An approach to the Intermediate Disturbance Hypothesis at the landscape scale:: the effects of hydrodynamic disturbance on phytoplankton communities

The River Durance and its main tributary, the Verdon, are two highly regulated rivers in south-eastern France. The course of both rivers is interrupted by a series of reservoirs with quite different hydrodynamic characteristics. Here the effects of a human-induced hydrodynamic disturbance on the phytoplankton community were studied for the first time at the landscape scale. The level of hydrodynamic disturbance occurring in these regulated rivers varies depending on the landscape unit under investigation, the depth and area of the reservoirs and on how the systems are managed. A statistical analysis showed that the level of hydrodynamic disturbance in each reservoir could be evaluated by the residence time. At the community level, the Intermediate Disturbance Hypothesis (IDH) predicts maximum diversity at an intermediate disturbance frequency and intensity. The validity of the IDH was confirmed by the number of algal species and the total number of cells: the community with the largest numbers was found to be associated with intermediate disturbance levels. The composition of the algal community showed conspicuous variations along the hydrodynamic disturbance gradient and explanations for these variations are proposed.