# Towards carbon neutrality by 2040 in La Rochelle metropolitan area (France): quantifying the role of wetlands and littoral zone in the capture and sequestration of blue carbon

Christine Dupuy, Hélène Agogué, Benjamin Amann, Frédéric Azémar, Nicolas Becu, Lauriane Bergeon, Xavier Bertin, Pierrick Bocher, Emilie Bout, Isabelle Brenon, et al.

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Christine Dupuy, Hélène Agogué, Benjamin Amann, Frédéric Azémar, Nicolas Becu, et al. Towards carbon neutrality by 2040 in La Rochelle metropolitan area (France): quantifying the role of wetlands and littoral zone in the capture and sequestration of blue carbon. ECSA 59 Using the best scientific knowledge for the sustainable management of estuaries and coastal seas, Sep 2022, San Sebastian, Spain., 2022. hal-03777579

HAL Id: hal-03777579

https://cnrs.hal.science/hal-03777579

Submitted on 14 Sep 2022

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# Towards carbon neutrality by 2040 in La Rochelle metropolitan area (France): quantifying the role of wetlands and littoral zone in the capture and sequestration of blue carbon

**Consortium of LRTZC (La Rochelle Territoire Zéro Carbon) project:**

Dupuy C.¹, Agogué H.¹, Amann B.¹, Azémar F.², Becu N.¹, Bergeon L.¹, Bertin X.¹, Bocher P.¹, Bout E.³, Brenon I.¹, Carpentier A.⁴, Ceaux S.², Chaumillon E.¹, Choquet C.⁵, Colin, B.¹, Deborde J.⁶, Dubillot E.¹, Emery C.¹, Ferrari S.⁷, Gaucherel C.⁸, Geairon P.⁶, Gilbert S.², Jeannin M.⁹, Jourde J.¹, Kalenitchenko D.¹, Lachaussée N.¹, Lacoue-Labarthe T.¹, Lanneluc I.¹, Lavaud L.¹, Lavaud S.⁷, Lefrancois C.¹, Le Fouest V.¹, Le Fur I.⁶, Long N.¹, Mahieux PY.⁹, Mayen J.⁶, Marais C.¹,⁹, Metzger E.¹⁰, Moncelon R.¹, Ouisse V.¹¹, Péreau JC.⁷, Pétillon J.¹²,¹³, Philippine O.¹⁴, Pineau P.¹, Pignon-Mussaud C.¹, Polsenaere P.⁶, Sabot R.⁹, Refait P.⁹, Reveillac E.¹, Robin FX.¹⁴, Rouquette H.¹⁵, Sable S.¹, Sauriau P.-G.¹, Tackx M.², Turcry P.⁹, Vagner M.¹⁶, Vincent J.¹, Volto N.¹

1 UMR 7266 Littoral Environnement et Sociétés (LIENSs), CNRS - La Rochelle University, France, 2 UMR 5245 (CNRS-UPS-INPT), Laboratoire Ecologie Fonctionnelle et Environnement- Toulouse University (Paul Sabatier), France, 3 La Rochelle metropolitan area, France, 4 University of Rennes, UMR BOREA (MNHN), France, 5 MIA - La Rochelle University, France, 6 IFREMER, Littoral, Laboratoire Environnement et Ressources des Pertuis Charentais (LER-PC), France, 7 Univ. Bordeaux, CNRS, BSE UMR 6060, France, 8 AMAP-INRAE, CIRAD, CNRS, IRD, Univ. Montpellier, France, 9 LASIE, CNRS - La Rochelle University, France, 10 LPG, University of Angers, CNRS, France, 11 MARBEC, Univ. Montpellier, Ifremer, IRD, CNRS, Sète, France, 12 UMR 6553 Ecosystèmes, Biodiversité, Evolution (ECOBIO), CNRS - University of Rennes, Rennes, France, 13 Institute for Coastal and Marine Research - Nelson Mandela University, Port Elizabeth, South Africa, 14 Union des Marais de la Charente-Maritime, France, 15 La Rochelle town, France, 16 LEMAR, CNRS-IFREMER - University of Brest, France

## Importance of aquatic ecosystems in the capture and sequestration of blue carbon

Rising greenhouse gas emissions are causing increasing worldwide impacts and changes on climate patterns, sea level, food production, human lives and livelihoods. Maintaining or improving the ability of coastal aquatic ecosystems and oceans to remove CO₂ from the atmosphere is a crucial aspect for climate mitigation. The vegetated coastal ecosystems are able to catch and to sequestrate carbon, called Blue Carbon. These ecosystems are key exchange zones that mediate the biogeochemical cycles across the continent, the ocean and the atmosphere. Given the importance of these ecosystems in biogeochemical cycles and their sensitivity to natural and anthropogenic pressures, the carbon cycle within and between compartments and across the interfaces (e.g. atmosphere, ocean...) needs to be addressed.

