Every supernatant containing recruited cyanobacteria was fixed in 1 persent glutaraldehyde. Rapid disruption(< 1 min) of the colonial structure was produced by low-power ultrasonic vibration (20 kHz). Isolated cells were then counted in triplicate under a microscope at x400 magnification using a Thoma counting chamber. The basal area of the microcosm and the time that had elapsed since the last sampling were then used to calculate a recruitment rate per day and per square meter.