Chapter 1: Reef Life
From Microcosm Aquarium Explorer
Reef Origins
Oases in a Watery Desert
My first impression was of having landed in a scene of utter destruction or perhaps a place that had never seen life. The underwater site was strewn with cracked boulders that might have been cast down in fury from the heavens. Rusty brown deposits of iron oxide had settled on every exposed surface and filled every crack. Trapped gases escaped fissures in the massive rocks, sending bits of debris swirling, and bubbled slowly toward the surface. No living thing came into view to lend a sense of earthliness to the scene. We could have been on Mars, so otherworldly was the setting.
In reality, we were diving on an underwater volcano in a remote area of Indonesia. As I explored further, my eyes gradually became accustomed to the monochromatic scene and began searching the sediment for traces of the reef organisms that usually proliferate in such shallow waters. I didn’t have to go far. Amid this desolation, the first signs of stirring life were everywhere—a variety of algae, first links in the ocean’s food chains, were thriving under a layer of volcanic detritus. On the algae were tiny sea stars, nudibranchs, worms, and other invertebrates. Random clumps of sponges and corals managed to survive despite the adverse conditions. We were witnessing a new reef in the making—life forms assembling themselves in ways that have astonished and awed humans throughout history.
Coral Reef Paradox
To ancient mariners, coral reefs were deadly hazards where ships foundered and souls were lost. To tropical islanders, reefs are life itself, giving safe harbor from the violent seas and yielding a ready harvest of fishes, crustaceans, mollusks, and other readily caught and collected foods and materials. To early scientists, corals and the reefs they formed were a profound bafflement.
Thriving only in warm, clear tropical waters, reefs and their proliferation of life forms seem to defy the usual rules of nature. In other realms, plants and animals congregate in areas with obvious sources of fertility, such as good soils, nutrient-rich rivers and streams, and adequate light. In the shallow, pristine waters where coral reefs occur, nutrient levels are so low as to be virtually undetectable. How can this be?
For centuries, humankind grappled with this paradox. Corals were initially classified as minerals, because of their stony skeletons, then plants, and finally as the animals that they are. It was the discovery in 1881 of zooxanthellae, symbiotic algae cells found living in the flesh of corals and other reef animals, that led to the ongoing unravelling of the mystery.
Reduced to elemental simplicity, the single-celled algae proliferate in the sun-drenched conditions of the tropical shallows, deriving their primary energy from solar radiation and finding protective shelter in the tissues of various reef animals. Synthesizing an excess of energy-rich products, the algae ooze nutrients into the flesh of their host corals, which use this energy to drive a series of complex reactions in which calcium is extracted from seawater and deposited as hardened coral skeletons.
What Is A Reef
What exactly is a coral reef? To the geologist, a coral reef is a mass or ridge of calcium carbonate that has been built up over time by the secretions of stony corals and other aquatic organisms, including calcareous algae and shelled mollusks. Over time, the rocky foundation of the reef is fused into a stony mass that may be hundreds of feet thick. In the case of the Great Barrier Reef, the biological construction extends some 1,900 km (1,140 mi.) in length with almost 3,000 individual reefs.
A coral reef is not just a heap of inanimate mineral material. It swarms with life and represents one of the most complex ecosystems known, composed of thousands of plants and animals that have evolved intricately intertwined lives in one of the most competitive and species-rich habitats in the world.
As these so-called stony corals grow, many complex processes are put into motion. Habitat is created, attracting all manner of aquatic life with essential shelter from open-water predators. Where these plants and animals gather, there are wastes, which serve to feed other populations of plants and animals. In time, a reef community evolves: a collection of corals, sponges, algae, fishes, and a proliferation of other life forms that exploit every nook and cranny in the reef labyrinth. Like an oasis in a desert, a coral reef is a fertile hub in the ocean. It is a spawning ground for many fishes and invertebrates and therefore generates tremendous quantities of plankton, drifting organisms of various sizes from microscopic algal cells to larval fishes to small invertebrates that are all important food sources for commercially valuable oceanic species.
Reef Parameters
Coral reefs do not pop up just anywhere. As anyone who has traversed sections of the Caribbean or the tropical Indo-Pacific by plane or boat will know, there are vast expanses of warm seawater absolutely devoid of reefs. Some tropical islands are ringed with coral, others are surrounded by deep blue sea. A single island may boast luxurious coral growth along one coast, and nothing but open water off its opposite shore. This is not simply nature acting in a random manner.
Several conditions or environmental parameters are necessary for healthy reef development. Certainly one of the most important requirements for coral growth is warm water. Coral reefs are found in areas with average temperatures ranging from 21 to 29 degrees C (69 to 85 degrees F), although they can live at temperatures outside this range. The worldwide average temperature on a coral reef is 27.1 degrees C (81.7 degrees F). Corals can become stressed and die if temperatures vary much above or below the normal levels for their location. Ideal temperatures are found between 30-degrees North latitude and 30-degrees South, a band roughly 7,040 km (4,400 mi.) wide encircling the globe.
Excellent water conditions are also a must for coral growth. Constant surges from wave action provide vital oxygenation of the water, prevent overheating and stagnation, and flush wastes from the surface of sessile life. Water clarity is essential, because solar rays must be able to penetrate the water and sustain the vitality of zooxanthellae. Turbid conditions block sunlight and are often coupled with the presence of runoff from terrestrial sources or pollution. Sediment flowing into the sea from rivers, streams, or sites of erosion all work against coral health and the existence of reefs. Nutrients, such as wastes from development or agriculture, can spur the proliferation of algae that outcompetes and overtakes many reef-building corals. Most corals are sensitive to chemical toxins, and few reefs are able to thrive near areas of industrial development or commercial shipping. Reef waters are exceptionally clean, with a pH of 8.2 and virtually undetectable levels of such nutrients as nitrate and phosphate that are common in areas rich with organic wastes.
Constant levels of salinity or salt content, typically 35 to 37 parts per thousand (specific gravity of 1.024 to 1.027), depending on the reef and the water temperature) are vitally important, and areas subjected to wide seasonal variations because of currents or runoff from floods are poorly suited to reef organisms. Finally, reef-building corals need a firm footing in relatively shallow water—generally no deeper than 30 m (100 ft.)—to establish themselves. The majority of reef systems are found anchored on continental shelves (shelf reefs) or out in deeper water where they grow in association with oceanic islands.
Perfect conditions for corals are far from uniform in the world’s tropical seas. Coral reefs are estimated to cover less than one quarter of one percent of the total marine environment on earth. Shallow reefs, as mapped by satellite, total some 255,000 km2 (98,450 mi.2)—an area not much larger than Great Britain.... [For full text, please see the book, Reef Life.]
From: Reef Life