EISORS - Toulouse, July 17th-21th, 2000.

Poster : Heterogeneity of the large river systems and biodiversity. A case study on a six order section of the Garonne River. Garcia XF, Laville H.


The biodiversity pattern on a 6th order section of the Garonne River was studied according to the modern concept of the biodiversity of large hydrosystems. This break down it into three components : two related to the spatio-temporal heterogeneity of the system - structural and functional biodiversities - and one related to its species richness as a result of the two first - compositional biodiversity.

Seventeen environmental parameters as well as chironomid populations have been studied during twelve months (1996-1997) in four sites of the Garonne river : the main channel and three sites located on side channels differently connected to the main channel. Three observation scales of the diversity patterns have been taking into account : the site scale using collections of drifting pupal exuviae and the micro-habitat scale using collections of benthic pupae, the section scale being the integrative observation scale.

A total of 17 747 pupal skins and 4 019 pupae were sorted. 137 species or taxa were identified, in which 14 are new records for the French fauna.

The structural biodiversity of the Garonne ecosystem is strongly linked both to the isolation of morphologically particular sites due to the embankment of the main channel and to the seasonal processes (functional biodiversity) generated in the connected sites by the fluvial dynamic. Then it appears that the specific diversity at the section and at the site scales is influenced both by the spatial heterogeneity of sites and by their level of disturbance related to their degree of connectivity.

At the micro-habitat scale, the influence of the structural and functional biodiversities on the species richness is conceptually similar, but express differently as shown by the highest chironomid diversity due to the vernal development of Callitriche sp in the oxbow.

This case study emphasised on the complex scale (space and time) of the biodiversity pattern as well as the ability of the chironomid populations to adapt themselves to the rapidity of the events.