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) Congo Basin 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. Congo Basin 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. ▶ High variability in river characteristics across the basin.
Borges et al. (unpubl) Data from Kisangani- Kinshasa transect Congo Basin 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. ▶ High variability in river characteristics across the basin.
Borges et al. (unpubl) Data from Kisangani- ▶ New data collected every two Kinshasa transect 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
Discharge at Kisangani
14000 Average annual discharge (2005-2014): Congo River, Kisangani 6800 m³ s-1 12000
) ) 10000
-1
-1
s ~17.5 % of the total basin flow s
3
3 (39200 m3s-1 for 2008-2012) 8000
6000
Discharge (m Discharge
Discharge (m Discharge Qmax/Qmin is moderate (2.6) 4000
2000 High flows: November-June Oubangui, Bangui Low flows: July-October 0 12-2012 04-2013 08-2013 12-2013 04-2014 08-2014 12-2014
Verification/establishment of discharge rating curve Seasonality in biogeochemical pools & proxies Total Alkalinity, d13C-DIC
Discharge Total Alkalinity (mmol L-1) 14000 1.0 . Higher TA at low discharge
. Hysteresis 0.9 )
-1 . Correlation with d13C-DIC 12000
) 0.8
-1
Influence of weathering s
3 Much lower d13C-DIC in Tshopo R. 10000 0.7 0.6
. If we take TA ≈ DIC (…) 8000 0.5
-1 Discharge (m 0.4 annual DIC flux: 1500-1600 Tg C y 6000 AlkalinityTotal (mmol L 0.3 Compare with estimate for Kinshasa: 3700 Tg C y-1 (Wang et al. 2014) 4000 0.2 dec apr aug dec apr aug dec
Tshopo River mainstem Congo River 1.0
)
-4 -1 0.9 -6 0.8 -8 0.7 -10 0.6 -12 0.5 -14 0.4
C-DIC (‰) C-DIC 0.3
Total Alkalinity (mmol L 13 -16 0.2 -18 4000 6000 8000 10000 12000 14000 -20 Discharge (m3s-1) -22 0.0 0.2 0.4 0.6 0.8 1.0 Total Alkalinity (mmol L-1) Seasonality in biogeochemical pools & proxies Total Alkalinity, d13C-DIC
. Data compiled from across the Congo Basin (>600 paired measurements)
Congo @Kisangani Tshopo River Transcongo data Oubangui + tributaries Rivers, L. Kivu & L. Edward catchments COBAFISH dataset
0
-5
-10
-15
C-DIC (‰) C-DIC
13 -20
-25
-30 0.001 0.01 0.1 1 10 100 TA (mmol L-1) Seasonality in biogeochemical pools & proxies Suspended sediment and POC
Discharge TSM (mg L-1) 14000 120 . Much higher supended sediment
loads than most other studied sites in 100 the Congo Basin (20-120 mg L-1) 12000
)
-1
)
s 80 -1
3 . Complex hysteresis, but general 10000 increase with Q 60 8000 annual TSM flux: 12.3 – 13.8 Pg y-1 40 L (mg TSM
Discharge (m Discharge 6000 Basin area: 972,000 km² 20 TSM yield: 12.7 – 14.2 T km-2 y-1 4000 0 dec apr aug dec apr aug dec annual POC flux: 813 Tg C y-1 POC yield: 0.837 T C km-2 y-1 120 100
)
6 -1 80
60 5
) 40
TSM (mg L (mg TSM
-1 4 20
3 0
POC (mg L 4000 6000 8000 10000 12000 14000 2 Discharge (m3s-1)
1 0 20 40 60 80 100 120 TSM (mg L-1) Seasonality in biogeochemical pools & proxies Dissolved organic C
Discharge DOC (mg L-1) 14000 10 . DOC concentrations vary substantially (3-9 mg C L-1) with 9 discharge 12000
) 8
)
-1
s -1
3 annual DOC flux: 1271 Tg C y-1 10000 7 DOC yield: 1.308 T C km-2 y-1 6
8000 5
DOC (mg L (mg DOC
Discharge (m Discharge 4 6000 3
4000 2 39 % 92 % dec apr aug dec apr aug dec dense forest dense forest -24 10 9
) 8
-26 -1 7 6 -28 5
DOC (mg L (mg DOC 4
C (‰) 3
13 -30 2 4000 6000 8000 10000 12000 14000 -32 Discharge (m3s-1) Congo Congo Tshopo Tshopo POC DOC POC DOC -34 Seasonality in biogeochemical pools & proxies OC sources in Congo Basin
