Ocean: Reflections on a Century of Exploration

4. Of Coral Reefs and Atolls: Stone Gardens of Tropical Seas

Why Study Reefs? Reefs under Siege Life in the Reef Rates of Growth Questions about Origins Origin of the Great Barrier Reef Sunken Reefs of the Mid-Pacific Reef-forming stony corals potentially can grow everywhere in the tropical and subtropical realm of the planet—about one-third of its surface —where the water is warm throughout the year (greater than 20 ⬚C). For stony corals to grow, the seafloor has to be shallow and the ground firm. Also, the water has to be clear, so that plenty of sunlight reaches the seafloor. The reason is that the coral animals bear inside their bodies, within the outermost cells, photosynthesizing microscopic algae (symbiotic dinoflagellates called zooxanthellae), which need light. This association between animals and green microbes (which gives most corals olive green and olive brown colors) is one of the most impressive symbiotic arrangements on the planet.1 It is one reason why many corals tend to look like plants (fig. 4.1). The symbiosis allows corals to flourish in the nutrient-starved warm-water deserts of the sea, by setting up ecosystems where animals (corals) capture nutrients through predation and recycle them with minimum loss to captive photosynthesizing organisms (dinoflagellates). The coraldino flagellate symbiosis makes the reefs and, thus, provides the basis for a proliferation of animals and plants that live within these stony gardens .2 Reefs are living oases in a vast desert. Tropical reefs are striking centers of marine diversity, across all types of reef organisms, led off by a great diversity in “coral” itself (fig. 4.2) and extending to fishes, snails, and other mollusks , crustaceans, echinoderms, bryozoans, foraminiferans, and many types of coralline algae.3 Reefs, as ecosystems, are in serious trouble in many parts of the world, wherever there is strong impact from human activities. Because of the requirement for shallow water, most reefs are next to land and are readily accessible to commercial exploitation, mainly fishing, but also collecting of corals and mollusks for the shell market. The removal of large fishes benefits the stony 95 FOUR Of Coral Reefs and Atolls STONE GARDENS OF TROPICAL SEAS coral’s competitors for space: algae, sponges, and soft coral. With fewer predators checking their expansion, they tend to take over. Runoff from agriculture, soil erosion, and sewage bring nutrients into the system, and this removes the competitive advantage of corals in a nutrient-starved environment. Algae, sponges, and tunicates take advantage. With a dwindling cover of coral, a reef system becomes degraded and finally “collapses,” that is, it changes irrevocably into a new and much less diverse system.4 Another, more general threat to the health of coral reefs is coral bleaching, that is, the loss of photosynthesizing symbionts, which accompanies unusually high water temperatures. Bleaching events apparently have become more abundant and more widespread with the progressive global warming that characterizes climate change for the last quarter century. The process is not well understood. Invasive species, brought from far away in bilge waters, are a problem in some regions. Stony corals are the masons of the sea; they build enormous cities that offer food, shelter, substrate, and diversity of habitat. They have done so for many millions of years, and the 96 O F C O R A L R E E F S A N D AT O L L S Corallium "Madrepora" Millepora FIGURE 4.1. Many types of corals are shaped much like plants on land. FIGURE 4.2. Different types of organisms called “coral.” The gem-quality red coral named Corallium rubrum (by Linné), a reef-building stone coral (called Madrepora by Haeckel), a stony hydrozoan known as “fire coral” because of its painful sting upon touching it. remains of ancient coral cities are now found in mountain belts wherever the shallow deposits of warm seas partook in the collision of great plates on the surface of Earth, to make new land. Thus, a hike in the Alps may well include a picnic on a reef structure made in the Mesozoic! Some continents have large accumulations of reef rubble off their tropical shores. The biggest of these accumulations is off northeastern Australia and forms the foundations for the Great Barrier Reef. Tropical islands extend their girth by acquiring a thick mantle of coral rock, bearing active fringing reefs and barrier reefs in the sunlit zone at the top of the volcanic structure (fig. 4...