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Distribution and cycling of carbon in the Tana river basin, from headwaters to the Tana river delta

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Distribution and cycling of carbon in the Tana

River Basin (Kenya), from headwaters to the

Tana River delta.

Data compilations on carbon fluxes in freshwater systems

invariably

indicate

that

tropical

regions

are

severely

underrepresented. Large-scale studies on carbon processing along

the flowpath of river basins have to date been concentrated on a very

limited number of systems, e.g., the Orinoco basin, the

Ganges-Brahmaputra, and an extensive body of work on the Amazon river

basin.

In this study, we present data on various biogeochemical

characteristics of a tropical river basin (Tana river, Kenya), along the

flowpath from high-altitude headwater streams in perennial catchment

areas, down to the lower river meandering through semi-arid plains.

Using a large suite of parameters both on particulate and dissolved

carbon pools and nutrients, we present a first basin-wide view on the

inputs and processing of carbon in this tropical river basin.

Introduction

Steven Bouillon

1,2, 3

, G.Abril

4

, A.V. Borges

5

, E. Chevalier

2

, F. Dehairs

2

, R.

Merckx

1

, C. Osburn

6

, K. Van Oost

7

, & J.J. Middelburg

3

1: Katholieke Universiteit Leuven, Dept. of Earth & Environmental Sciences, Kasteelpark Arenberg 20, B-3001 Leuven, Belgium. E-mail: steven.bouillon@ees.kuleuven.be; 2: Dept. of Analytical and Environmental Chemistry, Vrije Universiteit Brussel, Belgium; 3 : Netherlands Institute of Ecology, Centre for Estuarine and Marine Ecology, Yerseke, the Netherlands; 4: Environnements et Paléoenvironnements Océaniques, Université Bordeaux 1, France; 5: Unité d’Océanographie Chimique, Université de Liège, Belgium; 6: Department of Marine, Earth,& Atmospheric Sciences, NC State University, Raleigh, USA; 7: Dept. of Physical Geography, Université Catholique de Louvain, Belgium.

Sites

The Tana River originates in the vicinity of Mt. Kenya and is

the longest river system in Kenya (~1000 km), with a catchment

area of ~120,000 km². The three main perennial headwater

regions are located in high-altitude regions, i.e. the Aberdare

range, the S and E slopes of Mt. Kenya, and the Nyambene Hills.

A number of hydroelectric power dams have been constructed

along the river since the late 1960’s, the largest of which is

Masinga Dam. Sampling took place in February 2008, i.e. during

dry season (low river flow) conditions. Samples were taken in the

three headwater regions, along several points on the main Tana

river, and surface water was taken at 4 sites along a transect on

the largest of the reservoirs, Masinga Dam.

Surface waters and soil samples at selected locations were

analysed

for

a

wide

range

of

physico-chemical

and

biogeochemical parameters – the focus here is on carbon pools

and their stable and radiocarbon isotope composition.

R

e

s

u

lt

s

&

D

is

c

u

s

s

io

n

Acknowledgements

Thanks are due to Pieter van Rijswijk, Peter van Breugel, Marco Houtekamer (NIOO-CEME) and Mathieu Boudin (KIK, Brussels) for lab assistence, to Stéphanie Duvail, Olivier Hamerlynck and Jean Albergel for help in organising logistics, and to Gabriel Kung`u for his cheerful company and help during fieldwork. This work was supported by the

FWO-Vlaanderen (contracts G.0118.02,

G.0651.09)

http://www.vub.ac.be/ANCH http://www.nioo.knaw.nl/CEME/

►Surface waters of Masinga Dam appear highly

productive, with much lower pCO2(378 ± 57 ppm)

compared to the main river, high %O2 (111 ± 3 %) ,

and low δδδδ18O-O2(+19.2 ± 0.2 ‰).

POC is dominated by phytoplankton biomass

with low δ13C, while DOC appears to retain its

original δδδδ13C signature, suggesting high turnover rates of phytoplankton-derived DOC and/or low reactivity of terrestrially-derived DOC.

► Comparison of nutrient concentrations in the main

inflow and outflow indicates a high degree of

nutrient retention: >50% for DIN, >70% for DIP, and ~40% for Si.

►δ13C-POC below Masinga relatively low (-25.2 ‰)

reflecting a contribution of reservoir phytoplankton. This signature rapidly shifts to -21.5 ‰, with a concurrent loss of POC (%POC/TSM from 4.6 to 2.0

%), consistent with rapid selective loss of 13

C-depleted phytoplankton.

Methane

Suspended matter and particulate organic carbon

Distribution and sources of POC and DOC

Effects of Masinga Dam

pCO

2

and indicators of aquatic

metabolism

http://ees.kuleuven.be

Chania river, Aberdares

Tana river, Kora (~600 m altitude)

Strong downstream increase in TSM from Masinga to coast (>20x), despite lack of

tributary inputs during sampling season: indicates important sediment storage and

remobilization

Increase in TSM due to resuspension coincides with decrease in %POC/TSM, i.e.

indicates net loss of particle-associated organic carbon during period of within-river storage.

