Variations of dissolved greenhouse gases (CO2, CH4, N2O) in the Congo River network overwhelmingly driven by fluvial wetland connectivity

(1)

Variations of dissolved greenhouse gases (CO

₂

, CH

₄

, N

₂

O) in

the Congo River network overwhelmingly driven by fluvial

wetland connectivity

Alberto V. Borges

(2)

Lab presentation

• Measurements of greenhouse gases in aquatic enviroments

• Inland waters, coastal waters, oceanic waters & marine cryosphere

• Versatile, compact and rugged equipment for harsh environments

• Carbon dioxide (CO

₂

) by infra-red & lazer gas analysers (Li-Cor & LGR)

• Continuous (surface) and discrete (profiles)

• Nitrous oxide (N

₂

O) concentration by GC

• N

₂

O isotopes by lazer spectrometry (LGR)

• Dimethylsulfide (DMS) by GC

(3)

How important are emissions of greenhouse-gas

from inland waters ?

(4)

Introduction

(5)

5

9.1±0.5 PgC y

-1

+

0.9±0.7 PgC y

-1

2.6±1.0 PgC y

-1

26%

Calculated as the residual of all other flux components

5.0±0.2 PgC y

-1

50%

24%

2.4±0.5 PgC y

-1

Average of 5 models

Global anthropogenic CO

₂

fluxes in 2010 (PgC y

-1

_{= 10}

15

_{gC y}

-1

₎

www.globalcarbonproject.org/

(6)

Introduction

(7)

Introduction

(8)

Introduction

R

a

y

m

o

n

d

e

t

a

l.

(

2

0

1

3 )

L

a

u

e

rw

a

ld

e

t

a

l.

(

2

0

1

5 )

5 data !

pCO

₂

= 277,076 ppm ?

(9)

Introduction

Raymond et al. (2013) & Lauerwald et al. (2015) used pCO

₂

computed

from pH and total alkalinity

(10)

Where’s the river/lake CO

₂

coming from ?

(11)

(12)

Introduction

(13)

Introduction

(14)

(15)

Introduction

0.7-2.9

(16)

(17)

(18)

Congo river

(19)

(20)

(21)

Congo

Wetland

GLC 2000

Humid

Forest

Savannah

(22)

Congo

WHRC

Wetland

Humid

Forest

Savannah

(23)

Wetland

= flooded forest

(Tributary)

(24)

Wetland

= floating macrophytes

(Tributary)

(25)

Wetland

= floating macrophytes

(Congo mainstem)

(26)

Vossia cuspidata

« Hippo grass »

Salvinia auriculata

Azolla pinnata

Eichhornia crassipes

« water hyacinth »

Congo

(27)

Metamorphic

Courtesy of J. Hartmann

Siliciclastic and

unconsolidated

sed.

Congo

(28)

Ferralsols

Arenosols

Acrisols

Cambisols

Plinthosols

Gleysols

Congo

(29)

Kisangani

Kinshasa

1700 km

Congo

375 m

280 m

(30)

(31)

Cruises & Methods

164 stations

29 variables

> 23,000 continuous measurements

pCO

₂

, cond, temp, pH, O

₂

, TSM, cDOM

(32)

(33)

(34)

Results

0 20 40 60 80 100

Conductivity (µS cm

-1

₎

High W (Dec. 2013) Falling W (June 2014) 3 4 5 6 7 8 15 16 17 18 19 20 21 22 23 24 25

← Dowstream Longitude (°E) Upstream →

pH

(35)

Results

0 5000 10000 15000 20000

pCO

₂

(ppm)

High W (Dec. 2013)

Falling W (June 2014) 0 20 40 60 80 100 120 15 16 17 18 19 20 21 22 23 24 25

← Dowstream Longitude (°E) Upstream →

%O

₂

(%)

(36)

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 0 200 400 600 800 1000 1200 1400 1600 ← Downstream Distance from Kinshasa (km) Upstream →

pCO₂(ppm) Mainstem High W Tributaries High W Mainstem Falling W Tributaries Falling W

Results

« Cuvette Centrale »

(Wetland)

Savannah

(37)

Results

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 0 200 400 600 800 1000 1200 1400 1600 ← Downstream Distance from Kinshasa (km) Upstream →

pCO₂(ppm) Mainstem High W Tributaries High W Tributaries Falling W Tributaries Falling W Mainstem High W Tributaries High W Mainstem Falling W Tributaries Falling W

(38)

0 10000 20000 30000 40000 50000 60000 0 200 400 600 800 1000 1200 1400 1600 ← Downstream Distance from Kinshasa (km) Upstream →

CH₄ (nmol L-1₎ Mainstem High W Tributaries High W Mainstem Falling W Tributaries Falling W

Results

(39)

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 0 200 400 600 800 1000 1200 1400 1600 ← Downstream Distance from Kinshasa (km) Upstream →

Results

« Cuvette Centrale »

(Wetland)

Savannah

(40)

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 0 200 400 600 800 1000 1200 1400 1600 ← Downstream Distance from Kinshasa (km) Upstream →

Results

« Cuvette Centrale »

(Wetland)

Savannah

(41)

0 5 10 15 20 25 30 0 200 400 600 800 1000 1200 1400 1600

← Downstream Distance from Kinshasa (km) Upstream → N₂O (nmol L-1₎ Mainstem High W Tributaries High W Mainstem Falling W Tributaries Falling W

Results

Denitrification

(42)

