Sur la répartition spatiale des Crustacés dans le sous-écoulement d'un ruisseau des Pyrénées

Abstract

The crustacean assemblages of ruisseau de Lachein (Pyrénées) underflow were studied on a site of 75 m2 at a depth of 0,6 m. Four main zones are created by the surface flow, i.e. two gravel banks, a channel and a stagnant water zone. The porosity (20 - 30 %) is higher in the gravel banks than in the channel. It is also higher upstream than dowstream. The grain size differs in relation to the four zones, the gravel banks possessing a larger amount of elements measuring 1 to 8 mm.The underflow and the brook have close chemical characteristics varying in the same manner through the season. Oxygen levels are close to saturation in the water surface, but are between 0,02 % and 84,7 % in the underflow where their values are directly affected by the influence of the surface flow. The proportion of organic matter (4 - 13,6 %) follows a seasonal pattern, the lowest mean ratio being that of the channel.Among the 50 sp. and sub. sp. of Crustacea in the underflow, 22 are stygobiont (hypogeous) taxa. These taxa show a strong faunistic similarity with the community of the Baget karstic system from which the ruisseau de Lachein originates. This fauna is characterized by a stable organisation throughout the entire year and is dominated by Copepoda, particularly Harpacticoida representing 60 % of the population. The average density is 369 animals/10 l. The Malacostraca and the Harpacticoida maintain a variable spatial distribution with eurytope and stenotope species. Among stenotope species, some taxa are restricted to the region with the highest amount of O2, whereas others are limited to weakly oxygenated areas.The different zones defined by the surface flow have a direct influence on the density as well as on the percentage of harpacticoid hypogeous forms. © Masson, 1988.