Coral-to-macroalgae phase shift

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The transition of a reef from a state dominated by living coral to one dominated by fleshy macroalgae, and — the part that makes it interesting — its tendency not to reverse when the original disturbance is removed. Fleshy algae occupy bare substrate quickly after a mortality event, and once established they inhibit coral recruitment directly by shading and abrasion and indirectly by altering the microbial and chemical environment that settling larvae respond to. The shift is therefore not merely a change of dominance but a change of the reef's response to future conditions: the same reef, in the same water, now recruits coral more slowly than it did. Grazing pressure is the principal thing that stops it. The Caribbean after the 1983-84 urchin die-off is the canonical case; the general lesson is that reef recovery depends on a herbivore community that is usually invisible in accounts of bleaching.

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