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The nature and origin of the crustacean zooplankton of Sahelian Africa, with a note on the Limnomedusae
In: Hydrobiologia. Springer: The Hague. ISSN 0018-8158; e-ISSN 1573-5117, meer
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tropical zooplankton Cladocera Calanoida Cyclopoida Limnomedusa Sahel Pleistocene climate change biogeography |
| Auteurs | | Top |
- Dumont, H.J., meer
- Verheye, H.M.
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| Abstract |
The zooplankton of the major Sahel river basins Nile, Shari (Chad), Niger, and Senegal, is different from that found in the Sahara and in Equatorial Africa. Similarities and differences between the individual basins are numerous as well. Many species are shared by the Nile and Lake Chad, by Lake Chad and the Niger (plus Senegal), or occur in all four basins, or are restricted (endemic) to only one basin.These patterns are identical to patterns found in fish, molluscs, and macrophytes and show that crustacean zooplankton obeys the same laws of dispersal as these groups, in spite of its apparent preadaptation to passive dispersal.The patterns can be explained by the climatic fluctuations of the Upper Pleistocene and Holocene. Following a general dry period over Africa between 20 000 and 13 000 BP, high river and lake levels prevailed between 12 500 and 8 400 BP. This was the period of maximum faunal interchange between all basins, and even with the Zaire basin. After a regression (8 000–7 000 BP), wetter conditions returned around 6 000 BP, but the Sahel itself remained dry, although its rivers and lakes, fed by waters of southern origin, showed higher levels than today. They flooded large areas of the southern Sahara, permitting aquatic animals and plants to reach the Adrar of Mauretania, the Tibesti, and the Ennedi mountains.Since 3 000 BP, present day conditions developed. This last period is characterized by species extinctions, as exemplified by the droughts in Lake Chad in historical times, and in spite of the tremendous diversity still extant here today. Between 6 000 BP and the present, however, very little speciation took place, and faunal exchange between basins was very limited. |
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