Alaotra wetlands, Madagascar (photo: Pina Lammers); Madagascar Conservation & Development

Lake Alaotra wetlands: how long can Madagascar´s most important rice and fish production region withstand the anthropogenic pressure?

Pina L. Lammers, Torsten Richter, Patrick O. Waeber, Jasmin Mantilla-Contreras


The Alaotra wetlands represent the biggest lake and wetland complex in Madagascar and are home of several endemic species. The region constitutes the largest rice production area and inland fishery of Madagascar. Rice and fish are the main local sources of income. While the population has increased fivefold during the last 40 years, the growing need for resources is continuously increasing the pressure on the wetland system. In this study, vegetation and water parameters were collected within three sites differing by level of degradation in order to evaluate the current ecological state of the wetland. The results show that high levels of ongoing anthropogenic disturbance are favoring the formation of a new plant community in the fringe area of the marsh belt. This area is now dominated by invasive species such as the water hyacinth (Eichhornia crassipes) which shows a mean coverage up to 53% and water ferns (Salvinia spp.) with a mean coverage up to 31.4%. Lake water levels were very low and decreased during the dry season to a mean level of only 3 cm in the littoral zone. Signs of eutrophication like hypoxia (mean saturation of only 22%), increased phosphate concentrations (1.18 mg L-1 ) and black colored, foul smelling water were observed. Under a likely scenario of growing anthropogenic pressures, it remains unclear what the current trends will bring for the wetland’s future.


La zone humide de l’Alaotra représente le plus grand lac et complexe de zones humides de Madagascar hébergeant plusieurs espèces endémiques. La région constitue la première production de riz et de pêche dulcicole de l’île. Le riz et les ressources piscicoles sont les principales sources de revenus locaux. L’effectif de la population humaine locale a été quintuplé au cours des dernières 40 années et les besoins en ressources ont augmenté en conséquence de sorte que les pressions sur la zone humide ont été exacerbées. Dans cette étude, des paramètres portant sur la végétation et l’eau du lac ont été collectés dans trois sites qui diffèrent par le niveau de dégradation pour évaluer l’état écologique actuel de la zone humide. Les résultats montrent que le niveau élevé des perturbations anthropiques favorise la formation d’une nouvelle communauté végétale sur la ceinture du marais. Cette zone est dominée par des espèces de plantes envahissantes à l’exemple de la jacinthe d’eau (Eichhornia crassipes) dont la couverture moyenne est de 53% et de la fougère d’eau (Salvinia spp.) avec une couverture moyenne de 31,4%. Les niveaux du lac sont bas et diminuent jusqu’à 3 cm de moyenne dans la zone littorale pendant la saison sèche. Des signes d’eutrophisation comme l’hypoxie (saturation moyenne de 22%), l’augmentation de la concentration de phosphate (1,18 mg L-1 ) et la présence d’eau de couleur noire et nauséabonde ont été observés. Dans un possible scénario avec une augmentation des pressions humaines, les tendances actuelles restent difficiles à comprendre pour l’avenir de la zone humide.


Alaotra wetlands; human disturbance; invasive species; eutrophication; drawdown

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