Assessment of the Betsiboka estuary mangrove areas 

from 1989 to 2019




WIOMSA 12th symposium, poster presentation, October 13th, 2-3pm

THEME XXIII: Mangroves ecosystems

Mangroves, Madagascar @ Yacinthe Lesecq

Authors

Ravaka RANDRIANATOANDRO, University of Antananarivo, Department of Geography

Éric DELAITRE, French Institute for Sustainable Development, UMR Espace-Dev

Samuel RAZANAKA, Centre National de Recherches sur l'Environnement

Dominique HERVE, French Institute for Sustainable Development, UMR SENS

Background

The Betsiboka estuary mangrove, one of the most extensive mangroves in Madagascar, is of socio-economic and environmental value that justifies spatial monitoring.

The spatiotemporal dynamic of the geomorphological formation of the Betsiboka estuary and its colonization by the mangrove has been reconstituted between 1989 and 2019 on the basis of eight LANDSAT images taken during the dry season at low tide, in such a way as to capture as much as possible of the emerged area. From this, we have devised maps of the land cover with three classes: water, bare soil / salt-marsh / non-mangrove vegetation, and mangrove, a map of changes in the land cover (1989 to 2019) and maps assessing the extent of the mangrove areas (extension and loss). In addition, on the basis of these maps, twelve transects were positioned from upstream to downstream, the same distance apart, perpendicularly to the main axis of the river, and including the two banks. This cartographical system embedded in a GIS provides a basis for quantifying the specific dynamics of the three land cover classes for each transect.

Over this period of 30 years, 1) the various branches of the river have become silted up upstream to the extent of complete disappearance mainly through an extensive accretion of islets, and 2) downstream, several islets have developed from the drying out of the shallows, 3) the surface area of mangrove has extended on the new emerged land downstream with a simultaneous loss due to the expansion of mangrove to salt marsh, mainly upstream, with recently in addition frequent clear cutting for the manufacture of charcoal and the collection of timber and firewood. According to the assessment of surface areas over 30 years, the decline of the mangrove has been greater than the gain: 46% of the mangrove (91km2) has been lost, while 11% of water bodies and 7% of the bare soil have become mangroves (48km²).

Nevertheless, improvements in the discrimination of the processes are required to be able to distinguish the two causes of decline. The natural decline results from the expansion of salt marsh or the mortality of the mangrove trees owing to the natural evolution of the physical and chemical conditions, especially those of the soil, which have become unfavorable for their survival. The human-induced decline is due to the attraction of the mangroves for migrant populations newly settled in the region, which find there a major source of additional income, despite the ban on cutting mangroves through the territory of Madagascar.  

Summary map of the dynamics of the estuary between 1989 and 2019 @ Ravaka RANDRIANATOANDRO

Keywords


Mangrove, remote sensing, land cover dynamic, LANDSAT