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

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

wetland connectivity Alberto V. Borges

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Lab presentation

• Measurements of CO₂, CH₄, N₂O in aquatic enviroments

• Personal emphasis on African lakes and rivers (since 2007)

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Natural wetlands (217) Freshwaters (40) Geological-land (36) Wild animals (15) Termites (11) Oceans (18)

Natural sources

Cattle (101)

Land fills & waste (63) Rice (36)

Fossil fuels (96)

Biomass & biofuel burning (35)

Anthropogenic sources

Tropospheric OH- ₍₅₂₈₎ Soils (28) Stratospheric Cl (25) Stratospheric loss (51)

Sinks

Sources and sinks of CH₄ in Tg CH₄ yr-1

Kirschke et al. (2013) Nature Geoscience, 6, 813-823

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Natural wetlands (217) Freshwaters (40) Geological-land (36) Wild animals (15) Termites (11) Oceans (18) Cattle (101)

Biomass & biofuel burning (35) Tropospheric OH- ₍₅₂₈₎

Tropospheric OH- ₍₂₈₎

Stratospheric Cl (25) Stratospheric loss (51)

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Natural wetlands (217) Freshwaters (40) Geological-land (36) Wild animals (15) Termites (11) Oceans (18) Cattle (101)

Biomass & biofuel burning (35) Tropospheric OH- ₍₅₂₈₎

Tropospheric OH- ₍₂₈₎

Stratospheric Cl (25) Stratospheric loss (51)

100 Lakes 30 Rivers Introduction

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Introduction www.globalcarbonproject.org/ 9

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 CO₂ fluxes in 2010 (PgC y-1 _{= 10}15 _{gC y}-1₎

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Introduction

River CO₂ global emission

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Introduction

River CO₂ global emission

1.8 PgC yr-1 _{(Raymond et al. 2013)}

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Introduction

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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 !

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Tropical Rivers:

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Highest CO₂ emissions

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Lowest data coverage

-

(Lowest confidence in CO₂ data quality) Introduction

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Congo

Congo river basin

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Congo

Congo river basin

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Metamorphic Courtesy of J. Hartmann Siliciclastic and unconsolidated sed. Congo

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Congo Wetland GLC 2000 Humid Forest Savannah Savannah

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Wetland

= flooded forest

(Tributary)

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Wetland

= floating macrophytes

(Tributary)

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Wetland

= floating macrophytes

(Congo mainstem)

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Vossia cuspidata

« Hippo grass »

Eichhornia crassipes

« water hyacinth »

Salvinia auriculata

Azolla pinnata

Congo

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Kisangani Kinshasa 1700 km Congo 375 m 280 m

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Cruises & Methods

164 stations 29 variables

> 23,000 continuous measurements pCO₂, cond, temp, pH, O₂, TSM, cDOM

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Spatial variations of CO₂ Results

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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

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Results 0 5000 10000 15000 20000

pCO₂ (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 →

%O₂ (%)

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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

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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 → CH₄ (nmol L-1₎ Mainstem High W Tributaries High W Mainstem Falling W Tributaries Falling W Results « Cuvette Centrale » (Wetland) Savannah

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

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Metabolism versus CO₂ emissions Results

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Results Net heterotrophic R >> P CO₂ emission >> R Lateral CO₂ inputs >> in-stream R

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Results

CO₂ and CH₄ in rivers & streams of the Congo seem to be mainly related to wetland inputs

Based on:

- Spatial patterns (in/out of the Cuvette Centrale)

- Metabolic measurements

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Results

CO₂ emission from Congo rivers-streams

FCO₂ = k H ΔpCO₂

FCO₂ = air-water CO₂ flux

H = Henry’s constant = f(temperature)

ΔpCO₂ = air-water gradient of pCO₂ (measured)

k = gas transfer velocity

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Results

CO₂ emission from Congo rivers-streams Stream surface area = length x width

length = Hydrosheds

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CO₂ emission from rivers 130 times higher than C export of terrestrial NEE to rivers ? What ?? Results

CO₂ emission from Congo rivers-streams = 251 TgC yr-1

Net ecosystem exchange (NEE) Congo forests + savannahs = 77 TgC yr-1

CO₂ emission from rivers 3 times higher than terrestrial NEE ??? Export of C from soils to rivers

= 2-3% of NEE

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Results

CO₂ 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 C leaked from wetlands = 400 TgC yr-1

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Congo & other African rivers Results

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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

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Congo versus Amazon Results

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Results Amazon Congo Catchment area (km2) 6,025,735 3,705,222 Slope (°) 1.4 0.6 Discharge (km3 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 (km2) 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 > > > > > <

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Results p C O 2 ( p p m ) C H 4 ( n m o l L -1 ) pCO₂ _{is ± similar} CH₄ is 3-4 times higher in Congo

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

Q_max:Q_min = 2.85 Qmax:Qmin = 1.99

H_max - H_min = 10-12 m H_max - H_min = 3-4 m Wetlands only in floodplains

Not in river channels

Wetlands in river channels

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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 ) pCO₂ = 277,076 ppm ?

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Introduction

Raymond et al. (2013) & Lauerwald et al. (2015) used pCO₂ computed from pH and total alkalinity