For my first fifty dives a reef was a wall of colour. Somewhere after that it started resolving into parts that need each other, and I have not been able to unsee it since.

The dependency chain

Coral is an animal hosting an algae. The algae photosynthesises and feeds the animal; the animal gives it shelter and nitrogen. Warm the water a few degrees for a few weeks and that arrangement breaks — the coral expels the algae, loses its colour, and starts starving. Bleaching is not a disease. It is a partnership ending.

Above that sits everything else. Parrotfish graze the algae that would otherwise smother young coral. Groupers keep the grazers in check. Remove the big predators and the grazers boom, then crash, and the algae wins. The reef does not fail at the coral first; it fails wherever you pulled a piece out.

What you can see on one dive

• Rubble versus structure. Broken flat rubble means something killed the framework — a storm, a bleaching event, dynamite. Living reefs are three-dimensional because that is what growth looks like.
• Fish size, not fish count. Plenty of small fish and no large ones is the signature of a fished reef, and it looks abundant to a visitor.
• Algae on hard surfaces. Fuzzy green over dead coral means the grazers are gone or overwhelmed.

Why the systems framing helps

Because it stops you reading a reef as pretty or not pretty. A brown, low-visibility, unphotogenic reef with big fish and complex structure is in far better health than a bright one with none. I spend my working life looking at systems where the visible symptom is a long way from the cause, and this turned out to be the same discipline in warmer water.