Spatio-temporal variation of the structure of the ephemeropterofauna in Sejenane stream (Ichkeul, northern Tunisia)
The spatio-temporal variation in the community structure of Ephemeroptera was studied over a period seven of months (from May to November 2003) at four sampling stations along the Sejenane stream (tributary of the lake Ichkeul, northern Tunisia). The Ephemeroptera preferentially populate the stations located upstream of the Sejenane dam, due to the lower mineralization, better water oxygenation, coarser substrate texture and a relatively faster current that characterize these waters. The structure of the Ephemeroptera population is sharply unbalanced in favour of a single species, Caenis luctuosa, which is characterized by a marked phenotypic plasticity; it is the most prolific, the most plentiful and the most frequent species at the four studied stations. The spatial distribution of the Ephemeroptera is directly conditioned by the quantity of dissolved salts. From upstream to downstream, the population structure changes markedly, presumably in response to the rise in the salinity of the water. Analysis of the dendrogramme of interbiotope similarity separated the four stations into two biotopes. However, the affinity between these two biotopes is high (48%), suggesting that the Sejenane dam does not represent a strong ecological barrier (Ij > 33%) for Ephemeroptera, despite the fact that it separates two stream reaches that differ markedly at the morpho-dynamic and physico-chemical levels.