THE INTERMEDIATE DISTURBANCE HYPOTHESIS - APPLICATION OF THIS CONCEPTTO THE RESPONSE OF EPILITHON IN A REGULATED MEDITERRANEAN RIVER (LOWER-DURANCE, SOUTHEASTERN FRANCE)

Hydrological regime is responsible for the most significant disturbanc es occurring in the regulated lower part of the River Durance. The pre sent study was an attempt at determining the response of epilithon to the hydrodynamic disturbances occurring in this river. Three patterns of hydrodynamic disturbance resulting from three different types of re gime were analysed. Under low stable compensation flow regimes, distur bance frequency and intensity were considered to be low. Under constan t release flow regimes, disturbance frequency and intensity were consi dered to be severe. High compensation flow regimes with infrequent rel eases were considered intermediate. The validity of the intermediate D isturbance Hypothesis (IDH) was confirmed for the number of algal spec ies and the autotrophic index (an index for estimating the autotrophic versus heterotrophic balance). The results show that when the disturb ance was of intermediate severity, the autotrophic and heterotrophic c ommunities were balanced, which was not the case at either low or high hydrodynamic disturbance levels The results allow to define the habit at of the epilithic algal species. Colonizer and competitor species we re identified on the basis of the IDH. Several obviously ubiquious and resistant species were recorded at all three disturbance levels.