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Spatial distribution of the firmly attached epiphytic algae was investigated directly on the leaves of one Apiaceae (Apium nodiflorum L.) commonly found in chalky Mediterranean rivers, at two sampling dates (summer and autumn). The composition and structure of the epiphytic communities of the samples were determined at different levels: along the leaves, on both their sides and within each leaflet (n ¼ 13). Forty-eight leaflets from four randomly selected plants were cut along the longitudinal and vertical axes so as to define 16 microhabitats located on the edge, the centre and the vein. Only a few species of algae were found to be constantly present whatever the sampling date, and dominated all microzones (n ¼ 768). Their presence seemed to depend, rather, on the architectural structure of the plant. The other algae represented the major part of the epiphytic community and were only occasionally present. Those were more subject to temporal and environmental variations and less specific of the substrate. The epiphytes showed considerable spatial heterogeneity within their microhabitats, with an evident preference for the centre and the edge of the leaves, whereas two inhospitable spaces were the vein and apex. On the other hand, when the study was performed at a larger scale (along the leaf), the epiphytic populations of the youngest leaves (near the surface) and the eldest ones (close to the sediments) were homogenous, hence suggesting that other factors, independent from the physiology of the plant, may have an impact on epiphytic distribution.

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Une étude de la distribution spatiale épiphytique sur une plante aquatique commune des sources karstiques méditerranéennes a été effectuée à l'échelle du microhabitat, sur différentes feuilles & Apium nodiflorum. Des découpages longitudinaux et transversaux sur toute la longueur du foliole ont été réalisés et analysés séparément.Hormis de fortes variations liées à chaque individu macrophytique, les premiers résultats, obtenus sur 48 folioles, prélevés en automne et en été, révèlent l'existence d'une microrépartition effective et non stochastique des epiphytes sur les différentes parties architecturales de la plante-hôte, ainsi qu'une homogénéité des communautés algales sur la hauteur totale de la plante.Cette approche méthodologique a permis de mettre en évidence des microzones représentatives de la richesse spécifique et de la biomasse totale du peuplement épiphytique. Cette approche pourrait faire l'objet d'études élargies à d'autres macrophytes et être utilisée par les limnologues comme outil d'observation du peuplement épiphytique en rivières. Methodological approach of a spatial distribution pattern of epiphytic algae on Apium nodiflorum leaves (L.) Lag. (Apiaceae)Keywords : Epiphytic algae, spatial distribution, methodological approach, Apium nodiflorum.A microhabitat scale study of spatial epiphytic distribution of a common Apiaceae of karstic mediterranean springs has been carried out, on the leaf system of Apium nodiflorum. Longitudinal and transverse zones along the leaf have been determined and separatetly analysed.In spite of a physiologic variability inherent in each sample, initial results, based on a 48 leaflets sample, for each stage, reveal a non-random distribution of epiphytes on different architectural elements of the host-plant. A homogeneity of algal communities is noted on the whole plant height from surface to base. The several scales necessary for this methodological study clearly show existence of microzones which are quantitatively and qualitatively representative for the whole epiphytic populations.These results, followed by later investigations on different plants must be useful as a technical approach for scientists and for water administrators in the river monitoring.