Since the quantity of cells recruited depends on the initial abundance of benthic cells and in order to compare recruitment between the different populations, we calculated the percentage of the initial benthic stock that had been recruited over the whole experiment, for each benthic population. To do this, the initial benthic cyanobacteria were isolated from the sediments using the method of Verspagen. Four milliliters of sediment was diluted 10-fold, and silica solution was added .The mixture was then centrifuged at 400 g for 20 min. One milliliters of every supernatant containing cyanobacteria was sonicated to dissociate the colonies, as previously described. Isolated cells were then counted under a microscope at x400 magnification with a Thoma counting chamber.