The impact of transfers on water quality and the disturbance regime in a reservoir.

The effect of water transfers between two reservoirs on the water quality of the receiving reservoir was investigated over a 9-year period (2000-2008). Different management strategies were implemented in term of the magnitude and timing of water transfers, i.e. the amount of transferred volume and the frequency at which transfers occurred. These different operational modes were analysed to determine changes in nutrient and metal concentrations, chlorophyll a, algal genera and biovolume. During high water transfers, chlorophyll a and total algal biovolume increased, with larger diatoms preferentially selected due to the high silica content of the pumped inflow and a significant shift in cyanobacteria genera occurring from Microcystis to nitrogen-fixing genera. The magnitude and timing of water transfers exerted a strong control on phytoplankton competition and disturbed the typical seasonal succession during low pumping years of a spring diatom bloom followed by summer cyanobacteria dominance: intensive and frequent water transfers resulted in dominance by diatoms for the whole year and effectively limited cyanobacteria summer growth. From this analysis, we identified iron concentration and diatom biovolume as the key water quality indicators to be included in any optimal management, able to control the transfer regime from both a water quantity and water quality prospective.