Decrease in %POC/TSM accompanied with decrease in ∆∆∆∆14C-POC: suggests

preferential loss of recent POC and/or preferential preservation of aged POC. 0 10 20 30 40 50 0 1000 2000 3000 Elevation (m) P O C /T S M ( % ) tributaries Masinga Tana 0 1 2 3 4 5 0 500 1000 P O C /T S M ( % ) -200 -150 -100 -50 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 POC/TSM (%) Elevation (m) ∆ 1 4C -P O C 0 100 200 300 400 500 600 0 1000 2000 3000 Elevation (m) T S M ( m g L -1 ) tributaries Masinga Tana downstream increase along main river

In agreement with the increase in TSM, POC increases drastically along the main Tana River, from ~0.6 mg L-1

downstream of Masinga to > 5 mg L-1at the lowest station sampled.

In contrast, DOC concentrations along the river continuum are remarkably stable at 0.98 ± 0.14 mg L-1, leading to

a large shift in DOC/POC ratios from 1.5 to <0.2 along this transect.

►Variations in DOC/POC are inversely related to TSM (as expected), but relationships are different for tributaries and

main Tana River.

River basin has mixed C3-C4 vegetation. At high altitude, C4 plants are restricted to a few tussock-forming species in

e.g. the Aberdares, at lower altitudes grasses in savannas are exclusively C4. Soil samples encompass the full range of C3-C4 continuum (-28.5 to -13.2 ‰).

δδδδ13C-POC varied between -26.5 and -22.7 ‰ for tributaries, between -25.2 and -21.2 ‰ along the main Tana river, and much more negative in Masinga (-29.0 ± 0.3 ‰) where it reflects dominance of phytoplankton.

►δδδδ13C-DOC ranged between -27.7 and -21.8 ‰ in tributaries, within a narrow range of between -24.0 and -22.7 ‰ along

the Tana river, and showed no marked depletion in 13C Masinga Dam (-23.9 ± 0.3 ‰).

► Thus: δ13C-DOC and δ13C-POC clearly uncoupled and showed a relatively weak relationship.

Bacterial carbon sources in

suspended matter, soils and

river sediments

y = 1.10x - 2.28 R2 = 0.75 -35 -30 -25 -20 -15 -10 -30 -25 -20 -15 -10 Soils Suspended matter Sediments Tana Delta Betsiboka 1:1 δ13C-bulk OC δ 1 3C -i+ a 1 5 :0 P L F A

Despite some scatter, δδδδ13C of bacterial markers generally

follow those of bulk carbon pools in both soils, TSM and riverine sediment. Consistent also with previous data from coastal mangrove (i.e. C3) sediments with C4 inputs [Ralison et al. Org. Geochem. 39: 1649-1658, 2008; and Bouillon et al. L&O 52: 46-59].

No clear indication for selective use of C3 or C4

material. 0 1 2 3 0 10 1000 TSM (mg L-1 ) D O C /P O C tributaries Masinga Tana -30 -28 -26 -24 -22 -20 -30 -28 -26 -24 -22 -20 δ13C-DOC tributaries Masinga Tana δ 1 3C -P O C ► CH4 concentrations in headwater

streams ranged typically between 25 and 92 nM, except for Muringato river (314 nM) and Mutundu river (410 nM), above atmospheric equilibrium (<3 nM) .

► In the main Tana river, CH4

concentrations were relatively high

upstream of Masinga Dam (505 nM) but lower below the reservoir (372 nM), and much lower in surface waters of the reservoir (51 ± 7 nM),suggesting a high

CH4oxidation activity(or emission to the

atmosphere) despite the likeliness of CH4

production in its anoxic waters.

► Along the middle and lower course, CH4

concentrations ranged between 54 and 387

nM, with the highest concentration

observed in the Tana Delta.

►Although no direct metabolism rate measurements were

made during this survey, several patterns suggest that

in-stream primary production can be significant, even in the highly turbid waters of the middle and lower Tana River:

• %O2was high in all tributaries, Masinga Dam, and typically

only slightly undersaturated along most of the Tana River. • δδδδ18O-O2was consistently lower than the value expected for equilibrium with atmospheric O2(i.e., +24.2‰), and ranged between +19.0 and +23.0 ‰ – suggesting contributions

by isotopically light O2from photosynthesis (δ18O between -5.6

and -2.7 ‰).

• pCO2in headwater streams ranged between 110 and 1020

ppm, and ranged between 500 and 660 ppm along most of the

main Tana River, except for high pCO2 at the outflow of

Masinga and towards the tidal delta (3570 and 1540 ppm,

resp.). While generally oversaturated, these pCO2levels are in

the lower range of those observed in other tropical river systems.

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