Biogeochemistry of the Congo River: annual transport fluxes and sources of carbon in the upper Congo River (Kisangani, DRC Congo)

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Biogeochemistry of the Congo River: annual

transport fluxes and sources of carbon in the upper

Congo River (Kisangani, DRC Congo)

Steven Bouillon, Zita Kelemen (Earth & Environmental Sciences, KU Leuven, Belgium)

Alberto V. Borges, Thibault Lambert, François Darchambeau (Chemical Oceanography

Unit, Univ. de Liège, Belgium)

Ernest Tambwe, Alfred Toengaho Sembaito, Taylor Mambo (Université de Kisangani,

Centre de Surveillance de la Biodiversité, DRC Congo)

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2nd_{largest river basin in the world}

2nd_{in terms of total annual discharge}

▶ Laraque et al. (2009): most recent

synthesis of material fluxes in the basin. Highlights lack of data: regular sampling has been restricted to Kinshasa/Brazzaville (lower mainstem), Bangui (Oubangui), and some rivers in the smaller right bank

tributaries (e.g. Sangha), much of the data from 1980’s & 1990’s.

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Borges et al. (unpubl)

Data from Kisangani-Kinshasa transect

▶ High variability in river characteristics across the basin.

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▶ New data collected every two weeks (since Dec. 2012, ongoing) -mainstem Congo River at Kisangani -Tshopo River at Kisangani

Catchment areas:

Congo Basin: 3,500,000 km²

Upstream of Kisangani: 972,000 km² Tshopo: 58,000 km²

Borges et al. (unpubl)

Data from Kisangani-Kinshasa transect

▶ High variability in river characteristics across the basin.

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Verification/establishment of discharge rating curve

12-2012 04-2013 08-2013 12-2013 04-2014 08-2014 12-2014 Di sch a rg e (m 3 s -1 ) 0 2000 4000 6000 8000 10000 12000 14000 12-2012 04-2013 08-2013 12-2013 04-2014 08-2014 12-2014 D isch a rg e (m 3 s -1 ) 0 2000 4000 6000 8000 10000 12000 14000

Congo River, Kisangani

Oubangui, Bangui

Average annual discharge (2005-2014): 6800 m³ s-1

~17.5 % of the total basin flow (39200 m3_s-1_{for 2008-2012)}

Q_max/Q_min is moderate (2.6) High flows: November-June Low flows: July-October

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Discharge (m3s-1) 4000 6000 8000 10000 12000 14000 T ot al A lkalinit y (mm ol L -1 ) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

dec apr aug dec apr aug dec

Dis c har ge ( m 3 s -1 ) 4000 6000 8000 10000 12000 14000 T otal Al k ali nit y (mmol L -1 ) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Discharge Total Alkalinity (mmol L-1)

Seasonality in biogeochemical pools & proxies

Total Alkalinity,

d

13

_C-DIC

Total Alkalinity (mmol L-1)

0.0 0.2 0.4 0.6 0.8 1.0 13 C -D IC (‰ ) -22 -20 -18 -16 -14 -12 -10 -8 -6 -4

mainstem Congo River Tshopo River

 Higher TA at low discharge  Hysteresis

 Correlation with d13_C-DIC

Influence of weathering

Much lower d13_{C-DIC in Tshopo R.}

 If we take TA ≈ DIC (…)

annual DIC flux: 1500-1600 Tg C y-1

Compare with estimate for Kinshasa: 3700 Tg C y-1_{(Wang et al. 2014)}

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TA (mmol L

-1

)

0.001 0.01 0.1 1 10 100 13

C-DIC

(‰)

-30 -25 -20 -15 -10 -5 0 Congo @Kisangani Tshopo River Transcongo data Oubangui + tributaries

Rivers, L. Kivu & L. Edward catchments COBAFISH dataset

Seasonality in biogeochemical pools & proxies

Total Alkalinity,

d

13

_C-DIC

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Seasonality in biogeochemical pools & proxies

Suspended sediment and POC

Discharge (m3s-1) 4000 6000 8000 10000 12000 14000 T S M (mg L -1 ) 0 20 40 60 80 100 120

D is c ha rge ( m 3 s -1 ) 4000 6000 8000 10000 12000 14000 T SM ( mg L -1 ) 0 20 40 60 80 100 120 Discharge TSM (mg L-1)

 Much higher supended sediment loads than most other studied sites in the Congo Basin (20-120 mg L-1₎

