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La Tour du Valat

Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Mediterranean-type streams supplied from karstic aquifers Ecological and hydrobiological characteristics
Auteurs :	Jean Giudicelli, Auteur ; Georges Olivari, Auteur
Type de document :	Article
Dans :	Ecologia Mediterranea (36(1) 2010)
Article en page(s) :	25-44
Résumé :	Seasonnality and variability in rainfall is the principal attribute of Mediterranean-type climate. As a consequence, Mediterranean-type streams exhibit strong seasonnal variability which can lead to extreme conditions of flooding and drying.Large areas of karst geology occur in the Mediterranean region, consequently large amounts of water may be stored in subterranean aquifers; so, many karstic streams occur in Mediterranean-climate areas. Karstic streams are lotic habitats supplied from underground water in contrast to surface run-off, and this gives them some characteristics quite different from those of other true Mediterranean streams; mainly, water conditions are more regular and uniform.Karstic streams, vary little in flow volume and temperature all year round (the temperature fluctuates by only a few degrees, even in areas with cold winters and warm summers; summer temperatures seldom exceed 20 degrees C). Moreover, water issuing from sedimentary rocks, such as limestones, contains large amount of dissolved salts, notably calcium bicarbonate; these streams are liable to deposit calcium carbonate (tufa concretions).The present paper concerns four karstic watercourses in Provence (South-East France); it relates the influence of the ground water supplying on hydrology, ecology and hydrobiology of streams issuing from limestone formations. The peculiarities of these streams and the specificities of their animal community, analized through the knowledge of their macroinvertebrate fauna, are pointed out.In the studied karstic streams, the invertebrate community is dominated in numbers by taxa (Crustacea Gammarus, Coleoptera Elmidae, Gasteropoda) regularly found in strongly mineralized water-courses with high Ca contents; these taxa have always been found associated. In addition to that, two species of Diptera Simuliids, Simulium xanthinum and S. galloprovinciale, occur on tufa (from Spain to Libanon).One of the most noteworthy faunal peculiarity, common to karstic hydrologic regime watercourses, is the occcurence of a lot of cold-stenothermous invertebrates which usually live at higher altitudes in mountain streams and in springs. Their occurrence in Mediterranean streams at middle and low altitudes is atypic. This peculiarity is correlated with thermal regime of the karstic streams which appear as cold enclaves in the Mediterranean region.The ecology of 11 cold-stenothermous species (1 Planarian, 3 Ephemeroptera, 2 Plecoptera, 4 Trichoptera, 1 Blepharicerid) which are both inhabitants of karstic and of non-karstic streams in the same mediterranean region (South-East France) is compared.The invertebrate community of karstic streams also contains some endemic species. The most worthy of note is Rhyacophila vallisclausae, a caddisfly (Trichoptera) which lives in the upper reach of the river Sorgue. This species, belonging to the European group of R. vulgaris, is clearly distinguished from all species in this group by the strongly marked brachyptery of specimens; it appears to be a relic remnant of an ice age fauna which could survive in an aquatic environment characterized by its remarquable cold stenothermy.

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