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A comprehensive biogeochemical record and annual flux estimates for the Sabaki River (Kenya)

Trent R. Marwick, Fredrick Tamooh, Bernard Ogwoka, Alberto V. Borges, François Darchambeau, Steven Bouillon
2018 Biogeosciences  
<p><strong>Abstract.</strong> Inland waters impart considerable influence on nutrient cycling and budget estimates across local, regional and global scales, whilst anthropogenic pressures, such as rising populations and the appropriation of land and water resources, are undoubtedly modulating the flux of carbon (C), nitrogen (N) and phosphorus (P) between terrestrial biomes to inland waters, and the subsequent flux of these nutrients to the marine and atmospheric domains. Here, we present a
more » ... e, we present a 2-year biogeochemical record (October 2011–December 2013) at biweekly sampling resolution for the lower Sabaki River, Kenya, and provide estimates for suspended sediment and nutrient export fluxes from the lower Sabaki River under pre-dam conditions, and in light of the approved construction of the Thwake Multipurpose Dam on its upper reaches (Athi River). Erratic seasonal variation was typical for most parameters, with generally poor correlation between discharge and material concentrations, and stable isotope values of C (<i>δ</i><sup>13</sup>C) and N (<i>δ</i><sup>15</sup>N). Although high total suspended matter (TSM) concentrations are reported here (up to ∼<span class="thinspace"></span>3.8<span class="thinspace"></span>g<span class="thinspace"></span>L<sup>−1</sup>), peak concentrations of TSM rarely coincided with peak discharge. The contribution of particulate organic C (POC) to the TSM pool indicates a wide biannual variation in suspended sediment load from OC poor (0.3<span class="thinspace"></span>%) to OC rich (14.9<span class="thinspace"></span>%), with the highest %POC occurring when discharge is &amp;lt;<span class="thinspace"></span>100<span class="thinspace"></span>m<sup>3</sup><span class="thinspace"></span>s<sup>−1</sup> and at lower TSM concentrations. The consistent <sup>15</sup>N enrichment of the particulate nitrogen (PN) pool compared to other river systems indicates anthropogenic N loading is a year-round driver of N export from the Sabaki Basin. The lower Sabaki River was consistently oversaturated in dissolved methane (CH<sub>4</sub>; from 499 to 135<span class="thinspace"></span>111<span class="thinspace"></span>%) and nitrous oxide (N<sub>2</sub>O; 100 to 463<span class="thinspace"></span>%) relative to atmospheric concentrations. Wet season flows (October–December and March–May) carried &amp;gt;<span class="thinspace"></span>80<span class="thinspace"></span>% of the total load for TSM (∼ 86<span class="thinspace"></span>%), POC (∼<span class="thinspace"></span>89<span class="thinspace"></span>%), dissolved organic carbon (DOC; ∼ 81<span class="thinspace"></span>%), PN (∼ 89<span class="thinspace"></span>%) and particulate phosphorus (TPP; ∼<span class="thinspace"></span>82<span class="thinspace"></span>%), with &amp;gt;<span class="thinspace"></span>50<span class="thinspace"></span>% of each fraction exported during the long wet season (March–May). Our estimated sediment yield (85<span class="thinspace"></span>Mg<span class="thinspace"></span>km<sup>−2</sup><span class="thinspace"></span>yr<sup>−1</sup>) is relatively low on the global scale and is considerably less than the recently reported average sediment yield of ∼ 630<span class="thinspace"></span>Mg<span class="thinspace"></span>km<sup>−2</sup><span class="thinspace"></span>yr<sup>−1</sup> for African river basins. Regardless, sediment and OC yields were all at least equivalent or greater than reported yields for the neighbouring dammed Tana River. Rapid pulses of heavily <sup>13</sup>C-enriched POC coincided with peak concentrations of PN, ammonium, CH<sub>4</sub> and low dissolved oxygen saturation, suggesting that large mammalian herbivores (e.g. hippopotami) may mediate the delivery of C<sub>4</sub> organic matter to the river during the dry season. Given recent projections for increasing dissolved nutrient export from African rivers, as well as the planned damming of the Athi River, these first estimates of material fluxes from the Sabaki River provide base-line data for future research initiatives assessing anthropogenic perturbation of the Sabaki Basin.</p>
doi:10.5194/bg-15-1683-2018 fatcat:ftsuxwyabranvgeqksm7aqjwha