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

• Inland waters, coastal waters, oceanic waters & marine cryosphere • Personal emphasis on African lakes and rivers

• Versatile, compact and rugged equipment for harsh environments • ULiège = small oceanography Dept, no limnology Dept

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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 ! 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

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Tropical Rivers:

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Highest CO₂ emissions

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Lowest data coverage

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Lowest confidence in pCO₂ computed from pH & TA Introduction

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Congo

Congo river bassin

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Congo

Congo river bassin

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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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Eichhornia crassipes

« water hyacinth »

Vossia cuspidata

« Hippo grass »

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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Results 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 0 200 400 600 800 1000 1200 1400 1600

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

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0 10000 20000 30000 40000 50000 60000 0 200 400 600 800 1000 1200 1400 1600

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

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0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 0 200 400 600 800 1000 1200 1400 1600

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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0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 0 200 400 600 800 1000 1200 1400 1600

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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Stable isotopic composition of DIC Results

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Results 1 3 C -D IC ( ‰ ) Increasing HCO₃ -Decreasing CO₂ (degassing) « only » CO₂

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Atmospheric CO₂ C₄ Plants C₃ Plants Results Increasing CO₂ Increasing C₄ plants Not savannah Macrophytes

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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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Results

CO₂ 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

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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 = 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

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