Fish Assemblage Structure on the Floodplain of an African River

In the Oueme River, a lowland river in Benin, Africa, artificial ponds constructed in the floodplain (whedos) are colonized during the high-water period by a presumably random sample of fishes from the river channel. As water slowly recedes from the floodplain, fishes are isolated in whedos until they are harvested near the end of the dry season. We surveyed fishes in whedos and adjacent main-channel and floodplain habitats during two low-water periods (2008 and 2009) and one falling-water period (2010-2011). In 2010-2011, we also measured a suite of physicochemical variables including dissolved oxygen, temperature, specific conductivity, and percent cover of aquatic vegetation. Whedos were covered with dense growth of aquatic vegetation, and dissolved oxygen concentrations were lower in whedos and a natural floodplain depression compared to the main channel. Multivariate analyses revealed that habitat types were distinct with regard to assemblage structure and abiotic conditions. Assemblages in whedos and natural floodplain depressions were differentiated from those of the river channel, with the floodplain habitats being dominated by piscivorous fishes that tolerate aquatic hypoxia. In this system, dispersal, aquatic hypoxia, and predation act in concert to shape local community structure.