Nematode communities of Lake Tana and other inland water bodies of Ethiopia

Abstract

Free-living nematodes from littoral benthic sediments of four lakes, two rivers and a hot spring in Ethiopia are studied. Populations of nematodes encountered are identified to the species level. The general nematode (generic and species) composition of the lakes, rivers and hot spring are appraised by giving special emphasis to the nematodes from L. Tana, i.e. three sites where different environmental factors come into play were considered. Gelda is a site close to a river inlet (River Gelda), Gedero is a site exposed to strong wind action while Zegie is a secluded site protected by a land mass from any strong wind action. Nematode communities were identified using classification and ordination techniques. The identified communities are then characterised with respect to species composition, biomass, density, diversity, size structure, feeding type and maturity index. Also, the vertical distribution of nematodes is examined at the community and species level. An attempt has been made to associate these variations with the considered environmental factors viz. site, water depth, sediment depth, median grain size and percentage of mud in the sediment. Median grain size and percentage of mud varied over the three sites with the lowest grain size and the highest percentage of mud in Gelda. The incoming river is regarded responsible for the high mud content at this site. The composition of the nematode communities from the lakes, rivers and hot spring by and large was typically freshwater. The highest number of species (40 species) was recorded from L. Tana (a lake relatively intensively sampled) whereas the lowest number was recorded from the saline lake, Abiyata (2 species). Both species in the latter belong to the genus Monhystrella. The complete dominance of species of this genus in L. Abiyata and its presence in the saline lakes and hot spring is argued to be associated with their opportunistic nature and their high temperature dependency and their capacity to withstand high osmotic stress conditions. The role played by salinity in the distribution of nematodes in inland water bodies is considered vital. The occurrence of two species of the genus Udonchus in River Abbay where an outlet of a waste disposal scheme of a textile factory is situated, and their absence in any other site coupled with the habitual presence of one of the two species in mineral waters are used to argue that the genus may be an indicator of some kind of environmental (chemical ?) stress. Five nematode communities are identified in L. Tana, viz. Gelda at 0.5 m, Gelda at 1.5 m, Zegie at 0.5 m, Zegie at 1.5 m and Gedero. The vulnerability of Gedero to strong wind action is maintained to be the main reason for the absence of a significant difference in nematode community at the two water depths at this site. The presence of few dominant species characterised four of the five communities. Factors that account for differences in species composition and abundance are discussed. Local conditions are thought to be important in the different communities in L. Tana. A strategy in niche partitioning is suggested to be the main driving force behind the composition of few most dominant species which (in L. Tana and other reviewed lakes) invariably was a combination of a deposit feeder(s) and an omnivore/predator(s) species. In general, nematode diversity in L. Tana was comparable to that found in oligotrophic and mesotrophic freshwater lakes. Nematode diversity varied over the different communities