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Compiled by Leon P. Zann
Great Barrier Reef Marine Park Authority, Townsville Queensland
Ocean Rescue 2000 Program
Department of the Environment, Sport and Territories, Canberra, 1995
ISBN 0 642 17399 0
J. E. N. Veron
Australian Institute of Marine Science
PMB no 3, Townsville MC, Qld 4810
Coral reefs are geological structures, yet they are made by living organisms. As such, they are the biggest and most conspicuous of all non-anthropogenic structures made by living organisms and have been a major physiographic feature of the earth's surface for the past 150 million years.
The key to this 'phenomenon' is a symbiotic relationship between plant and animal which allows the practically unlimited resources of sunlight and inorganic carbon to be used for the building of structures so large that they are the creators and controllers of their own macro-environment. Both energy (in the form of organic nutrients) and building materials (in the form of calcium carbonate or limestone) are products of the same process which is the nutrient base of all major terrestrial ecosystems: photosynthesis. In this sense, coral reefs are the marine equivalent of terrestrial forests.
This often-made analogy between reefs and forests readily extends to the diversity of organisms they house, for reefs and forests each provide the food and the environment, for the earth's greatest number and variety of species.
The world-wide distribution of coral reefs (and reef-building corals) is controlled primarily by bathymetry and temperature, for reefs can only develop in shallow, sun-lit waters where the temperature seldom falls below 18oC. Secondary constraints are related to Cenozoic geological history (eg the complete absence of reef building corals in the Mediterranean Ocean), surface circulation patterns (eg their paucity in the far eastern Pacific) and regional environments of which salinity (the absence of reefs in areas influenced by major rivers), substrate type (the paucity of reefs in extensive areas of soft terrigenous substrates) and nutrients are the most important. Clearly, surface circulation patterns and temperature are interlinked, as is bathymetry and the existence of extensive river deltas, soft terrigenous substrates and light availability (Veron 1986). The relative lack of nutrients is critical to long-term stability in reef development: reefs thrive around islands in the 'nutrient deserts' of the remote oceans; they also thrive around the more nutrient-rich waters of continental margins, but they appear to do so there only where natural ecological checks and balances do not permit corals to be outgrown by other organisms.
Like a patchwork of miniature forests, the coral reef is a microcosm of different communities, each separate, but linked to the next by a complex web of ecological interactions. These communities are distinctive because, on a single reef, they form a series of narrow bands or zones, each having a particular place in an array of rapidly changing environmental gradients.
Because coral reefs are constrained to shallow (less than 100 m depth) oceans, they are profoundly influenced by global climatic changes that affect not only ocean temperature and circulation patterns, but, much more importantly, ocean depth. Sea-level change has caused reefs to be repeatedly emerged and submerged throughout most of their geological history: thus, for example, only 20 000 years ago, all currently identifiable coral reefs were completely emerged as the sea level decreased to approximately -120 m. The coral reefs of today are living veneers on older formations, which are themselves based on successively older foundations and so on, back in geological time.
At least 50% of the world's coral reefs, covering an area of more than 300 000 km2, occur in the central Indo-Pacific. This region is the world's centre of reef coral diversity (Figure 1). The reasons involve a complex mixture of geological history, oceanography and biology but the principal reason concerns past and present sea
Millions of hectares of coral reef dominate much of the world's tropical coastline. These massive structures result from the accumulation and cementation of skeletons of innumerable corals and algae over thousands of years. Yet, imposing as they are, the existence of modern coral reefs is the result of a most intricate and subtle relationship between the coral polyp and the minute single-celled algae which live symbiotically within the cells of the polyp. Theses algae, commonly called zooxanthellae, belong to a group of unicelular brown plants known as dinoflagellates.
surface circulation patterns, for it is they which provide the means of long-distance dispersal for all groups of organisms capable of maintaining a planktonic existence.
Australia, because of its geographic position within the world's centre of marine diversity, is critical to the conservation of corals. Coral reefs mostly occur around the developing, over-populated countries or the world's tropics. Within major regions of the central Indo-Pacific 'Centre of Diversity', only western Micronesia, northern Papua New Guinea, Australia and (perhaps) Japan, currently have a low population pressure and/or the capacity to permanently regulate human impacts. These impacts are, as yet, poorly known: my own estimate is that 70% of all central Indo-Pacific coral reefs have been significantly degraded. This degradation has occurred primarily through over-fishing (which has effectively removed the top of the food pyramid in most South-east Asian and Japanese areas of reef), eutrophication and increased sedimentation (from urban outfall, deforestation, agricultural run-off and coastal zone development) and direct intrusive activities (principally through subsistence food gathering, particular mining practices, shell collecting and unregulated tourism).
