METHODICAL APPROACH TO DISTRIBUTION OF EPILITHIC AND DRIFTING ALGAE COMMUNITIES IN A FRENCH SUB-ALPINE RIVER - INFERENCES ON WATER-QUALITY ASSESSMENT

In a sub-alpine river, the Asse, with an unpredictable discharge regim e, chlorophyll, density and taxonomic diversity of both drifting and p eriphytic communities were studied from a methodical approach. The inv estigations based on stamping two close cross-sections to know their s patial distributions in a heterogeneous bed substrate showed a heterog eneous pattern of colonization mainly influenced by hydrological varia bles as the current velocity, the size of the substrata (gravel-pebble -boulder). For the drifting algae, mainly composed with benthic specie s temporarily unhooked to the substrata, the heterogeneity of the dist ribution was reduced in fast flow which induced a mixing of the whole mass of water. For the periphyton, some results indicated that the gre atest diversity and the highest chlorophyll concentration were found o n pebble substratum where the current velocity was moderate. These res ults have also shown that the classical methods used to day for invest igations were inadequate for most Mediterranean rivers. It was really difficult to estimate the water quality from classical index created f or use in regular large plain rivers.