0 5 10 15 20 25 30 0 200 400 600 800 1000 1200 1400 1600

← Downstream Distance from Kinshasa (km) Upstream → N₂O (nmol L-1₎

Results

Mainstem High W Tributaries High W Mainstem Falling W Tributaries Falling W

Denitrification

(43)

(44)

(45)

(46)

Results

0 20 40 60 80 100 120 0 5,000 10,000 15,000 20,000 25,000 %O₂ (%)

(47)

(48)

Results

2

3

4

5

6

7

8

9

10

0

100

200

300

400

800 1200

Strahler order

(49)

Results

1 3

C

-D

IC

(

‰

)

(50)

Results

CO

₂

emission from Congo rivers-streams

F

CO

₂

= k H ΔpCO

₂

F

CO

₂

air-water CO

₂

flux

H

Henry’s constant = f(temperature)

ΔpCO

₂

= air-water gradient of pCO

₂

(measured)

k

= gas transfer velocity

(51)

Results

CO

₂

emission from Congo rivers-streams

Stream surface area = length x width

length = Hydrosheds

(52)

Results

CO

₂

emission from Congo rivers-streams

= 251 TgC yr

-1

Net ecosystem exchange (NEE) Congo forests + savannahs

= 77 TgC yr

-1

???

Export of C from soils to rivers

= 2-3% of NEE

for terra firme forests

??????

(53)

Results

CO

₂

emission from Congo rivers-streams

= 251 TgC yr

-1

Mostly sustained by C leaked from wetlands ?

Export C from flooded forest in Amazon (Abril et al.)

+

Surface of flooded forest in Congo

= 400 TgC yr

-1

(54)

Congo & other African rivers

(55)

(56)

Results

0 25 50 75 100 125 150 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 250 %O₂ Congo Zambezi Tana AGS Rianila Betsiboka springs 1 10 100 1000 10000 100000 CH₄ (nmol L-1) 18000

(57)

Results

0 1000 2000 3000 4000 5000 C H4 ( n m o l L -1 ) _Nyong Zambezi Tana AGS Rianila Betsiboka Congo r2=0.94 p C O2 ( p p m ) 0 5 10 15 20 25 40 60 80 100 120

Wetland fraction of catchment (%)

% O2 ( % ) r2=0.96

(58)

Results

C

H

4

(

n

m

o

l

L

-1

)

C

H

4

(

n

m

o

l

L

-1

)

Steeper

→ short residence time → smaller flood areas → high exchange + air

Steeper

(59)

Results

p

C

O

2

(

p

m

)

p

C

O

2

(

p

m

)

Steeper

(60)

Congo versus Amazon

(61)

Results

Amazon

Congo

Catchment area (km

2

)

6,025,735

3,705,222

Slope (°)

1.4

0.6 Discharge (km

3

yr

-1

)

5,444

1,270

Specific discharge (L s

-1

km

-2

)

29

11 Precipitation (mm)

2,147

1,527

Air temperature (°C)

24.6

23.7 River-stream surface area (km

2

)

74,904

26,517

Wetland surface area (%)

14

10 Above ground biomass (Mg km

-2

)

909

748 Land cover

Dense Forest (%)

83

49 Mosaic Forest (%)

4

18 Woodland and shrubland (%)

4

27 Grassland (%)

5

3 Cropland/Bare soil (%)

4

2 >

>

<

(62)

(63)

Results

p

C

O

2

(

p

m

)

C

H

4

(

n

m

o

l

L

-1

)

pCO

₂

is ± similar

CH

₄

is 3-4 times

higher in Congo

(64)

Results

Q ( m 3 s -1 ) Q ( m 3 s -1 )

Q

_max

:Q

_min

= 2.85

Q

max

:Q

min

= 1.99

(65)

Results

Amazon

Congo

Flooded land = 80 % flooded forest

Numerous permanent & temporary lakes

Flooded land = 100 % flooded forest

Only a few large permanent lakes

(66)

Results

Amazon

Congo

Flooded land = 80 % flooded forest

Numerous permanent & temporary lakes

Flooded land = 100 % flooded forest

Only a few large permanent lakes

Seasonally inundated wetlands

Permanently inundated flooded forest

(67)

Results

Amazon

Congo

Flooded land = 80 % flooded forest

Numerous permanent & temporary lakes

Flooded land = 100 % flooded forest

Only a few large permanent lakes

Seasonally inundated wetlands

Permanently inundated flooded forest

Flooding from river overflow

Wetland water from upland runoff

(68)

Results

Amazon

Congo

Flooded land = 80 % flooded forest

Numerous permanent & temporary lakes

Flooded land = 100 % flooded forest

Only a few large permanent lakes

Seasonally inundated wetlands

Permanently inundated flooded forest

Flooding from river overflow

Wetland water from upland runoff

Macrophytes only present in floodplains

Extensive macrophyte meadows in river

channels (mainstem + tributaries)

(69)

Results

Using gas transfer velocity + river/stream areas from GIS of

Raymond et al. (2013)

+ GLWD (Lehner & Döll 2003)

0 5 10 15 0 2,000 4,000 6,000 8,000 10,000

Wetland fraction of catchment (%)

Amazon Congo Zambezi Betsiboka Rianila Tana-AGS Mainstem Large tributaries Small tributaries Small tributaries r2=0.96 Large tributaries r2=0.55 Mainstem r2=0.91

CO

₂

emissions from tropical

rivers = 1.8 PgC yr

-1

Raymond et al. (2013)

1.4 PgC yr

-1

Lauerwald et al. (2015)

0.5 PgC yr

-1

(70)

(71)

(72)