raised that the dams are trapping most sediment. Such degradation has the potential to negatively affect livelihoods in communities that have become reliant on the nutrient-rich sediment-laden floodplains.…”

Disproportionate Contribution of Riparian Inputs to Organic Carbon Pools in Freshwater Systems

“…However, determining the origin of riverine carbon (C) at the landscape scale is often hampered by a lack of appropriate proxies distinguishing various landscape units and their constituent organic carbon (OC) pools. Riverine OC pools constitute an amalgam of allochthonous (for example, vegetation and soils) and autochthonous (for example, phytoplankton, benthic algae, aquatic macrophytes) C sources, with the relative contributions of each to riverine OC pools increasingly disentangled through multi-proxy approaches, such as the use of C stable isotopes (δ 13 C) in combination with elemental (for example, C:N) and particulate OC (POC):Chlorophyll a (Chl a ) ratios (Hamilton and Lewis 1992; Kendall and others 2001; Bernardes and others 2004; Finlay and Kendall 2007; Tamooh and others 2012), and more recently the incorporation of GIS-based toolsets (Ballester and others 2013). …”

Spatiotemporal Variation in Particulate Organic Carbon Based on Long-Term MODIS Observations in Taihu Lake, China

“…This is similar to the inland water of the Chena River in Alaska, where the mean POC/SPM value is 0.158 ± 0.074 [74]. The value for Taihu Lake is much higher than it is for the Tana River, a highly turbid river in Kenya (0.042 above reservoir and 0.046 below dam, due to the negligible contribution of phytoplankton), [75,76] and for sediment in Taihu Lake itself [58]. Unsurprisingly, it is much less than that of clear ocean water (Pacific entrance, 0.19; Cabo Pilar area, 0.28) [71] and extremely eutrophic reservoirs (Masinga Reservoir, 0.326 ± 0.036) [75].…”