The Effect of perturbations on phytoplankton assemblages in a deep reservoir (Vouglans, France)

Disturbances, such as rainfall and hydrological management of Vouglans reservoir, seem to influence the establishment of the ecological system inside this man-made lake. In this paper, the main effect of these disturbances are changes in the z(mix):z(eu) ratio, with a shift in the composition and phytoplankton dynamics, and consequently in the Shannon's diversity. The seasonal changes found in the surface layers of Vouglans reservoir were typical for mesotrophic and temperate lakes, but some perturbations induced the 'reversion' to an earlier stage in the expected successional pattern during the stratification period. This early-state displayed an intensive proliferation of the filamentous ulotrichal Planctonema lauterbornii, which remained dominant in terms of abundance from the middle to the end of the stratification. The beginning of this period was characterized by the highest value of Shannon diversity index, followed by its marked decrease, whilst z(mix):z(eu) remained around 1. Despite the environmental variations which induced the reversion in this expected pattern, we found some sort of equilibrium, that is, stability in the assemblage simply expressed as a 'no-change situation', although not in the sense of equilibrium inside a competitive system. Moreover, peculiar freshwater red tides occurred in this reservoir involving Rhodomonas lacustris and Peridiniopsis durandi, whose dynamical events were also influenced by external forces.