<figure><img src="image_2.png" /></figure>

## Objective of LRTZC project: On the regional scale, target the ambition of carbon neutrality in La Rochelle metropolitan area by 2040 through a holistic approach (from measuring CO₂ to raising people's awareness and assessing the impact of exogenous natural factors)

<figure><img src="image_3.png" /><figcaption>9 types of experimental zones</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Diagnostic of carbon budget in La Rochelle metropolitan area</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>La Rochelle metropolitan area
170 000 inhabitants, 28 municipalities</figcaption></figure>

<figure><img src="image_6.png" /></figure>

### 8 axes to transform La Rochelle metropolitan area with a holistic approach

<figure><img src="image_7.png" /></figure>

Blue carbon

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Building

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Energy

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Soft mobility

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Industries

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Data

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Carbon cooperative

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People's awareness

### Objectives of the blue carbon axis:

* Assess and monitor carbon capture and sequestration on the wetlands and littoral zone: sink or a source of CO₂ to the atmosphere?

* Protect the littoral zone to the submersion and erosion

* Development of decision-making tools for scientists and stakeholders

* Support the territory actors towards the blue carbon mutation

* Replicability of the project in national and international metropolitan areas

### Innovations of LRTZC project:

Holistic and methodologic innovations on experimental zones to optimize circular economy and carbon footprint of the territory

Create a territorial data plate-form to evaluate all the actions and develop new actions and uses

Carbon cooperative: a new financial paradigm to mobilize all actors

Governance: implication of more than 130 partners, citizens, public and private structures

Societal innovation to support the cultural transformation of La Rochelle metropolitan area

<figure><img src="image_15.png" /></figure>

## First results on the Blue Carbon axis

### Atmospheric CO₂ fluxes in aquatic ecosystems

<figure><img src="image_16.png" /><figcaption>Freshwater marsh of Tasdon</figcaption></figure>

Methodology: seasonal 24h cycle measurements with sensors at 3 stations

<figure><img src="image_17.png" /><figcaption>Renaturation of the marsh</figcaption></figure>

* Between 2019 and 2021: significant decrease in water pCO₂ from oversaturated (CO₂ source) to near balance, undersaturated (CO₂ sink) waters following the renaturation and reconnection of the marsh to the coastal zone.

<figure><img src="image_18.png" /><figcaption>Salt marsh of Ré Island</figcaption></figure>

Methodology: Atmospheric Eddy Covariance system since summer 2019

In 2020:
Net ecosystem exchange (NEE):
-1,41 ± 3,61 µmolCO₂ m²/s

<figure><img src="image_19.png" /></figure>

CO₂ Sink: - 485 gC/m²/yr (- 4,85 tCeq/ha/yr or -17,78 tCO₂eq/ha/yr)

### Long-term carbon accumulation in aquatic ecosystems

<figure><img src="image_20.png" /><figcaption>Salt marshes of the Aiguillon Bay</figcaption></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

* High sediment accumulation rates >1 cm/yr lead to relatively high carbon accumulation rates in salt marshes of the region.

* Potential long-term carbon sequestration rates are within the order of 4 – 13 tCO₂eq/ha/yr.

### Nature-based solutions for littoral zone protection

<figure><img src="image_23.png" /><figcaption>Salt marsh</figcaption></figure>

Methodology: different instruments

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

* Vegetation accounts for 65% of wave dissipation across the salt marsh.

* Work in progress aims at quantifying the lowering of mean water level that accompanies wave propagation over vegetated bottom.

**All the partners of LRTZC project:** La Rochelle metropolitan area, La Rochelle town, Universities of La Rochelle, Toulouse, Rennes, Bordeaux, Montpellier, Angers, Brest, IFREMER (La Tremblade, Sète), Union des Marais de la Charente-Maritime, France and Institute for Coastal and Marine Research - Nelson Mandela University in South Africa

Contact: Christine Dupuy: cdupuy@univ-lr.fr

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Communauté d'Agglomération de La Rochelle

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BANQUE des TERRITOIRES

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LE GRAND PLAN D'INVESTISSEMENT

RÉGION Nouvelle-Aquitaine

<figure><img src="image_33.png" /></figure>

PORT DE PLAISANCE DE LA ROCHELLE

ADEME

<figure><img src="image_34.png" /></figure>