Carbon dioxide fluxes in Lake Kivu

Carbon dioxide fluxes in lake Kivu

Alberto V. Borges, Bruno Delille

Unité d’Océanographie Chimique, Université de Liège

Jean-Pierre Descy, François Darchambeau, Bruno Leporcq

Laboratoire d'Ecologie des Eaux Douces, Université de Namur

Pierre Servais

Ecologie des Systemes Aquatiques, Université Libre de Bruxelles

Marc LLirós

Institute of Aquatic Ecology, University of Girona

Steven Bouillon

Analytical and Environmental Chemistry, Vrije Universiteit Brussel

Gwenaël Abril

Environnements et Paléoenvironnements OCéaniques, Université Bordeaux 1

Brussels, 12 June 2007 http://www.co2.ulg.ac.be/

Global air-water CO₂ fluxes in aquatic systems (PgC yr-1 _{or 10}15 _{gC yr}-1₎

Oceans -1.22 Takahashi et al. (2007)

Rivers +0.30 Cole and Caraco (2001)

Lakes +0.14 Cole et al. (1994)

Surface oceans / surface lakes = 1470

Brussels, 12 June 2007

Atmospheric CO

fluxes in lakes

Brussels, 12 June 2007 http://www.co2.ulg.ac.be/

Borges et al. (2006) ECSS 70: 375-387

-75 -50 -25 0 25 50 75 C o n ti n e n ta l s h e lv e s E s tu a ri e s R iv e rs L a k e s D O C e x p o rt t o r iv e rs P O C e x p o rt t o r iv e rs R o c k w e a th e ri n g T e rr e s tr ia l b io s p h e re C O 2 fl u x e s ( T g C y r -1 o r 1 0 1 2 g C y r -1 ) Source of CO₂ to the atmosphere Sink of atmospheric CO₂ -68 -11 -9 -14 -66 +67 +21 +15

Atmospheric CO

fluxes in lakes

Brussels, 12 June 2007 http://www.co2.ulg.ac.be/

Sobek et al. (2005) GBC, 19, GB2003, doi:10.1029/2004GB002264 CO₂ emission of lakes driven by net heterotrophy fuelled by terrestrial (allochtonous) organic carbon inputs

Atmospheric equilibrium

Atmospheric CO

fluxes in lakes

Brussels, 12 June 2007 http://www.co2.ulg.ac.be/

Sobek et al. (2005) GBC, 19, GB2003, doi:10.1029/2004GB002264 Highest CO₂ emission from lakes observed in the tropics

Atmospheric equilibrium

Why lake Kivu ?

Brussels, 12 June 2007 http://www.co2.ulg.ac.be/

Lehner & Döll (2004) J. Hydrol. 296:1–22

0 100 200 300 400 -60 -30 0 30 60 90 surface area (103 km) L a ti tu d e ( °N )

Tropics and subtropics = 15.7% of total lake surface area African lakes = 9.3% of total lake surface area

= 0.8% of total of lakes measured for CO₂ = 0.3% of total of measurements for CO₂

Why lake Kivu ?

Brussels, 12 June 2007 http://www.co2.ulg.ac.be/

Schmid et al. (2005) G3 6(7):doi:10.1029/2004GC000892

Bottom high CO₂, higher temperature and salinity = geothermal inputs Bottom high CH₄ = degradation at depth of organic matter from surface

-0.5 0.5 1.5 2.5 3.5 -500 -450 -400 -350 -300 -250 -200 -150 -100 -50 0 δ δδ

δ13_{C DIC (per mil)}

Why lake Kivu ?

Brussels, 12 June 2007 http://www.co2.ulg.ac.be/

In bottom waters, increase of CH₄ of 15-20% and CO₂ of 10% in 30 yr Due to increase of primary production and/or sedimentation of organic carbon ?

Due to eutrophication ?

Due to removal of Daphnia curvirostris due to introduction of sardine Limnothrissa miodon ?

Schmid et al. (2005) G3 6(7):doi:10.1029/2004GC000892

How will industrial exploitation of CH₄ change nutrient, organic carbon and CO₂ cycling ?

Why lake Kivu ?

Brussels, 12 June 2007 http://www.co2.ulg.ac.be/

CAKI

Brussels, 12 June 2007 http://www.co2.ulg.ac.be/

Carbon and Nutrient cycles in lake Kivu (CAKI)

FNRS funded

Phytopl.

bacteria

DIC

zoopl.

