Wetlands influencing river biogeochemistry: the case study of the Zambezi and the Kafue Rivers

(1)

Wetlands influencing river

biogeochemistry: the case study of the

Zambezi and the Kafue Rivers

AFRIVAL Project (African River Basins)

http://ees.kuleuven.be/project/afrival/

EGU General Assembly 2014, Vienna

Cristian R. Teodoru

1 _{, Frank C. Nyoni}

2 _{, Imasiku Nyambe}

2 _{, Alberto V. Borges}

3 _{, and}

Steven Bouillon

1

1 _{K.U. Leuven, Belgium;}

2 _{University of Zambia, IWRM Center, Zambia,}

3 _{University of Liege, Belgium}

2 nd

_{International Conference “Water Resources and Wetlands”}

(2)

Introduction: AFRIVAL project (2009-2014)

AFRIVAL – ‘African river basin: catchment scale carbon fluxes and

transformation”

(

http://ees.kuleuven.be/project/afrival/

)

joint European Research Council Starting Grant project hosted at the Department

of Earth & Environmental Sciences (K.U. Leuven, Belgium) and the Chemical

Oceanography Unit (Université de Liège, Belgium)

5-year funding to explore the role of African rivers in carbon cycling

Fieldwork within AFRIVAL has taking place in:

• Kenya: Tana & Sabaki Rivers

• Niger: Niger River

• Gabon: Ogooué River

• Madagascar: Bestiboka and Rianila Rivers

• DRC Congo & Central African Republic: Congo River Basin

• Zambia & Mozambique: Zambezi River Basin

Tana River

Ogooué River

(3)

Study site: the Zambezi River

The Zambezi River – general characteristics

•

4 th

_{largest in Africa and the largest flowing in to the Indian Ocean (from Africa)}

• Total length: > 3000 km; Drainage basin: ~ 1.4 x 10

6 _km

2 _{(shared by 8 countries)}

• Average annual discharge at Zambezi Delta: 3800-4130 m

3 _s

-1

• 2 large reservoirs: Kariba (5580 km

2 _{; 180 km}

3 _{), and Cahora Bassa (2670 km}

2 _{; 52 km}

3 ₎

• 2 major wetlands: Barotse Floodplains (7700 km

2 _{), and Chobe Swamps (1500 km}

2 ₎

Kariba Reservoir

Victoria Falls

Zambezi source

Barotse Floodplains

Chobe Swamps

Barotse Floodplains

(4)

Study site: the Kafue River

The Kafue River – general characteristics

• Main tributary of the Zambezi River (entirely within Zambia)

• Total length: > 1500 km; Drainage basin: ~ 156,000 km

2

• Average annual discharge at the confluence with Zambezi: 370 m

3 _s

-1

• 2 reservoirs: Itezhi Tehzi (365 km

2 _{; 5.5 km}

3 _{), and Kafue Gorge (13 km}

2 _{; 0.8 km}

3 ₎

• Major wetlands: Lukanga Swamps (2100 km

2 _{), and Kafue Flats (6500 km}

2 ₎

(5)

General characteristics: Climate & Rainfall

• Climate is classified as humid subtropical or tropical wet and dry

• Annual rainfall varies with latitude: 1400 mm in N to 400-500 mm in S (mean average

rainfall for entire basin: 940 mm)

• Two seasons:

1. Wet season (Oct/Nov – Apr) corresponding to summer, with 95% of annual rainfall (900 mm)

2. Dry season (May – Sep/Oct), corresponding to winter, with 5% of annual rainfall (40 mm).

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May-Sep

(40 mm)

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[

m

]

Wet Season

Oct-Apr

(900 mm)

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General characteristics: Hydrological cycle

• Driven by seasonality in rainfall patterns resulting in a bimodal distribution with a single

main peak flood (max. Q: Apr/May) and min. flow in Oct/Nov

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0

500 1000

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2000

dry season

Kafue (Hook Bridge)

Kafue Gorge Dam

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is

c

h

a

rg

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[

m

3

s

-1

]

Data [mm/dd/yy]

Zambezi River

Kafue River

wet season

0 1000

2000

3000

4000

5000

Zambezi (Victoria Falls)

Kariba Dam

D

is

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[

m

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(7)

