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This is the largest lake in Madagascar, situated 170 km north-east of Antananarivo and 7 km north of Ambatondrazaka. It lies in a tectonic basin 40 km long and 9.5 km wide, located between the Grand Angavo escarpment in the west and the Mangoro–Alaotra escarpment in the east. Its depth is 1.0–2.5 m during the low-water season and 4 m during the high-water season. The water is fresh, and turbid with suspended silt due to intense erosion of the deforested hills that surround the lake. The lake is fed by the Sasomanga and Sahabe rivers in the south, and by the Sahamaloto and Anony rivers in the north-west. The only drainage river, the Maningory, flows out of the north-eastern part of the lake. The lake-bottom sediments consist of thick layers of sand, black mud and grey clay. Natural, permanent marshes occur along the lake margin, especially in the southern part of the site. The largest are located to the south and east of the Antanamalaza peninsula and in the part drained by the Maningory. There are also wet grasslands. The lake vegetation was formerly dominated by water-lilies Nymphaea, but these have decreased in extent due to competition with non-native water-hyacinth Eichhornia and water-fern Salvinia. In the marshes, sedges Cyperus and reeds Phragmites dominate. Secondary savanna covers the surrounding hills.
See Box and Tables 2 and 3 for key species. Eighty species are known from the site, of which 13 are endemic to Madagascar. Lake Alaotra is an exceptional site for waterbirds. It formerly held two highly threatened endemic species, which were probably confined to the lake, but which may now be extinct: Tachybaptus rufolavatus and Aythya innotata. The former was last recorded in 1985 and the latter in 1991. Furthermore, the flocks of Anas melleri that occur on the lake constitute the largest congregations known for this species, and Ardea humbloti also occurs regularly (in its only eastern site), though it is not proven to have bred.
Non-bird biodiversity: Lemur: Hapalemur griseus alaotrensis (CR). Carnivore: Salanoia concolor (VU).
BirdLife International (2020) Important Bird Areas factsheet: Lake Alaotra NPA. Downloaded from http://www.birdlife.org on 04/04/2020.