They are called black reefs. They are a marine vision of Tolkien’s Mordor, and they happen when a boatload of iron lands in an ecosystem that is generally poor in nutrients. The iron acts as fertiliser for fleshy algae, which grow so vigorously that even grazing fish can’t trim them back fast enough. The algae then trigger Rohwer’s cycle: more DOC, more microbes, more pathogens, more disease, more dead corals.
Boats kill coral
