Fig. 3. Hypoxic-preconditioning on a coral reef platform like that of Heron Island. (A) At very low tides, the water on the platform gets cut off from the surrounding ocean, essentially forming a very large tide pool. If this happens at night, the respiration of the reef organisms will make the water hypoxic, particularly on calm nights with little wave action. (B) The tide chart shows a period where the tides become lower and lower over a few days. As a result, the time that the water on the reef platform is disconnected from the ocean will increase in length for each subsequent night, causing the nocturnal hypoxic episodes to become longer and longer and therefore increasingly severe. Such `natural preconditioning periods' occur once or twice per month.