CO

POC

POC

FUNDP

ULg

ULB

δ13_{C DIC measurements : S. Bouillon / F. Dehairs / J.J. Middelburg} CH₄ measurements : G. Abril

CAKI

Brussels, 12 June 2007 http://www.co2.ulg.ac.be/

Daily variations of pCO

in surface waters

Brussels, 12 June 2007

http://www.co2.ulg.ac.be/

Daily variations in surface waters

Bukavu

Kabuno

Ishungu

pCO₂, temperature, salinity and %O₂ during 24 h at hourly intervals

1m, 5m, 10m, 20m Tiring at times…

Brussels, 12 June 2007

http://www.co2.ulg.ac.be/

Daily variations in surface waters

13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00

pCO₂@in-situ temp (ppm)

13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 570 580 590 600 610 620 night-time 1 m 5 m 10 m

Brussels, 12 June 2007

http://www.co2.ulg.ac.be/

Daily variations in surface waters

13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00

pCO₂@in-situ temp (ppm)

13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 570 580 590 600 610 620 night-time 1 m 5 m 10 m 13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 temperature (°C) 13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 24.0 24.5 25.0 25.5 night-time 1 m 5 m 10 m

Brussels, 12 June 2007

http://www.co2.ulg.ac.be/

Daily variations in surface waters

13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00

pCO₂@in-situ temp (ppm)

13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 570 580 590 600 610 620 night-time 1 m 5 m 10 m 13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 temperature (°C) 13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 24.0 24.5 25.0 25.5 night-time 1 m 5 m 10 m 13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 pCO₂@23°C (ppm) 13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 535 545 555 565 575 585 night-time 1 m 5 m 10 m

Brussels, 12 June 2007

http://www.co2.ulg.ac.be/

Daily variations in surface waters

13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00

pCO₂@in-situ temp (ppm)

13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 570 580 590 600 610 620 night-time 1 m 5 m 10 m 13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 temperature (°C) 13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 24.0 24.5 25.0 25.5 night-time 1 m 5 m 10 m 13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 pCO₂@23°C (ppm) 13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 535 545 555 565 575 585 night-time 1 m 5 m 10 m 13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 %O₂ (%) 13:00 17:00 21:00 01:00 05:00 09:00 13:00 17:00 82 84 86 88 90 92 94 night-time 1 m 5 m 10 m

Brussels, 12 June 2007

http://www.co2.ulg.ac.be/

Daily variations in surface waters

Next step : computation of metabolic process rates (gross primary production, community respiration), by mass balance of CO₂

Spatial variability in surface waters

Bukavu Kabuno Ishungu Kibuye

Spatial variability

Net particulate primary production (NPPP) and bacterial respiration (BR)

Spatial variability

Variability of ± 5% Variability of ± 5% Variability of ± 27%

Air pCO₂ = 380 Air CH₄ = 2

≈ _{4 x}

Spatial variability

Spatial variability

Spatial variability

δ13_{C DIC (per mil)}

d e p th ( m )

Spatial variability

δ13_{C DIC (per mil)}

d e p th ( m ) at 320 m in

Spatial variability

δ13_{C DIC (per mil)}

d e p th ( m )

Suggests a much larger contribution of geothermal inputs to whole water column including surface water, and to dynamics of inorganic C chemistry in Kabuno bassin than rest of Kivu lake.

at 320 m in

Kabuno is shallower (100m), receives no river water and exchanges little water with Kivu lake (connexion 10 m deep).

Spatial variability

δ13_{C DIC (per mil)}

d e p th ( m ) at 320 m in

the «main lake»

Suggests a much larger contribution of geothermal inputs to whole water column including surface water, and to dynamics of inorganic C chemistry in Kabuno bassin than rest of Kivu lake.

Comparison with other lakes

0

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lake rank for highest CH

C

H

(

n

m

o

l

l

)

Surface CH

Global comparison

Brussels, 12 June 2007 http://www.co2.ulg.ac.be/

Other CH₄ data from Bastviken et al. (2004) GBC, 18, GB4009

K a b u n o r a n k 3 6 o f 4 9 “ m a in l a k e ” r a n k 4 7 o f 4 9

Surface pCO

Global comparison

Brussels, 12 June 2007 http://www.co2.ulg.ac.be/

Other pCO₂ data from Sobek et al. (2005) GBC, 19, GB2003

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lake rank for highest pCO

p

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p

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Surface pCO

Global comparison

Surface pCO

African comparison

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lake rank for highest pCO

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p

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Other pCO₂ data from Sobek et al. (2005) GBC, 19, GB2003 Kabuno rank 3 of 78

Comparison with lake Tanganyika

Brussels, 12 June 2007 http://www.co2.ulg.ac.be/

salinity pCO₂ TA DIC Ref.

Tanganyika 0.6 279 6541 5899 Graig (1974)

"main" Kivu 1.0 615 12970 11980 CAKI data

Kabuno 1.3 14210 16860 17240 CAKI data

atmosphere - 380 - -

-Suggests a much larger contribution of geothermal inputs to surface water dynamics of inorganic C chemistry in lake Kivu than in lake Tanganyika related to geomorphology (depth, volume, river inputs)