The now often-made distinction between acute and chronic impacts on reefs is intuitively useful. Acute impacts, whether anthropogenic or not, are generally limited in area (the starfish Acanthaster and, to a lesser extent, the gastropod Drupella outbreaks being the dramatic exceptions) and are often associated with widespread local death of corals. Chronic impacts are generally sub-lethal, long-term and environmental. It is the latter which are of principal importance to coral reefs and which present the main challenge for scientific study. Environmental deterioration of the type that has so widely affected European forests opens a Pandora's box of present and future possibilities for coral reefs, among which is their capacity to cope with the synergistic effects of multiple chronic influences (such as eutrophication together with over-fishing) and their capacity to recover from acute impacts while under the influence of chronic ones.
Figure 1: Isopangeneric contours of hermatypic (reef-building) coral diversity of the world ( from Veron 1992). The Indo-Pacific centre of diversity is highlighted.
The result of anthropogenic influences are perhaps best seen by international comparisons. Truly pristine reefs, such as those of the remote outer northern Great Barrier Reef (GBR), some parts of the Coral Sea, and the reefs off the North West Shelf are inhabited by sharks and other big predators, turtles, whale-sharks and marine mammals in numbers that are seldom seen in the central and southern GBR (some specifics excepted) and which are rare anywhere in South-east Asia. Similar comparisons are valid for most collectable objects of value or interest, notably the big and/or valuable molluscs. Putting Australian reefs in a broader context is a subjective undertaking, but it appears clear that in the next few generations, they will play a critical role in the conservation of a significant proportion of the species of the world's centre of diversity.
Australia's coral reefs form seven distinctive groups:
The Solitary Islands (30oS) (Clayton 1989) adjacent to the central New South Wales coast, are a group of rugged islands that do not have coral reefs as such, but do have a combination of reefal and non-reefal biota that is not found elsewhere in the world. This biota includes 53 species of reef corals and 280 species of fish of which 80% are tropical. North Solitary Island has very large populations of giant anemones and attendant clown fishes. The fauna of the Solitary Island has long been largely unprotected but recently was made a marine reserve.
Lord Howe Island, a spectacularly mountainous national park, is situated on a submerged volcanic seamount of the Lord Howe Island Rise. The reef which extends for approximately 6 km along the island's western side is the world's southern-most coral reef. The outer slope, broken by three passages, rises from a sandy sea floor at 15-20 m depth. The reef is dominated by algae with tropical affinities. The biota includes 65 species of coral (some in temporary populations) and 427 species of fish. The fish species also have primarily tropical affinities (Francis in press).
Elizabeth and Middleton reefs (Hutchings 1992) are large platform reefs, also on seamounts of the Lord Howe Island Rise and only 95 km north of Lord Howe Island. In all essential characteristics, these coral reefs closely resemble those of the tropics yet they exist far to the south in very marginal conditions for reef development. They are much less accessible than the Lord Howe Island reef and have not been as well studied. Their intrinsic interest is nevertheless enormous, because of their environment and isolation. One hundred and twenty-two species of reef coral, which include most species found at Lord Howe Island, have been recorded. Elizabeth Reef was one of the first eastern Australian reefs to have a major Acanthaster outbreak in the 1980s. Since then both reefs have been extensively damaged with the result that abundant coral is now restricted to the reef lagoons.
There are no other limestone reefs south of the GBR although reef fauna and flora may occur in great abundance at some coastal localities, notably the little-studied 'Flinders Reef' off Brisbane. Flinders 'reef' is actually a sandstone outcrop, but has a diversity of corals which rivals that of Elizabeth and Middleton reefs. The same is likely to be true of other benthic groups on the outcrop, although most await study.
The GBR is the largest single coral reef in the world. It is not the most diverse in terms of species (Indonesian and Philippine reefs have greater numbers of coral species) but it is extremely diverse in terms of reef types, habitats and environmental regimes. Why this is so, is because the GBR is large enough to extend from the low latitude tropics to subtropical zones, to have regions with very different climates (wind patterns and rainfall), tidal regimes, water qualities, bathymetry, island types, substrata and even geological histories. To some extent the GBR faunas have regional identities, but in general there is more variation across the GBR, than there is down its length. This variation is because the western (inshore) edge is dominated by shallow seas with terrigenous substrates and is exposed to periodic river run-off and consequent low salinity and high turbidity. Also, high (continental) islands occur only in inshore regions of the GBR, and it is these islands which provide much of the habitat diversity.
