XIVth International Symposium on Chironomidae - Rio de Janeiro, August 29th-September 2th, 2000.

Oral presentation : Biodiversity pattern in two strongly reduced summer pools from the large sandy river Loire system (France). Garcia XF, Laville H.

Abstract

The sandy River Loire, in its six order section, shows a wide range of waterbodies related to the main channel.

The Chironomid populations of two oxbows have been sampled monthly in 1996 with additional fortnightly samples from April to August, using both pupal skin collections and benthic collections of pupae.

Firstly, the species composition and population structure of these oxbows appear to be influenced by the winter and spring connection pattern to the main channel.

In contrast, from June to September, the oxbows are completely isolated from the main channel and show a strong reduction in size over a few weeks. The resulting severe conditions are demonstrated by high values of conductivity and alkalinity, revealing a chronic excess of carbon dioxide, as well as a decrease of oxygen content.

Surprisingly, the biodiversity, strongly influenced by rare species, is higher under these extreme conditions.

The faunal composition of main channel and oxbow populations shows a great number of similar species during the period of isolation. This, underlines two aspects of the ecology of the large river Loire system : the real potamic status of its main channel which offers viable conditions for lentic species and the colonising exchanges between the fluvial landscape components facilitated by partially similar ecological conditions.

Nevertheless, the biodiversity of the oxbows also supports many characteristic species.

The study of the micro-distribution of their populations reveals a poor diversification of the habitat, mainly roots and muddy sand, with greatest diversity amongst the tree's roots on the eroded banks. The higher biodiversity encountered under these drastic ecological conditions relative to the more diversifying ecological conditions of the main channel cannot be attributed to habitat heterogeneity.

Using the b diversity index, we note that the turnover of species is higher in the reduced waterbodies.

The discussion deals with the role played by a dynamic colonisation in the increase of biodiversity and deals with the factors responsible. The necessary meta-population like-functioning of the rare species populations implicated in the high colonising dynamic is also discussed.

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