Comparative ecology of Chironomid populations from potamal zones of the rivers Garonne and Loire (France)

Heterogeneity of large rivers and biodiversity


The biodiversity of a potamal section (<1 km) of two French rivers - the middle Loire and the Garonne in its overflowing part - was studied and compared according to its three components identified on hydrosystem level : structural, functional and compositional components. The first two come under the spatio-temporal heterogeneity of the river and the third comes under specific diversity.

Collection of Chironomids was carried out using two different complementary sampling methods and on two different spatial scales, namely, collection of pupal exuviae with a drift net (site scale) and collection of pupae with a Surber net (micro-habitat scale). This enables 187 species to be recorded, 144 on the Loire and 137 on the Garonne. Amongst these, 23 mostly potamophilous, are new to the latest list of French Diptera Chironomids established by Laville & Serra-Tosio (1996). Such a wide faunistic richness highlights the taxonomic interest of sampling the potamal zones of large European rivers.

The spatio-temporal dynamics of the environmental parameters and Chironomid communities of the two rivers were studied separately in order to identify the characteristics of each river, as much mesologically as faunistically.

The fluvial dynamics associated to the degree of submersion of side zones (duration and frequency) are the driving force behind the structural and functional biodiversity of the hydrosystems. The heterogeneity of the river results from the synergy between these two components of biodiversity. This synergy is expressed on all time and space scales (section, site and micro-habitat).

The compositional biodiversity or specific diversity of Chironomids is undoubtedly conditioned by the heterogeneity of the river but shows varied and complex patterns of expression which are dependant on the scale considered. These patterns are subject to the geomorphological particularities of each hydrosystem as we have seen in the comparison carried out between the Loire and the Garonne. The high specific diversities observed in the sections studied both in the Loire and in the Garonne result from different mechanisms which are not necessarily exclusive :

- in the Loire, specific diversity is generated by the dynamics of the connection between the main channel and the side channels. Specific diversity is thus mostly generated by the functional biodiversity of the hydrosystem

- in the Garonne, specific diversity is generated by the juxtaposition of habitats with marked typological characteristics. Specific diversity is thus mostly generated by the structural biodiversity of the hydrosystem.

These differences, at section level, can equally be observed on site and micro-habitat level.

The study of chironomid communities on a micro-habitat scale highlights the ability of most potamophilous chironomids larvae to use the vast range of environmental resources, as well as their ability to adapt to particular situations such as sudden variations in discharge.

Board of examiners                                                                                                                                                                                      
P. Auriol, Professor, University of Toulouse - France Foreman
J.P. Berton, Professor, University of Tours - France Examiner
W.P. Coffman, Professor, University of Pittsburg - USA First examiner
P. Langton, Affiliated researcher, University Museum of Zoology, Cambridge - UK           Examiner
H. Laville, Researcher, CNRS, Toulouse - France Supervisor
N. Prat, Professor, University of Barcelona - Spain First examiner
B. Rossaro, Profesor, University of Milano - Italy First examiner
J.N. Tourenq, Researcher, CNRS, Toulouse - France Examiner