The Capricorn and Bunker reefs are the southernmost reefs of the GBR and are among the best known. The region as a whole is characterised by well-defined, distinctly elevated platform reefs with entire, steeply sloping sides. Inter-reefal water is moderately deep. Many reefs have vegetated cays which are much sought after by visitors. Faunistically, the reefs are very uniform with the same zones or community types being repeated from one reef to the next. Because of this uniformity, the overall diversity of corals is low (and so probably is the diversity of most other faunal and floral groups) compared with other major regions of the GBR.
Figure 2: The relationship between the coral faunas of Australia based on presence/absence data. Links (nodes) in the dendrogram are measures of dissimilarity, the more remote the node the greater the dissimilarity. The diagram shoes that ther is a high degree of uniformity in the tropics and that tropical species become attenuated towards higher latitudes. This broad pattern is repeated by reef species of most major groups. However, unlike corals, most major groups show replacement of tropical species by temperate ones, rather than attenuation.
Figure 3: The distribution of coral reefs around the Australian coastline.
The Swain Reefs and Pompey Complex extend further from the coast than any other part of the GBR and have, until recent times, been known only from a brief description in Maxwell's (1968) Atlas of the Great Barrier Reef. Seen from the air, the Pompey Complex is a spectacular panorama of interlocking reefs, channels, sandbars and lagoons, all set in the highest tidal range of the GBR and forming immense barriers to tidal water movement. So-called 'deltaic' reefs of the outer 'hard line' of the Pompeys resemble river deltas in reverse, the deltas being solid limestone and the tributaries being 'U'-shaped channels carrying extremely strong, reversing, tidal currents. The Swain complex forms a southward pointing wedge, the reefs on both sides of this wedge having exposed outer faces and protected inner margins. The two sides are ecologically dissimilar: the eastern side has several sparsely vegetated cays. The increased habitat diversity displayed in the Swain Reefs and Pompey Complex is reflected in their having a higher diversity of corals compared with the diversity in the Capricorn/Bunker Reefs.
The central GBR is a vast area, primarily characterised by lower reefs than those further south or further north, and by the absence of cays and well-defined outer barrier reefs. Perhaps the best-studied aspect of it is the change in fauna that takes place across the continental shelf from inshore to offshore in response to major environmental gradients. The relatively shallow, turbid, terrigenous coastal waters that are protected from strong wave action and subject to seasonal river flooding (and attendant pulses of silt and organic nutrients), support a reefal and inter-reefal fauna and flora of a very different character to that found offshore. The complex of high islands of the Whitsunday and Lindeman groups have a very high diversity of benthic fauna - perhaps the highest diversity of the GBR - and certainly one of the most varied.
The continental shelf is narrowest in the northern section of the GBR. It is here that the Queensland Trough forms, deepening as it extends northwards. 'Ribbon reefs' occur where the trough and GBR shelf meet. The ribbon reefs follow the shelf-edge break all the way to Torres Strait (720 km) and form the most conspicuous physiographic feature of the whole GBR. On the eastern side where they are very exposed to ocean swells, the ribbon reefs plunge steeply into the abyssal depths of the Queensland Trough. Although the water is very clear, the lower slopes are too deep for SCUBA divers to explore and almost nothing is known about them.
Inside the ribbon reefs is a band of open water mostly devoid of reefs where, particularly in the vicinity of passes between the reefs, the substrate consists of enormous mounds of Halimeda (Roberts & Macintyre 1988). The mid-shelf is occupied by extensive areas of reefs with roughly parallel east-west margins, cut in the past by rivers at low sea levels. The inner shelf contains a wealth of reef types, high islands and coral cays, many of them heavily vegetated.
A research station on Lizard Island, a high island on the mid-shelf, provides the only land-based access to the northern and far northern GBR. Inaccessibility of this enormous region has thus truncated even the most basic faunistic and descriptive studies.
The continental shelf widens in the far northern GBR, but the extensive shelf-edge reefs remain. Raine Island is a special place by any standards, having the largest green turtle rookery in the world and some of the largest sea bird rookeries as well. The outer barrier in the far north abounds with life in dramatic abundance no longer seen in the south. Numerous large nearshore reefs are found only in this region.
