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The inland water fishes of Africa

 | 
Didier Paugy
, 
Christian Levêque
, 
Olga Otero

The diversity of aquatic environments

Jacques Lemoalle

Résumé

The long-term survival of a fish species in an aquatic system depends on whether that system provides conditions that favour, among other requirements, the species’ growth and reproduction. These conditions are grouped into two categories:
• the physico-chemical (or abiotic) environment deriving from the geological context and the climatic conditions as well as the aquatic landscape as perceived by the fish;
• the trophic (biotic) environment defined by the interactions between species, notably the predator-prey relationships during the successive stages of the fish development.
These habitat characteristics must be considered in both their spatial and temporal dynamics: the survival of a cohort or population depends on the synchronization between the needs of an eco-phase and the characteristics of its environment (Lévêque, 1995a).
The physico-chemical quality of both lotic and lentic aquatic systems is indirectly but rather precisely regulated by their geographic location. This takes place through complex interactions of different environmental variables (figure 1.1). The abiotic conditions (seasonality and spatial distribution of turbulence, oxygen, temperature, salinity and transparency, habitat structure) that have a direct influence on fish communities depend on two constraints: the local climatic conditions and the geology of the basin. In general, an aquatic system is a component of a landscape, which itself depends on the same two constraints. It is within these interactions between terrestrial landscapes and the aquatic environment that we will discuss, in brief, the variety of African environments, first by dividing Africa into two large regions, and then reviewing the main characteristics of the aquatic systems encountered in each region.

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Extrait du texte

FIGURE 1.1. An example of a network of environmental abiotic characteristics that determine the quality of a lacustrine system. The proximal variables (bottom line of the figure) have a more direct action on the composition of communities.

Image

The role of physico-chemical variables: temperature and salinity

The water temperature of rivers and shallow lakes are normally close to the mean air temperature, in contrast with deeper lakes where temperature is unevenly distributed over depth in the water column. The diurnal and seasonal solar irradiance cycle variations result in delayed cycles of temperature in the water column and alternation of stratification and mixing.

The main factors that determine the temperature of water are latitude and altitude, with a local influence from atmospheric circulation. The latter explains why climate is often more variable than expected in equatorial regions of Africa where seasonality is expected to be minimal. The temperatures observed in different shallo...

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