The influence of external perturbations on the functional composition of phytoplankton in a Mediterranean reservoir

The changes in abundance and composition experienced by phytoplankton communities in lakes and reservoirs occur in response to variations in the physical (light climate or energy) and the chemical (nutrient availability or resources) constraints for algal growth. Mediterranean reservoirs are very dynamic systems, subject to frequent changes in the physical environment as a result of water management operations, which suggests that phytoplankton communities might also undergo frequent changes. The phytoplankton community composition, abundance and seasonal dynamics of El Gergal, a medium-size Mediterranean reservoir, is analyzed and interpreted in terms of changes in the nutrient-energy balance. It is demonstrated that the seasonal scale changes in the physical environment trigger the seasonal predictable autogenic dynamics of the phytoplankton community. In addition, frequent short-term external perturbations of the physical environment may also induce allogenic shifts and reversions in the succession. The physical changes occur mainly as a result of variations in the outflows. Results are discussed in terms of phytoplankton functional groups life cycle strategies and water quality management.