coral reefs Coral reefs are masses of calcareous rock deposited by living organisms (not all of them corals), representatives of which commonly inhabit reef tops a little below the low-tide level of the ocean.
At present, the principal hermatypic (or reef-building) organisms are the calcifying rhodophytes (red algae), molluscs, sponges, polychaetes, and cnidarians. The latter include the corals; the most important contributors to the growth of coral reefs are scleractinian corals, together with a few octocorallians and hydrocorallians (Table 1). The Scleractinia are an order of corals, mostly colonial, which have a calcareous external skeleton with radial partitions or septa. The Octocorallia form fan-shaped colonies with interconnecting branches. The Hydrocorallina also have calcareous skeletons.
Reef-building corals are animals containing soft parts called polyps that contain symbiotic algae. One of the major constituents of a polyp is calcium bicarbonate, which is broken down by the coral into calcium carbonate and carbonic acid. The latter is then broken down into carbon dioxide and water. The carbon dioxide is taken by the algae which, through photosynthesis, produce metabolites that feed the polyp. The process of extracting carbonic acid from calcium bicarbonate entails the precipitation of calcium carbonate, a process that is responsible for the construction of coral reefs:
Ca (HCO3)2 | | CaCO3 + | H2CO3 | | CO2 + H2O. |
Calcium bicarbonate | | calcium Carbonate (precipitated) | carbonic acid | | carbon water dioxide (removed by algae |
The whole process of coral-reef build-up is therefore driven by the demand for carbon dioxide of the algae which live in symbiosis with corals. The calcium carbonate produced in the reaction above is precipitated around the polyp, where it forms a variety of calcareous structures that become incorporated into the reef upon which younger corals will ultimately grow.
Most corals live only within the photic zone, that is, the upper 20 to 30 metres of the ocean into which enough light can penetrate for their symbiotic algae to photosynthesize. Reef-building or hermatypic corals live only in tropical seas, where temperature, salinity, and lack of turbid water are conducive to their existence.
So much emphasis has been placed on large, readily visible corals in reef-building that their role may have been overestimated. The role of calcifying rhodophytes (the crustose coralline algae which build impressive algal ridges and trottoirs (narrow features) along many tropical reef edges today) has been increasingly upgraded in importance during the past thirty years. Unlike corals, such algae are not confined to tropical waters but are involved in organic reef construction elsewhere. The paucity of corals in many emerged Quaternary reefs supports the view that the importance of corals as reef-builders may have been overestimated; the 1945 account by Harry Ladd and J. E. Hoffmeister of the limestones of the high Lau islands in the South Pacific is a good example. These geologists demonstrated that most (of the few) corals in Lau limestones were not in growth positions and had not therefore contributed significantly to building up the reefs.
Whatever the relative contributions of various groups of organisms to reef construction—a contribution which is unlikely to be globally uniform—reefs remain among the largest organic structures on Earth: the Great Barrier Reef of Australia can, for example, be seen from the Moon.
All reef builders require an initial surface upon which to grow, together with favourable oceanographic conditions. At the end of the Last Glacial (Würm), sea level rose from about 120m below its present position. This postglacial rise in sealevel provided ideal situations for upward reef growth (or re-growth). Warming ocean waters increased nutrients from enhanced organic activity and allowed reefs to become established on the flanks of larger edifices (see
oceanic islands). Subsequent upward growth was associated with a rise in sea level: the reef top needed to remain within the photic zone for the reef to survive. The resulting reef types have been characterized as ‘keep-up’, ‘catch-up’, and ‘give-up’ reefs. In the Pacific, for example, most reefs were probably catch-up reefs but a few were keep-up reefs and can be used for the accurate calibration of postglacial changes in sea level. To judge from the many submarine banks, some of which are known to be submerged reefs, in the Indian Ocean and around the Bahamas in the Caribbean, many reefs were also unable to grow upwards at the same rate as sea level rose, and have thus been submerged.
The absence of late Holocene emerged reefs in many places has given rise to the belief that contemporary sea level in these areas never exceeded its present level—as it is known to have done elsewhere. For the western Pacific, this view is largely incorrect, since the reefs in the areas where no emerged Holocene reef is found today are either occupied by catch-up reefs or by keep-up reefs which have been planed down to sea level since their emergence.
Since sea level stabilized around the middle to late Holocene, most coral reefs have extended laterally rather than vertically. This has meant a change in the dominant coral genera on many reefs, which in turn has led to a facies change in reef material. Studies of reef facies have made it possible to distinguish fossil reefs that grew vertically from those that grew horizontally. In consequence, a much clearer relationship between reef growth and sea-level change has been deduced. This understanding is important when the ability of many reefs to respond to future sea-level rise by growing upwards is assessed; many reefs will require a major species change before they can grow upwards because the branching corals that create a reef framework are generally not as abundant as they were during the period of postglacial rise in sea level.