In Torres Strait, the outer barrier reefs become broken up into a series of 'deltaic' formations similar to reefs of the Pompey Complex, and then an almost impenetrable line of 'dissected' reefs. Inside this barrier line is an aggregation of reef complexes, high islands and cays of great variety. The sea becomes progressively more shallow and turbid towards the west, finally forming the Warrior Reefs which are essentially vast mud flats fringed in the east by coral. In terms of interest and variety, both above and below water, Torres Strait and the far northern outer barrier has, in my view, no equal anywhere on the GBR.
The Western Coral Sea essentially can be divided latitudinally into three parts. To the north, and not far removed from the GBR, are Ashmore Reef, Portlock Reefs and Eastern Fields. Each of these groups is very different from the others, Ashmore Reef being atoll-like. Very deep, 'empty' ocean to the south separates these groups from the central Coral Sea which contains the widely dispersed reefs of the Queensland plateau. Some of these reefs have cays. Further to the south are even more isolated reefs including Marion, Kenn, Frederick, Cato and Wreck reefs. Of all of these reefs, Flinders Reefs nearest to Townsville are the best studied, but even these have only been the subject of expeditionary cruises.
There is a scattering of little-known fringing reefs along most of the complex coastline of the Northern Territory. The shallow, turbid waters of the eastern Arafura Sea are not conducive to coral reef growth and what reefs there are, are mostly shallow. Reef development increases to the west and reaches modest diversity in the vicinity of Essington Peninsula. Strong tidal currents dominate the environment in this region and increase towards the west. The sponge-dominated soft bottom communities of western Northern Territory have, in general, attracted more interest than the sparsely developed reefs there.
Cocos (Keeling) Atoll in the eastern Indian Ocean is Australia's only true atoll. Most scientific interest is in its geomorphology - especially as influenced by sea-level changes - and its isolation, which is of particular interest in faunistic studies. Due to its long occupation and recent expeditions, the atoll's fauna is moderately well known. Christmas Island to the east is a high mountainous island with a plunging shoreline. Its reefal fauna is similar to that of Cocos (Keeling) Atoll.
Ashmore Reef (Australian National Parks and Wildlife Service 1989), 350 km off the Kimberley coast on the outer edge of the Sahul Shelf, is basically a large sedimentary accumulation with reef patches. Nevertheless, it has the highest diversity in Western Australia of corals, sea snakes and probably most other major reefal taxa. Scott Reef, Seringapatam Reef and Rowley Shoals are all 'shelf-edge atolls', a reef type not found in the east of Australia. They are visually spectacular due to clear oceanic water with a high tidal range, and each has its own distinctive characteristics. Again, knowledge of them is mostly limited to faunistic studies (Berry 1986).
Coral reef fishes have complex life cycles involving two or more developmental stages that live in very different habitats. With rare exceptions, reef fishes do not protect offspring through their full development so that most species spend some part of their early life history in a planktonic refuge where they may be dispersed to new locations. Recruitment of juveniles from larval sources to reefs is highly variable and has given rise to debate about whether the size of adult populations is primarily determined by success of juvenile settlement or post-settlement history (Doherty1991).
Compared with the reefs of the east coast, these have been much neglected scientifically and remain little-known. Western Australian reefs are interestingly distributed down the coast in a series of 'stepping-stones', each connected to the other by the southward flowing Leeuwin Current. The influence of this current results is a chain of geographically and environmentally discrete localities forming a natural setting for long-distance dispersion of reefal fauna from Indonesia.
The reefs of the Kimberley coast are only superficially explored. They exist in turbid waters constantly mixed by large tidal fluctuations. Of all coastal reefs of north-western Australia, those of the Dampier Archipelago (Simpson 1988) off the Pilbara coast are the best known and probably the most diverse. With an inshore muddy environment and offshore clear waters, all mixed by strong tidal currents, the archipelago has a range of marine environments probably unmatched by any other area of similar size in Australia.
The 230 km-long Ningaloo Reefs (May et al. 1988), by far Australia's biggest fringing reef, is situated at the barren far western extremity of the country where the continental slope is closest to the Western Australian coast. The Ningaloo Reefs are readily accessible to visitors from Perth and, until recently, were heavily fished.
The Houtman Abrolhos, situated approximately 400 km north of Perth, are the most southerly reefs of the Indian Ocean (29oS) and are some of the most interesting coral reefs of Australia. Although they form the southern distribution limit of most Western Australian coral species, the corals show few signs of environmental stress and in some areas they form the most luxuriant communities to be found on any Australian reef. Curiously, Acanthaster has never dispersed to these islands although on the east coast it has reached the Solitary Island and Lord Howe Island. In other areas, corals grow with kelp and Sargassum seaweed in an extraordinary mixture of the tropical and temperate biota.
