What do algal blooms do?
Green algal blooms produce very large changes in oxygen concentration in the surface water over 24 hours. Plants produce oxygen by photosynthesis in excess of its use for their respiration (breathing, like us) during the daylight hours, but at night they just respire. So during mid-afternoon dam surface water becomes supersaturated with oxygen from an algal bloom, but overnight the oxygen falls to a very low level by sunrise. Some algae are present in even the most turbid dams. At very high nutrient levels (polluted dams), green algae give way to blue-green algae which produce toxins. Some blue-green algae can move up and down in the water with light change; at sunrise they can be seen momentarily as a dense, dark matt at the surface, which rapidly disappears as the sun comes up. You’ve probably read about all this in relation to Peel-Harvey and the suburban lakes in Perth, not to mention the Darling River going blue-green in early 1993. Nutrient enrichment is also called “eutrophicati