 Complex hysteresis, but general increase with Q

annual TSM flux: 12.3 – 13.8 Pg y-1

Basin area: 972,000 km²

TSM yield: 12.7 – 14.2 T km-2_y-1

annual POC flux: 813 Tg C y-1

POC yield: 0.837 T C km-2_y-1 TSM (mg L-1) 0 20 40 60 80 100 120 POC (mg L -1 ) 1 2 3 4 5 6

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Seasonality in biogeochemical pools & proxies

Dissolved organic C

Discharge (m3s-1) 4000 6000 8000 10000 12000 14000 D OC (mg L -1 ) 2 3 4 5 6 7 8 9 10

Dis c ha rge ( m 3 s -1 ) 4000 6000 8000 10000 12000 14000 DO C ( mg L -1 ) 2 3 4 5 6 7 8 9 10 Discharge DOC (mg L-1)

 DOC concentrations vary

substantially (3-9 mg C L-1_{) with}

discharge

annual DOC flux: 1271 Tg C y-1

DOC yield: 1.308 T C km-2_y-1 13 C (‰) -34 -32 -30 -28 -26 -24 Congo POC Congo DOC Tshopo POC Tshopo DOC 92 % dense forest 39 % dense forest

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Seasonality in biogeochemical pools & proxies

OC sources in Congo Basin

13 C (‰) -34 -32 -30 -28 -26 -24 Congo POC Congo DOC Tshopo POC Tshopo DOC 92 % dense forest 39 % dense forest

Kivu Rivers

POC DOC

-18

-20

-22

C4 inputs in Kivu Highlands

Similar range as observed in other

mainstem sites and large tributaries

(e.g., downstream section/Kinshasa,

and Oubangui River), but:

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Seasonality in biogeochemical pools & proxies

OC sources in Congo Basin

Distance from Kisangani (km)

0 200 400 600 800 1000 1200 1400 1600 1800 13 C-PO C ( ‰) -40 -38 -36 -34 -32 -30 -28 -26 -24

Mainstem, high waters (December 2013) Mainstem, low waters (June 2014)

Only minor downstream

gradient in mainstem

d

13

C-POC

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Seasonality in biogeochemical pools & proxies

OC sources in Congo Basin

Distance from Kisangani (km)

0 200 400 600 800 1000 1200 1400 1600 1800 13 C -PO C ( ‰) -40 -38 -36 -34 -32 -30 -28 -26 -24

Tributaries, high waters (December 2013) Tributaries, low waters (June 2014)



13

_{C-depleted POC in}

central tributaries: phyto-

and/or CH

₄

oxidizers

Also evidence for CH

₄

-derived C in some

invertebrates and fish

species

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Seasonality in biogeochemical pools & proxies

OC sources in Congo Basin

Distance from Kisangani (km)

0 200 400 600 800 1000 1200 1400 1600 1800 13 C -D O C ( ‰) -40 -38 -36 -34 -32 -30 -28 -26 -24

Only minor downstream

gradient in mainstem

d

13

₂

O

4000 6000 8000 10000 12000 14000 CH 4 ( nM) 0 20 40 60 80 100 120 Discharge (m3s-1) Borges et al., submitted

Relatively modest CH

₄

concentrations (and hence,

diffusive fluxes)

No strong seasonality

Data from rest of the basin and other African basins show

variations covering 5 orders of magnitude.

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CH 4 ( nM ) 0 20 40 60 80 100 120 Discharge (m3s-1) 4000 6000 8000 10000 12000 14000 N 2 O ( nM ) 6 7 8 9 10 11 12

Seasonality in biogeochemical pools & proxies

GHG – CH

₄

& N

₂

O

Borges et al.,

submitted

Relatively modest N

₂

O concentrations (and

hence, diffusive fluxes), only slightly above

saturation.

Increases with discharge

Data from rest of the basin and other African

basins show variations covering 3 orders of

magnitude, high levels only in anthropogenically

8.8 Oubangui

873

189 1072

3.3-5.0

Mpoko

300

900

7.7 Ngoko

900 3900

15.3 Kisangani

1595

837 1308

12.7-14.2

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Congo Basin C fluxes: how well do we know them ?

Sediment yields according to Laraque et al.

(2009) synthesis

Measured Y

_sediment

in Kisangani (12.7-14.2 T

km

-2

_y

-1

_{) much higher than those predicted}

(7.6 T km

-2

_y

-1

₎

Transport fluxes are only available for a very

limited # of sites, inter-annual variability

insufficiently covered - long-term datasets

lacking.

Poor network of hydrological gauging

stations.

Sampling across gradients of catchment

characteristics reveals huge variability in

aquatic

biogeochemical

functioning,

C

sources, and GHG sink/source strength.

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