The complex distribution of fishes among the 3000 reefs of the GBR is driven by cross-shelf gradients in water quality. The major determinant is the extent of intrusion of oceanic water into the continental shelf . This intrusion is influenced by the width of the shelf, the proximity of reefs to the shelf-edge and the extent of an outer barrier. It varies greatly from region to region (Williams 1991).
The GBR is arguably the most valued part of Australia's natural inheritance. Its importance to life on this planet and its intrinsic value to future Australians is beyond measure. Unlike most of the other great natural wonders of this earth, the GBR has nothing of the robustness we naturally associate with vast and apparently pristine regions. It is, as aforementioned, only a veneer of life on a limestone foundation and, in my opinion, that veneer is fragile and as sensitive to environmental degradation as is any other ecosystem on earth. It is the challenge of the present and future to preserve that veneer for all time, and do so in the face of human usage that appears likely to undergo an exponential increase.
Some would place the value of the GBR to individual Australians second only to its value to World Heritage. Most Australians, and indeed most educated people from any country who take an active interest in global issues, would place a high value on the conservation of the GBR. This value, I believe, is part of our national and international culture and is thus difficult to describe and impossible to define.
It is clear that Australian coral reefs in general are only at the dawn of international tourism. The GBR, as no other reef region in the world, offers true wilderness areas of vast proportions. As yet, the remoteness of these regions has preserved them almost completely from the tourist industry. How long this will last is guesswork, for future projections of tourist numbers and activities are inevitably prone to error because of the difficulty of predicting technological advances in transport and accommodation (such as high-speed aluminium catamarans and floating hotels), not to mention international economics.
At this point in time, most of the reef tourist industry is catering for a combination of speed and ease of access. In these respects there are many other places in the Indo-Pacific which effectively compete with the GBR. In perhaps a decade or less, this situation will almost certainly change as better informed visitors demand more personal experience, more 'adventure' and better access to remoteness and the unknown. In a decade or so beyond that, the place Australia will have in the global conservation of coral reefs will create management issues that can only be imagined at present.
In October 1981, the GBR was inscribed on the World Heritage list as it satisfies all criteria set out in Article 2 of the World Heritage Convention: an example of a major stage in the earth's evolutionary history; an outstanding example of geological processes, biological evolution and human-environment interactions; a place with unique, rare and superlative natural phenomena; and a place which provides habitats for rare and endangered species of plants and animals (Great Barrier Reef Marine Park Authority in prep.). At present the GBR region supports directly economic activity estimated to be worth at least $1 billion annually to the Australian economy.
The conservation of our natural heritage in the face of threatened impacts of the next technological and economic generation is the overwhelming issue of today. Assuming a continuation of the present level of management, key future impacts are to be from the same sources as at present: extractive industries, coastal zone development and, in the west, the oil industry.
Considerable difference of opinion exists concerning the impact of fishing on the GBR. While some studies have claimed significant effects of fishing on some reef fishes, there has not always been agreement on interpretation of the limited data available. I confine my own view, derived from my comparisons with other countries, to a simple statement of what I believe is obvious: there is no permanent protection of fish of any sort throughout almost all of the Australia and South-east Asian Nations (ASEAN) region; that long-term conservation within the Indo-Pacific centre of diversity is rapidly becoming dependent on Australian legislation; and that the central and southern GBR is nowhere near as 'pristine' as is sometimes claimed.
The two principal aspects of environmental impact resulting from coastal zone development, which are now profoundly affecting reefs world-wide, are the sedimentary environment and the nutrient environment. Corals, being immobile and sensitive to environmental deterioration, are affected by sediment directly through physical contact and the energetic cost to the organism of removal, and indirectly through the effect of turbidity on light attenuation. Increase in sedimentation on coastal reefs comes primarily from soil erosion after deforestation and from agriculture, coastal construction and dredging. The nutrient environment is perhaps more complex, nutrient increase coming from sewage and agricultural outfall as well from the nutrient content of sediment resulting from increased erosion, all of which may affect ecological checks and balances.
This is a consideration for the future, especially in relation to the North West Shelf in general and Scott Reef in particular.
These are global in nature and largely beyond the scope of this article. They vary greatly from one region or situation to another, but in the long-term the major ones are likely to be:
Two decades ago, the GBR was one of the most poorly known of the world's major reef systems and all publications on it would fit onto a small bookshelf. Very little had been published about any of the other coral reefs of eastern Australia and virtually nothing known or published about the reefs of Western Australia other than Dakin's (1917) book on the Houtman Abrolhos and the geological studies of Fairbr