Over the last two decades, measurements from oceanographic cruises in the Southern Indian Ocean show that surface water fCO2 levels and acidity have increased faster in regions with phytoplankton blooms (Crozet and Kerguelen) compared to high-nutrient, low-chlorophyll regions. The increases are primarily explained by rising total carbon levels associated with the anthropogenic signal. At Kerguelen and in high-nutrient, low-chlorophyll areas, fCO2 rose about 2 μatm/yr and pH fell 0.002 yr-1, consistent with atmospheric and global ocean trends. Near Crozet, fCO2 increased 3-5 μatm/yr and pH dropped more sharply

1. CO2 increase and ocean acidification
in the Southern Indian Ocean
over the last two decades
Coraline Leseurre, Claire Lo Monaco, Gilles Reverdin,
Nicolas Metzl, Jonathan Fin, Claude Mignon
Laboratoire d’Océanographie et du Climat :
Expérimentations et Approches Numériques
(LOCEAN) – Paris, France
coraline.leseurre@locean-ipsl.upmc.fr
17 Sept 2020

2. OISO : Océan Indien Service d’Observation
Gabriel, 2019 Leseurre, 2018
https://caraus.ipsl.jussieu.fr/oiso-campagnes.html
Marion Dufresne
• 1 to 2 oceanographic cruises per year
• Continuous sampling (surface water)
• Station sampling (20s; water column)
• Temporal monitoring of oceanic CO2 parameters
• Southern Indian Ocean
• Since 1998
pCO2
AT
CT
SST
SubAntarctic Front (South to North)
→ Carbon Parameters change
2

3. OISO area
fCO2 observations: SOCAT version v2020 (release, June 2020)
3
Bakker et al. (2016, 2020)

4. Observations of fCO2: A global view in the South-West Indian Ocean
SOCAT Monthly data available in the
South-West Indian ocean (1991-2019)
Mainly OISO in 1998-2019
Using all data, all seasons: fCO2 ↗ + 1.9 µatm yr-1
Imply ↘ in pH
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5. Example of fCO2 / pH data synthesis
Takahashi et al., 2014
Drake Passage
2002 – 2012
+1.5 µatm yr-1 to +2.2 µatm yr-1
-0.0018 yr-1 to -0.0027 yr-1
Bates et al., 2014
Time Series Stations
1983 – 2013
+1.3 µatm yr-1 to +3.0 µatm yr-1
-0.0013 yr-1 to -0.0026 yr-1
Lauvset et al., 2015
Oceanic Biomes
1991 – 2011
+0.2 µamt yr-1 to +3.5 µatm yr-1
+0.0013 yr-1 to -0.0026 yr-1
Continue this work and separate the processes for other datasets
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6. Biogeochemistry of the study area → impact of Fe limitation
HNLC
High Nutrient
Low Chlorophyll
Fe and light
limitations
Phytoplankton
blooms
Fe fertilization
(Crozet and Kerguelen)
Madagascar
Antarctic
Crozet
Kerguelen
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7. Results in summer: 2 different trends (1998-2019) Mixed Layer
Kerguelen
Crozet
HNLC
fCO2 atm
+2 µatm yr-1
fCO2 measurements and
calculations in agreement
ocean fCO2 trend =
atmospheric trend
-0.002 yr-1
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Calculated (AT / CT)
Measured

8. Results in summer: 2 different trends (1998-2019) Mixed Layer
Kerguelen
Crozet
HNLC
fCO2 atm
+2 µatm yr-1
fCO2 measurements and
calculations in agreement
-0.002 yr-1
+3 to +5 µatm yr-1
-0.003 to -0.006 yr-1
Faster trends
Phytoplankton blooms
8
Calculated (AT / CT)
Measured

9. Results in summer: 2 different trends (1998-2019) Mixed Layer
Kerguelen
Crozet
HNLC
Phytoplankton blooms
pH ↘ explained by ↗ Total Carbon (CT)
+0.5 to +0.9 µmol kg-1 yr-1 +1.4 to +2.1 µmol kg-1 yr-1
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10. Results in summer: 2 different trends (1998-2019)
Kerguelen
Crozet
↗ CT (MLD) ≈ ↗ CT (WW) ≈ ↗ Anthropogenic Carbon
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HNLC

11. Results in summer: 2 different trends (1998-2019)
Kerguelen
Crozet
Phytoplankton blooms ↗ CT (MLD) > ↗ CT (WW) ≈ ↗ Anthropogenic Carbon
↗ CT (MLD) ≈ ↗ CT (WW) ≈ ↗ Anthropogenic Carbon
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HNLC

12. Take Home Message
Kerguelen
Crozet
HNLC
Phytoplankton blooms
Mainly explained by ↗ CT
associated to anthropogenic signal
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↗ fCO2
oc and ↘ pH
Order of the global ocean Faster trends
Secondary mechanism during summer
Changes in productivity or timing of blooms ?
HNLC Blooms
+2 µatm yr-1
-0.002 yr-1
+3 to 5 µatm yr-1 -0.003 to -0.006 yr-1

13. CO2 increase and ocean acidification
in the Southern Indian Ocean
over the last two decades
Coraline Leseurre, Claire Lo Monaco, Gilles Reverdin,
Nicolas Metzl, Jonathan Fin, Claude Mignon
Laboratoire d’Océanographie et du Climat :
Expérimentations et Approches Numériques
(LOCEAN) – Paris, France
coraline.leseurre@locean-ipsl.upmc.fr
Thank you for your attention