March 14, 2021
… grazing for healthy reefs
The plant-like things that grow underwater are often lumped together as “seaweed”. However, the sea grasses (turtle grass and manatee grass) have true roots, an internal circulation system, flowers and seeds. Algae are simpler, and there’s a staggering variety of them. On Jamaica’s reefs, there are also too many of them and they are smothering the bits of live coral we have left.
Some algae are just single cells, almost too small to see. Encrusting algae use minerals from sea water to form a sort of mortar that binds them together. Macroalgae are considerably bigger and look more like plants. Most of the varieties overgrowing our reefs are fleshy or filamentous (hair-like) green and brown algae and sargassum, which tears loose in the autumn storms and floats on the surface in dense mats.
The seaweeds make two important contributions to life in the sea. First, they use sunlight and the nutrients and carbon dioxide in the water to build and feed their bodies, and give off the oxygen that fish and other marine creatures need. Second, they provide food for a wide range of herbivorous (vegetable-eating) creatures, from tiny shrimp to half-ton manatees.
Each of these creatures eats only a few kinds of algae, so it takes a good variety of grazers to keep the reefs clean. Most parrotfish, for example, are primarily scrapers, eating encrusting algae and leaving a clean, bare surface where new corals can get established. (In the process, they produce enormous amounts of new sand to build beaches.) Crabs, shrimp and snails are very effective at cleaning the slimy and turf-like algae that creep onto both live coral and sea grass.
The surgeonfish family (surgeons, doctors and tangs) and the angels and damselfishes are true grazers, eating fleshy and filamentous algae. Sea turtles prefer grass but both green and hawksbill turtles have been regularly observed eating fleshy algae. A damselfish who finds a good patch of his favourite food will defend it against other fish and “weed” it, pulling out and dumping anything else that tries to grow there.
Long-spine (diadema) sea urchins will eat almost all kinds of soft-bodied algae. They’ve been described as “marine lawnmowers”, and their mass die-off in the early 1980’s was a serious blow to Caribbean reefs. Over-fishing had wiped out most of the parrots and other grazing fish. Other urchin species survived the die-off but didn’t take advantage of the increase in available grazing.
Diadema populations are recovering, and in some places have almost regained their former levels. However, problems remain. First, these urchins graze like goats, stripping an area completely bare. Without enough big snappers, groupers and triggerfish to keep their numbers under control they can create an “urchin desert” with no food left for anybody and not much oxygen in the water. Also, the clean ocean bottom that grazing creates won’t help to restore the reef if there are not enough live corals spawning in the area.
A healthy reef needs enough cleaners, scrapers and grazers to keep algae from taking over. Variety is important. A 2008 Caribbean study found that areas with several kinds of grazers increased their coral cover by 20% over ten months, while areas with only one species or none at all lost 30%. Big fish are important – they make more efficient lawnmowers and they’re safer from predators. And it’s vital to have enough predators to keep the grazers from wiping out the algae completely.
More kinds of fish, bigger fish and more live corals are key objectives of protected areas like the Marine Park. Studies of the Exuma Cays reserve in the Bahamas suggest that protection works. Even with a lot more top-level predators in the protected area, increased grazing (especially by large parrotfish) allowed coral cover to increase twice as fast inside the reserve as it did in nearby areas.
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