Older fossil reefs, emerged and submerged, have been successfully used to detect Quaternary environmental changes. Methods have included oxygen-isotope analyses, which enable ocean palaeotemperatures to be known, and various techniques for determining past ocean productivity levels, which can be linked to palaeotemperature, ocean sediment levels, and other oceanographic variables. Drilling and dating of fossil reefs have allowed chronologies of reef build-up, tectonic and eustatic (sea-level) change to become precisely known. Reconstruction of Cenozoic sea-level changes has been made possible by drilling of Midway and other atolls in the North Pacific.
A classification of reefs, based on their geographical relationship to the land masses from which they have grown, is still appropriate.
Fringing reefs are those that fringe the coast of a landmass. They are often ephemeral, young, and may be highly localized in occurrence along a particular coast. They are usually characterized by an outer reef edge capped by an algal ridge, a broad reef flat, and a sand-floored ‘boat channel’ close to the shore. Within late Holocene times, most fringing reefs have been growing seawards. Many fringing reefs grow along shores which are protected by barrier reefs and are thus characterized by organisms that are best adapted to low wave-energy conditions.
Barrier reefs occur at greater distances from the shore than fringing reefs and are commonly separated from it by a wide deep lagoon. Barrier reefs tend to be broader, older, and more continuous than fringing reefs; the Beqa barrier reef of Fiji stretches unbroken for more than 37 km; that off Mayotte in the Indian Ocean for around 18 km. The largest barrier-reef system in the world is the Great Barrier Reef, which extends 2300 km along the east Australian coast, usually tens of kilometres offshore.
Atoll reefs (or ring reefs) rise from submerged volcanic foundations and often support small islands (
motu) of wave-borne detritus, sometimes armoured with beachrock or containing conglomerate platforms (
pakakota) or phosphate rock which cause them to resist wave erosion. Atoll reefs are essentially indistinguishable in form and species composition from barrier reefs except that they are confined to the flanks of submerged oceanic islands, whereas barrier reefs may also flank continents.
Many ancient (fossil) reefs show similar facies patterns to modern reefs. Some of the most closely-studied exhumed reefs are the Permian reefs of west Texas, the Devonian reefs of western Canada, Europe, and Australia, and the Triassic reefs of the European alpine province. Facies variations are also the cause of significant variations in the depth and thickness of freshwater lenses in high limestone islands like Niue. The Miocene reef limestone in Fiji illustrated in Fig. 1 shows evidence of a contemporary hiatus accompanied by
subaerial diagenesis.
In his 1842 classic book
Structure and distribution of coral reefs, Charles Darwin outlined the way in which coral reefs could grow upwards from submerging foundations. From this, it became clear that fringing reefs might be succeeded by barrier reefs and thence by atoll reefs. Later writers, particularly W. M. Davis, took this to mean that the three types were stages in an evolutionary succession and could thus be used to infer the stage of development that a particular reef had reached. Although partly a response to theories put forward which had questioned at the beginning of the twentieth century Darwin's ideas, Davis's views proved too inflexible and have since been challenged, principally by those concerned to incorporate the effects of sea-level changes in any explanatory framework of reef types.
Reefs in many parts of the world are currently under severe stress for a number of reasons. Direct human impacts include physical damage, pollution, and sedimentation. The rise of ocean-water temperature in many places has combined with other sources of stress to produce coral bleaching, a phenomenon in which polyps eject their symbiotic algae, resulting in coral death. The effects of predatory organisms, particularly
Acanthaster, can be devastating in the short term, although perhaps important in long-term regeneration. Reef damage resulting from storm surges, particularly those associated with tropical cyclones (hurricanes) and tsunamis, and from earthquakes can be catastrophic.
Most authorities are agreed that, should sea level rise in the future, reefs will be able to make optimal responses in many places only if stress levels are reduced. There is consequently an urgent pragmatic need for reef conservation apart from a desire to protect this unique ecosystem. Should sea level rise and reefs be unable to respond to exert the same degree of protection along landward coasts that they do now, many of these coasts will be subject to greatly increased wave attack and erosion, particularly during storms.
Patrick D. Nunn
Bibliography
Darwin, C. R. (1842) Structure and distribution of coral reefs. Smith, Elder, London.
Guilcher, A. (1988) Coral reef geomorphology. John Wiley and Sons, New York.
Jones, O. A. and Endean, R. (eds) (1973) Biology and geology of coral reefs, Vol. 1. Academic Press, New York.
Nunn, P. D. (1994) Oceanic islands. Blackwell, Oxford.
Nunn, P. D. (1999) Environment change in the Pacific Basin. John Wiley and Sons, Chichester.
Wiens, H. J. (1962) Atoll environment and ecology. Yale University Press, New Haven.