Methods: Sampling strategy

• 3 sampling campaigns: Wet (Feb-Apr) 2012, Wet (Jan-Apr) 2013, Dry (Oct-Dec) 2013

• 56 sampling sites: 26 along Zambezi (Kariba & CB Res.), 13 along the Kafue (ITT Res.),

and 17 on different tributaries

1 0 /1 /2 0 1 0 1 1 /1 /2 0 1 0 1 2 /1 /2 0 1 0 1 /1 /2 0 1 1 2 /1 /2 0 1 1 3 /1 /2 0 1 1 4 /1 /2 0 1 1 5 /1 /2 0 1 1 6 /1 /2 0 1 1 7 /1 /2 0 1 1 8 /1 /2 0 1 1 9 /1 /2 0 1 1 1 0 /1 /2 0 1 1 1 1 /1 /2 0 1 1 1 2 /1 /2 0 1 1 1 /1 /2 0 1 2 2 /1 /2 0 1 2 3 /1 /2 0 1 2 4 /1 /2 0 1 2 5 /1 /2 0 1 2 6 /1 /2 0 1 2 7 /1 /2 0 1 2 8 /1 /2 0 1 2 9 /1 /2 0 1 2 1 0 /1 /2 0 1 2 1 1 /1 /2 0 1 2 1 2 /1 /2 0 1 2 1 /1 /2 0 1 3 2 /1 /2 0 1 3 3 /1 /2 0 1 3 4 /1 /2 0 1 3 5 /1 /2 0 1 3 6 /1 /2 0 1 3 7 /1 /2 0 1 3 8 /1 /2 0 1 3 9 /1 /2 0 1 3 1 0 /1 /2 0 1 3 1 1 /1 /2 0 1 3 1 2 /1 /2 0 1 3 1 /1 /2 0 1 4 0 1000 2000 3000 4000 5000 6000 7000 8000 2010-2011 Wet Season D is c h a rg e [ m 3 s -1] Date [mm/dd/yy] Victoria Falls Kariba Dam Dry Season

biweekly monitoring: Zambezi and Kafue

Wet 2012 D ry 2 0 1 3 2011-2012 2012-2013 Wet 2013

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Methods: Measured parameters

• Physico-chemical: pH, O

₂

, t°, conductivity, Total Alkalinity

• Total Suspended Matter (TSM) and sediment characterization

• Concentration and stable isotope (δ

13 _{C) composition of POC, DOC, DIC,}

• Aquatic metabolism: Bacterial respiration and primary production

• GHG (CO

₂

, CH

₄

, N

₂

O) concentrations and fluxes

• Radiocarbon isotopes dating (Δ

14 _C

POC

and Δ

14 C

DOC

)

Headspace Technique

Membrane Equilibrator

In-situ CO

₂

measurements

(9)

Results: pH spatio-temporal variability

0

500 1000

1500

2000

2500

3000

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

C.B. Res. (2012 wet) C.B. Res. (2013 dry) Kariba Res. (2012 wet)

Kariba Res. (2013 wet) kariba Res. (2013 dry)

p

H

[

N

B

S

]

Distance from source [km]

Zambezi (2012 wet) Zambezi (2013 wet) Zambezi (2013 dry) Barotse Floodplains Chobe Swamps

0

200

400

600

800 1000

1200

1400

1600

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

Kafue Gorge Reservoir

ITT Res. (2012 wet) ITT Res. (2013 wet) ITT Res. (2013 dry)

Lukanga Swamps

pH

[

N

B

S

]

Distance from source [km]

Kafue (2012 wet) Kafue (2013 wet) Kafue (2013 dry)

Kafue Flats

Zambezi River

Kafue River

pH decreases (more acidic) in-,

(10)

Results: DO spatio-temporal variability

Zambezi River

Kafue River

Significant DO decrease in-, and

downstream wetlands (below 2 mg L

-1

₎

0

500 1000

1500

2000

2500

3000

0

20

40

60

80

100

120

140

160

180

Kariba Res. (2012 wet) Kariba Res. (2013 wet) Kariba Res. (2013 dry)

C.B. Res. (2012 wet) C.B. Res. (2013 wet) C.B. Res. (2013 dry)

