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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 two year biogeochemical record (October 2011-December 2013) at bi-weekly sampling resolution for the lower Sabaki River, Kenya, and provide estimates for suspended sediment and nutrient export fluxes from the Athi-Galana-Sabaki (A-G-S) river basin under pre-dam conditions, and in light of the approved construction of the Thwake Multi-purpose Dam on the Athi River. Erratic seasonal variation was typical for most parameters, with generally poor correlation between discharge and material concentrations and stable isotopic signatures of C (δ13C) and N (δ15N). Although high total suspended matter (TSM) concentrations are reported here (up to ~ 3.8 g L-1), peak concentrations of TSM rarely coincided with peak discharge. The contribution of particulate organic C (POC) to the TSM pool indicates a wide bi-annual variation in suspended sediment load from OC-poor (0.3 %) to OC-rich (14.9 %), with the highest %POC occurring when discharge is < 100 m3 s-1 and at lower TSM concentrations. The consistent 15N enrichment of the PN pool compared to other river systems indicates anthropogenic N-loading is a year-round driver of N export from the A-G-S basin. The Sabaki River was consistently oversaturated in dissolved methane (CH4; from 499 % to 135 111 %) and nitrous oxide (N2O; 100 % to 463 %) relative to atmospheric concentrations. We estimate export fluxes to the coastal zone of 4.0 Tg yr-1, 70.6 Gg C yr-1, 9.4 Gg N yr-1, and 0.5 Gg P yr-1 for TSM, POC, and particulate forms of N (PN) and total P (TPP), respectively, and fluxes of 24.1 Gg C yr-1, 6.6 Gg N yr-1, and 11.2 Gg P yr-1 for dissolved forms of organic C (DOC), inorganic N (DIN), and phosphate (PO43-). Wet season flows (October-December and March-May) carried > 80 % of the total load for TSM (~ 86 %), POC (~ 89 %), DOC (~ 81 %), PN (~ 89 %) and TPP (~ 82 %), with > 50 % of each fraction exported during the long wet season (March-May). Our estimated sediment yield of 85 Mg km-2 yr-1 is relatively low on the global scale and is considerably less than the recently reported average sediment yield of ~ 630 Mg km-2 yr-1 for African river basins. Regardless, sediment and OC yields were all at least equivalent or greater than reported yields for the neighbouring and flow-regulated Tana River. Rapid pulses of heavily 13C-enriched POC coincided with peak concentrations of PN, ammonium, CH4 and low dissolved oxygen saturation, lead to the suggestion that large mammalian herbivores (e.g. hippopotami) may mediate the delivery of C4 organic matter to the river during the dry season. Given recent projections for increasing dissolved nutrient export from African rivers, as well as planned flow regulation on 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 A-G-S river basin. [ABSTRACT FROM AUTHOR]

Quantification of sediment and carbon (C) fluxes in rivers with strong seasonal and inter-annual variability presents a challenge for global flux estimates as measurement periods are often too short to cover all hydrological conditions. We studied the dynamics of the Tana River (Kenya) from 2012 to 2014 through daily monitoring of sediment concentrations at three sites (Garissa, Tana River Primate Reserve and Garsen) and daily monitoring of C concentrations in Garissa and Garsen during three distinct seasons. In wet seasons, C fluxes were dominated by particulate organic C (POC) and decreased downstream. Dry season fluxes of dissolved inorganic C (DIC) and POC had a similar share in total C flux at both locations while POC fluxes increased downstream. The dissolved organic C (DOC) flux did not show strong spatial nor temporal variations. The construction of constituent rating curves with a bootstrap method in combination with daily discharge data (1942-2014) provided potential sediment and C flux ranges as a function of annual discharge. At low annual discharge, our estimates generally predict a net decrease of sediment and C storage between the upstream and downstream site. As the annual discharge increases, our simulations shift toward net retention. This analysis allowed us to infer how variations in discharge regime, related to climate or human impacts, may affect riverine fluxes. Overall, we estimate that retention was dominant: integration over all simulations resulted in an average net retention of sediment (~2.9 Mt yr-1), POC (~18000 tC yr-1), DOC (~920 tC yr-1) and DIC (~1200 tC yr-1) over the 73 years of discharge measurements. [ABSTRACT FROM AUTHOR]