Ravaka Randrianatoandro is a PhD student in geography at the University of Antananarivo. Her fields of interest are biogeography and geomatics. She is doing a thesis on the evolution of the mangrove ecosystem in the geomorphological context of the Betsiboka estuary. Her thesis is part of the Deltas - Madagascar component of the DIDEM programme. In 2020, as part of her thesis, she did an internship at the Maison de la télédétection UMR Espace-Dev in order to train in remote sensing thanks to a French government grant. Ravaka also teaches part-time Geographic Information System at the Department of Geography of the University of Antananarivo.
Ravaka, you work on the mangroves of the Betsiboka, can you explain your thesis project?
My thesis aims to analyse the relationship between geomorphology and the evolution and functioning of the mangrove in the Betsiboka estuary. My thesis is based on an empirical method of observation of the islets of the Betsiboka delta and a retrospective approach combining remote sensing and sedimentology techniques:
the observation of the islets is based on a description of the islets in terms of landscape, microtopography and plant formations following a transect.
The remote sensing part of the study aims to analyse the spatio-temporal dynamics and the bio-hydro-morpho-sedimentary functioning of the estuary.
The sedimentology part of the study aims to understand the sedimentation processes in the estuary and to highlight the relationships between the physico-chemical characteristics of the soil and the structure of the mangrove stands in the Betsiboka delta.M
Return from the field and landing in Marovoay, Madagascar @D. Hervé
Boat and mangrove on the Betsiboka delta @ IRD DIDEM Rijasolo
Why is this issue important in this region? What are the stakes?
The Betsiboka estuary, and in particular the mangrove of the delta nested in the estuary, presents important socio-economic and environmental challenges for the Boeny region. The environment is experiencing strong sedimentation due to massive deforestation upstream of the watershed. This has many impacts on the biodiversity of the environment. The hyper sedimentation in the estuary has favoured the creation of new islets which has allowed the extension of the mangrove. But this process is also the cause of tannification, i.e. the natural death of mangroves.
Studying mangrove-sedimentation interactions in the estuary will allow better management of the environment and the ecosystem. Some studies have already investigated the mangrove-sedimentation relationship in the Betsiboka estuary. My thesis seeks to go further and in more detail in the understanding, with the help of remote sensing.
How will your work feed the dialogue between science and decision makers that started in Mahajanga in DIDEM?
The work I have undertaken has shown that in terms of surface balance, the mangrove in the estuary does not always have a regressive dynamic, but it also has a progressive dynamic. This gain in mangrove area over time is a good thing for the conservation of the ecosystem, but the loss of mangrove area, whether of anthropogenic or natural origin, is also alarming in the estuary. The study of the functioning of the estuary using Sentinel2 images is currently being finalised. The bio-hydro-sedimentary dynamics of the estuary are analysed in this study. The expectation is that the results will help in the decision making process for mangrove reforestation projects in the estuary. In fact, mangrove reforestation must take into account the topography and the frequency of tidal flooding of the islands.
Landing on an islet in the Betsiboka estuary, Madagascar @D. Hervé
RANDRIANATOANDRO R. K., DELAITRE E., RAZANAKA S., HERVÉ D., 2022, « L’estuaire de la Betsiboka vue par les satellites d’observation de la terre », Poster scientifique, Nosy Be : Journée Internationale des Mangroves, MEED, MAEP, MESUPRES, 25 juillet 2022.
RANDRIANATOANDRO R. K., DELAITRE E., RAZANAKA S., HERVÉ D., 2021, « Impacts de l’hyper sédimentation sur la dynamique de la mangrove dans l’estuaire de la Betsiboka suivis par télédétection », Poster scientifique, Majunga : Journée Internationale des Mangroves, MEED, MAEP, MESUPRES, 24 juillet 2021.
RANDRIANATOANDRO R. K., 2022, « Dynamique spatio-temporelle des mangroves du delta du Betsiboka à partir d’images Landsat (1989-2019) », Rapport atelier 1 : Dialogue Science-Décideurs, Antananarivo, 30/01/2022, DIDEM/Deltas–Madagascar n°5, 10 p.
RANDRIANATOANDRO R. K., DELAITRE E., ROBISON L., RAZANAKA S., HERVÉ D., 2021, « Vérité terrain des images satellitaires et prélèvements de sol dans l’estuaire de la Betsiboka », Rapport de missions : Mangrove-Sédimentation, Antananarivo, 15/12/2021, DIDEM/Deltas-Madagascar n°1, 57 p.
RANDRIANATOANDRO R. K., DELAITRE E., RAZANAKA S., HERVÉ D., 2021, « Relations entre la dynamique de sédimentation, l’apparition des îlots et leur colonisation par la mangrove dans l’estuaire de la Betsiboka », Rapport d’activités : Mangrove-Sédimentation, Antananarivo, 15/12/2021, DIDEM/Deltas–Madagascar n°2, 16 p.