D

is

s

o

lv

e

d

O

x

y

e

n

[

%

]

Distance from source [km]

ZBZ (2012 wet) ZBZ (2013 wet) ZBZ (2013 dry) Barotse Floodplains Chobe Swamps

2 mg L

-1

0

200

400

600

800 1000 1200 1400 1600

0

20

40

60

80

100

120

140

160

180 D

is

s

o

lv

e

d

O

x

y

g

e

n

[

%

]

Distance from source [km]

Kafue (2012 wet) Kafue (2013 wet) Kafue (2013 dry) Kafue Flats Lukanga Swamps

2 mg L

-1

(11)

Results: CO

₂

spatio-temporal dynamics

0

500 1000

1500

2000

2500

3000

0 2000

4000

6000

8000

10000

12000

14000

_{C.B. Res. (2012 wet)} C.B. Res. (2013 wet) Kariba Res. (2012 wet)

Kariba Res. (2013 wet) Kariba Res. (2013 dry)

p

C

O

2

[

ppm

]

Distance from source [km]

Zambezi (2012 wet) Zambezi (2013 wet) Zambezi (2013 dry) Barotse Floodplains Chobe Swamps

Kariba & CB Dam

hypolimnetic

water release Shire >13000 ppm

Degassing Vic. Falls

0

200

400

600

800 1000 1200 1400 1600

0 2000

4000

6000

8000

10000

12000

14000

Lukanga Swamps atm. CO2

p

C

O

2

[

ppm

]

Distance from source [km]

Kafue (2012 wet) Kafue (2013 wet) Kafue (2013 dry) ITT Res. (2012 wet) ITT Res. (2013 wet) ITT Res. (2013 dry)

Kafue Flats

Kafue Gorge Reservoir

Zambezi River

Kafue River

Substantial increase in CO

₂

in-,

and downstream wetlands

(12)

Results: CH

₄

spatio-temporal dynamics

0

500 1000

1500

2000

2500

3000

1

10

100 1000

10000

100000

C.B. Res. (2012 wet) C.B. Res. (2013 dry) Kariba Res. (2012 wet)

Kariba Res. (2013 wet) Kariba Res. (2013 dry) Zambezi (2012 wet) Zambezi (2013 wet) Zambezi (2013 dry)

C

H

4

[

nm

ol

L

-1

]

Distance from source [km]

Barotse Floodplains Chobe Swamps

0

200

400

600

800 1000 1200 1400 1600

1

10

100 1000

10000

100000

C

H

4

[

nm

o

l

L

-1

]

Distance from source [km]

Kafue (2012 wet) Kafue (2013 wet) Kafue (2013 dry) ITT Res. (2012 wet) ITT Res. (2013 wet) ITT Res. (2013 dry)

Kafue Flats

Zambezi River

Kafue River

CH

₄

increases substantially in-,

(13)

Results: TSM & POC spatio-temporal variability

0

200

400

600

800 1000

1200

1400

1600

1

10

100 1000

T

S

M

[

m

g

L

-1

]

Distance from source [km]

Kafue (2012 wet) Kafue (2013 wet) Kafue (2013 dry) Kafue Flats Lukanga Swamps

0

500 1000

1500

2000

2500

3000

1

10

100 1000

C.B. Res. (2012 wet) C.B. Res. (2013 wet) Kariba Res. (2012 wet)

Kariba Res. (2013 wet) Kariba Res. (2013 dry) ZBZ (2012 wet) ZBZ (2013 wet) ZBZ (2013 dry)

T

S

M

[

m

g

L

-1

]

Distance from source [km]

Barotse Floodplains

Chobe Swamps

Zambezi River

Kafue River

UPSTREAM

DOWNSTREAM

6

8

10

12

14

16

18

20

22

24

26 %

P

O

C

o

f

T

S

M

[

%

]

Barotse Floodplains Chobe Swamps Lukanga Swamps Kafue Flats

2% increase

5% increase

11% increase

- No clear TSM trend

- Notable increase in the relative

contribution of POC to the TSM in-,

and downstream wetlands

(14)

Results: DOC spatio-temporal variability

0

500 1000

1500

2000

2500

3000

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

C.B. Res. (2012 wet) C.B. Res. (2013 wet) Kariba Res. (2012 wet)

