Tropical waters tend to be low in nutrients, like nitrogen and phosphorus, which are crucial to most forms of life. (This has to do with what’s called the thermal structure of the water column, and it’s why tropical waters are often so beautifully clear.) As a consequence, the seas in the tropics should be barren—the aqueous equivalent of deserts. Reefs are thus not just underwater rainforests; they are rainforests in a marine Sahara.
