Pluriannual morphological variability of phytoplankton in a highly productive Mediterranean reservoir (Lake Arancio, Southwestern Sicily)

The progressive decrease in water quality experienced by the Mediterranean, nutrient rich reservoir, Lake Arancio (Southwestern Sicily, Italy), has been accompanied by modifications in both phytoplankton shape and size. Since nutrient concentrations are always far from being limiting, in the present paper attention was focussed on the effects exerted by physical constraints (e.g., mixing, light availability) on the morphological variation observed in phytoplankton. The surface-volume ratio and its product with the maximal linear dimension were chosen as morphological descriptors to trace the trajectories followed by the dominant morphotypes in Lake Arancio. One of these descriptors was compared to the mixing depth-euphotic depth ratio, considered as a good descriptor of the underwater light climate. The strong influence exerted by light transmission and mixing regime on phytoplankton morphological traits and thus on its composition in this environment confirms the paramount importance of both light transmission and the peculiar hydrological regime to which reservoirs are subjected, especially in Mediterranean climate, and points out the need to integrate hydrology into ecological studies and management procedures.