Two sampling campaigns were carried out in the deepest part of the
Grangent reservoir (45 m depth): the first in November 2005, when three 50 m sediment cores were collected, and the first 2 cm corresponding to the surface layer were sampled and used to investigate the recently sediment cyanobacterial population of the year(within the last 3 months), the second in may 2006, when three 2m long sediment cores were collected. The dating of the sediment, previously performed using radioactive isotopes, allowed us to collect two Microcystis benthic populations by sampling sedimen layers containing Microcystis colonies originating from the blooms of either 2003 or 2005. During both campaigns, the cores were brought back to the lab, opened immediately and the sediment samples were stored at 4C in the dark for a few days, until the experiment began. Fifty liters of surface water was also sampled,filtered on 1.2 nanometer glass fiber filter and then stored at 4C in darkness until processed.