Boundaries

The Eastern Lowlands comprise a narrow strip of land flanking Madagascar’s eastern coast. 

Topography

Rivers flowing through the Eastern Lowlands have relatively flat profiles compared to their steep upper reaches, have moderate to slow flows, and are frequently turbid (probably a result of extensive upstream deforestation). 

Freshwater habitats

Within this narrow coastal plain, rivers often meander and terminate in chains of brackish lagoons (Aldegheri 1972). The substrate in most rivers ranges from coarse gravel to sand or mud, and the water is relatively warm compared to middle and upper reaches of the same rivers. Lowland Pandanus and palm swamps, as well as numerous estuarine habitats along the eastern coast, continue to support native fish faunas. 

Terrestrial habitats

Within this narrow coastal plain, rivers often meander and terminate in chains of brackish lagoons (Aldegheri 1972). The substrate in most rivers ranges from coarse gravel to sand or mud, and the water is relatively warm compared to middle and upper reaches of the same rivers. Lowland Pandanus and palm swamps, as well as numerous estuarine habitats along the eastern coast, continue to support native fish faunas. 

Justification for delineation

The upper and lower eastern basins have traditionally been considered as a single hydrographic region. Based on the results of recent ichthyofaunal surveys throughout eastern Madagascar, however, an elevational disjunct in species distributions has been documented (Reinthal & Stiassny 1991; Sparks & Stiassny 2003). Sparks and Stiassny (2003) record a total of 69 native fish species from basins in the Eastern Lowlands, and 51 species from the Eastern Highlands. In total, 57 native species are restricted to the Eastern Highlands [581] and Lowlands combined, whereas 48 native species are restricted to western basins. Thus, eastern drainages are richer than western basins. Within the eastern basins, the fish assemblages present in highland and lowland habitats differ considerably (Sparks & Stiassny 2003). Taking into account the substantial differences in their respective ichthyofaunal assemblages, the upper and lower eastern basins have been split at the edge of the coastal plain.

Level of taxonomic exploration

Fair. Of Madagascar’s five major aquatic ecoregions, the Eastern Highlands [581] and Eastern Lowlands are certainly the best studied. This is mainly due to a number of large towns and a network of regularly maintained roads along most of the eastern coast that permit access to these basins.