-18
-20
-22 39 % 92 % dense forest dense forest -24
-26
-28
C (‰)
13 -30 Kivu Rivers POC DOC -32 Congo Congo Tshopo Tshopo POC DOC POC DOC -34 Similar range as observed in other C4 inputs in Kivu Highlands mainstem sites and large tributaries (e.g., downstream section/Kinshasa, and Oubangui River), but: Seasonality in biogeochemical pools & proxies OC sources in Congo Basin
Mainstem, high waters (December 2013) Mainstem, low waters (June 2014)
-24 .Only minor downstream 13 -26 gradient in mainstem d C- POC -28 .No strong seasonality
-30
-32
C-POC (‰) C-POC
13 -34
-36
-38
-40 0 200 400 600 800 1000 1200 1400 1600 1800 Distance from Kisangani (km) Seasonality in biogeochemical pools & proxies OC sources in Congo Basin
Mainstem, high waters (December 2013) Mainstem, low waters (June 2014) Tributaries, high waters (December 2013) Tributaries, low waters (June 2014) -24 .13C-depleted POC in -26 central tributaries: phyto- and/or CH4oxidizers -28 .Also evidence for CH4- derived C in some -30 invertebrates and fish species -32
C-POC (‰) C-POC
13 -34
-36
-38
-40 0 200 400 600 800 1000 1200 1400 1600 1800 Distance from Kisangani (km) Seasonality in biogeochemical pools & proxies OC sources in Congo Basin
Mainstem, high waters (December 2013) Mainstem, low waters (June 2014)
-24 .Only minor downstream 13 -26 gradient in mainstem d C- POC -28 .Distinctly 13C-enriched DOC during low flow -30 conditions over most of the transect. -32
C-DOC (‰) C-DOC
13 -34
-36
-38
-40 0 200 400 600 800 1000 1200 1400 1600 1800 Distance from Kisangani (km) Seasonality in biogeochemical pools & proxies OC sources in Congo Basin
Mainstem, high waters (December 2013) Mainstem, low waters (June 2014) Tributaries, high waters (December 2013) Tributaries, low waters (June 2014) -24 .DOC in tributaries has 13 -26 fairly constant d C signatures – constrasts -28 with strong 13C-depletion observed in POC. -30
-32
C-DOC (‰) C-DOC
13 -34
-36
-38
-40 0 200 400 600 800 1000 1200 1400 1600 1800 Distance from Kisangani (km) Seasonality in biogeochemical pools & proxies GHG – CH4 & N2O
120 .Relatively modest CH concentrations (and hence, 100 4 diffusive fluxes) 80 .No strong seasonality
(nM) 60 4 .Data from rest of the basin and other African basins show
CH 40 variations covering 5 orders of magnitude.
20
0 4000 6000 8000 10000 12000 14000 Discharge (m3s-1)
Borges et al., submitted Seasonalityin biogeochemicalpools& proxies CH4 (nM) N2O (nM) 100 120 10 11 12 40 60 80 20 6 7 8 9 0 4000 6000 Discharge (m 8000 10000 3 s -1 ) 12000 14000 . impactedsystems. levelsanthropogenicallyin high magnitude, only basins show variations covering3 orders of . . saturation. diffusive hence, fluxes),slightlyonly above Relatively modest N Data from restAfrican other and basin of the Increases withdischarge
submitted Borges et al.,
2 O concentrations (and GHG
–
CH 4
& N 2 O
Congo Basin C fluxes: what do we know ?
DIC yield POC yield DOC yield TSM yield (kg km-2 y-1) (kg km-2 y-1) (kg km-2 y-1) (T km-2 y-1) Kinshasa 1060 600 3500 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 Seyler et al. (2006), Coynel et al. (2005), Wang et al. (2014) + our work on Oubangui and Congo @Kisangani Congo Basin C fluxes: how well do we know them ?
.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.
Sediment yields according to Laraque et al. (2009) synthesis
Measured Ysediment in Kisangani (12.7-14.2 T km-2 y-1) much higher than those predicted (7.6 T km-2 y-1) Acknowledgements