Quantitative and qualitative variability of epiphytic algae on one Apiaceae (Apium nodiflorum L.) in a karstic river (Southeast of France)

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 = 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 = 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.