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The inland water fishes of Africa

 | 
Didier Paugy
, 
Christian Levêque
, 
Olga Otero

Variability of climate and hydrological systems

Christian Levêque

Résumé

The role of hydrology in the structuring of aquatic ecology has long been well-acknowledged (Statzner & Highler, 1986; Junk et al., 1989; Lowe McConnell, 1987). Of equal importance is the understanding of the history of aquatic environments in order to accurately understand the present composition and structure of communities (Lévêque, 1997a). Ichthyologists therefore have granted much attention to the hydrological conditions of environments inhabited by fish. Hydrological variability resulting from the seasonal distribution of rains or inter-annual variations in precipitation has substantial consequences for the biology and dynamics of fish communities.
Rainfall is distributed very unevenly in Africa (figure 2.1). In parts of the west coast, for example, annual rainfall averages more than 1,250 centimetres. In contrast, more than half of Africa receives less than 50 centimetres) of rainfall yearly. The Sahara and the Namib Desert receive an average of less than 10-25 centimetres a year. In parts of the deserts, rain may not fall for six or seven years in a row. The annual rainfall is normally higher at the equator and diminishes towards the tropics, but nevertheless with local variations that can be linked to relief or to other geomorphologic factors.
The existence and the range of aquatic environments depend mainly on climatic conditions, particularly the link between gains and losses in water during seasonal cycles or in long-term changes at geological time scales. This results in a highly contrasted distribution of aquatic environments, with vast zones (40 to 50% of the continent) in which permanent hydrographical systems are extremely reduced or absent (figure 2.2; see also box “From Mega to Marshy Chad” and chapter The diversity of aquatic environments).

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Extrait du texte

FIGURE 2.1. Distribution of mean annual rainfall (from Balek, 1977).

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FIGURE 2.2. Comparison between the current geographical distribution of river and lacustrine systems (right) with that of the theoretical maxima that these systems attained during wetter periods (left) (from Dürr, 2003).

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Seasonal climatic variations and river discharge

Climatic conditions depend on the circulation of air currents driven by the energy budget of the earth-atmosphere system. The equator is a zone of updraft in which air currents cool during their rise and then descend at the subtropical anticyclone level. These in turn re-converge around the equator forming trade winds. The meteorological equator, or convergence zone of the trade winds, does not coincide with the exact position of the geographical equator, and is situated about 5°N during the boreal winter and about 10°N during the boreal summer.

During the year, the distribution of rainfall at different latitudes is uneven. At the equator it rains throug...

Auteur

Research director at IRD. He has published numerous works on the systematics, distribution, and ecology of West African fresh water fishes, and expanded the collections of the MNHN in Paris.

© IRD Éditions, 2017

Conditions d’utilisation : http://www.openedition.org/6540

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