A comprehensive biogeochemical record and annual flux estimates for the Sabaki River (Kenya)
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Date
2018-03
Authors
Marwick, Trent R
Tamooh, Fredrick
Ogwoka, Bernard
Borges, Alberto V
Darchambeau, François
Bouillon, Steven
Journal Title
Journal ISSN
Volume Title
Publisher
Biogeosciences
Abstract
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 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 ( 13C) and N ( 15N). Although high total
suspended matter (TSM) concentrations are reported here
(up to 3.8 g L1/, 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 %) to OC rich (14.9 %), with the highest %POC occurring
when discharge is < 100m3 s1 and at lower TSM concentrations.
The consistent 15N 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 (CH4; from 499 to
135 111 %) and nitrous oxide (N2O; 100 to 463 %) relative
to atmospheric concentrations. Wet season flows (October–
December and March–May) carried > 80% of the total load
for TSM ( 86 %), POC ( 89 %), dissolved organic carbon
(DOC; 81 %), PN ( 89 %) and particulate phosphorus
(TPP; 82 %), with > 50% of each fraction exported during
the long wet season (March–May). Our estimated sediment
yield (85Mgkm2 yr1/ is relatively low on the global
scale and is considerably less than the recently reported average
sediment yield of 630Mgkm2 yr1 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 13Cenriched
POC coincided with peak concentrations of PN,
ammonium, CH4 and low dissolved oxygen saturation, suggesting
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
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.
Description
article
Keywords
annual flux estimates, Sabaki River
Citation
Marwick, T. R., Tamooh, F., Ogwoka, B., Borges, A. V., Darchambeau, F., & Bouillon, S. (2018). A comprehensive biogeochemical record and annual flux estimates for the Sabaki River (Kenya). Biogeosciences, 15(6), 1683-1700.