Kariba Res. (2013 wet) Kariba res. (2013 dry) ZBZ (2012 wet) ZBZ (2013 wet) ZBZ (2013 dry)

D

O

C

[

m

g

L

-1

]

Distance from source [km]

Barotse Floodplains

Chobe Swamps

Zambezi River

Kafue River

DOC increases in-, and

downstream wetlands

0

200

400

600

800 1000

1200

1400

1600

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0 D

O

C

[

m

g

L

-1

]

Distance from source [km]

Kafue (2012 wet) Kafue (2013 wet) Kafue (2013 dry)

(15)

River / Reservoir

Area

CO

2

flux

CH

4

flux

CO

2

load

CH

4

load

Emission Deposition

[km

2

_]

_{[mg C m}

-2

_d

-1

_]

_{[kt C yr}

-1

_]

Kafue River without reservoirs

287 2962

20.0

310

2.1

312 -Itezhi Tezhi Reservoir

364 1102

18.1

146

2.4

149

16 Kafue Gorge Reservoir

13

956

17.4

5

0.1

5 1**

Kafue River with reservoirs

664 1903*

18.9*

461

4.6

466

17 Zambezi River without reservoirs

1879

4291

45.0 2943

30.8 2974

-Kariba Reservoir

5364

-191

4.8 -374

9.3 -364

120 Cahora Bassa Reservoir

2670

-356

1.4 -347

1.4 -346

60**

Zambezi River with reservoirs

9913

614*

11.5*

2222

41.5 2264

180 Zambezi & Kafue Rivers with reservoirs

10576

695*

12.0*

2683

46.1 2729

196 Results: Carbon Budget

Total C Yield: 7165 kt C yr

-1

Total C Yield: 23,485 kt C yr

-1

Emissions increase 7 fold

(16)

Concluding remarks

Wetlands/floodplains have large influence on river biogeochemisty:

Decreasing the pH and Dissolved Oxygen concentrations

Increasing the Temperature and Evaporation

Increasing CO

₂

and CH

₄

, DOC and POC concentrations

Highly productive ecosystems, wetlands are essential elements of carbon

cycle, capable of shifting significantly the balance between Emissions,

Storage and Transport components of carbon budgets.

Further research (more quantitative data) are needed to better constrain

the role of wetlands in both regional and global C budgets, which so far has

been largely overlooked.

(17)

Thank you for your attention!

(18)

The global carbon cycle

Global C budget studies are mostly limited to the three major carbon reservoirs

with little or no focus on inland waters

Major carbon

reservoirs

• Atmosphere

• Biosphere

• Ocean

(19)

Historical perception

Land 0.9 Pg C y

-1

_{Inland waters}

_{Ocean 0.9 Pg C y}

-1

River are passive “pipes” transporting to oceans terrestrial carbon

(Cole et al., 2007. Plumbing the global

carbon cycle: integrating inland waters into

the terrestrial carbon budget. Ecosystems 10,

172–185)

Introduction: Inland waters in global C cycle

Research on inland waters suggest that:

beside Transport

Emitting

Storing

(20)

Canadell & Raupach, 2008

Global NEP ~3.0 Pg C yr

-1

Terrestrial

Freshwater systems function as biogeochemical “hot spots”

(Aufdenkampe et al., 2011. Rivers key to coupling biogeochemical cycles between land, oceans and

atmosphere. Front. Ecol. Environ. 9, 53–60) based on Batin el al., 2009

3.3 (70%)

4.8 0.6 0.6 2.1

Introduction: Inland waters in global C cycle

More recent research

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Overlooked importance of wetlands

“Three-quarters of the world’s flooded land consists of temporary wetlands,

but the contribution of these productive ecosystems to the inland water

carbon budget has been largely overlooked”.

“Flooded forests and floating macrophytes provide,

through litterfall and submerged root respiration, a

total of 305±120 Tg C yr

-1

_{of atmospheric carbon to}

the waters”….“not significantly different from the CO

₂

outgassing flux of 210±60 Tg C yr

-1

_{. Central}

Amazonian waters thus receive at least as much

carbon from semi-aquatic plants as they emit to the

atmosphere”