Case Study: The Great Barrier Reef
The Great Barrier Reef (GBR) is a continental margin reef which has developed on the continental shelf, and is quite different in origin to the classic coral atolls of the nearby Pacific. It is the largest coral reef system in the world, with an outer perimeter about 2300 km in length and a total area approaching 350 000 km2 (figure 2.42). Although it forms a semicontinuous barrier in certain locations, the term barrier reef is misleading, because it is made up of over 2900 individual reefs, including about 750 fringing reefs attached to the mainland or numerous islands. The thousands of individual reefs range in size from small reef pinnacles up to reefs of 20 to 30 km long and more than 125 km2 in area. They vary in shape from flat platform reefs to the elongate ribbon reefs of the continental shelf margin, such as to the north-east of Cairns. In some places individual reefs are separated by channels 200 m wide, whereas in other places they could be 20 km apart. There are about 300 reef island sand cays, of which 87 are permanently vegetated. There are also about 600 continental or 'high' islands (see figure 2.43(a)).
Figure 2.42 The Great Barrier Reef, showing boundaries of the Marine Park and World Heritage Area
Source: Commonwealth of Australia 1993
Figure 2.43 Different reef and island types on the Great Barrier Reef
The variation in reef development can best be illustrated using three different sections of the Reef: the northern section from around Cairns up to 14°S, the central section south of Cairns and down to the Capricorn Channel, and the southern section off the coast from Gladstone.
The northern section of the reef is situated on one of the narrowest sections of the continental shelf, which in places is less than 50 km wide. The distinctive features of the area are related to this narrow, box-shaped shelf structure, which has generally less than 50 m of water across it. However, the water increases to greater than 1 km in depth within a kilometre of the outer reefs, where the continental shelf drops away steeply. Across the shelf in the northern section it is possible to recognise four major zones of reef development:
1 A line of fringing reefs which vary in size and have an irregular distribution along the coast. Their distribution is related to local hydrological conditions and sediment budgets.
2 An inner zone of Tow wooded island' reefs and small reefs with sand cays, generally within 20 km of the coast. These islands of reef origin generally include cemented deposits, extensive shingle ridges, spits or islets and vegetated sand cays, and often extensive mangrove development (see figure 2.43(b)). The southern limit of the low wooded islands is 16°30'S although some of the characteristic features are found on the high islands of continental origin further south.
3 A mid-shelf zone of reefs up to 26 km long and 12 km wide, often with narrow intervening passages. In the same zone are a number of smaller, irregularly spaced reefs, separated from others by up to 8 km of open water. These mid-shelf reefs are characterised by extensive reef flat development, algal rims, and often predominant orientation to the trade winds. Sand cays (see figure 2.43(c)) have developed on the lee side of a number of these reefs, but only a few are stable enough to maintain any vegetation.
4 A zone or line of 'ribbon reefs' (see figure 2.43(d)) less than 600 m wide but individually up to 28 km long with narrow intervening passages, and situated on the outer edge of the shelf. The southern limit of these is at 16°30'S. Behind the barrier is seemingly open water which often contains coral patches or 'bommies'. The water may be 50 m or so deep, in contrast to the steep drop off beyond the shelf edge reefs.
The central section of the Great Barrier Reef is defined here as the area between 17° and 23°S. This area shows marked contrast in the distribution and morphology of reefs. Nowhere in this region is there a zonation of inner shelf low wooded islands, mid-shelf reefs and linear ribbon reefs as described above for the northern section. However, many of the high islands (continental origin) have deposition features similar to the low wooded islands. In parts of the outer shelf, reefs form a complex, closely spaced reef pattern of 'hardline' reefs which to some extent parallels the continuity of the narrow ribbon reef development north of Cairns. In the northern part of the central section there is a narrow shelf, 50-100 km wide, which merges into the wider continental shelf off Townsville and Bowen (125-150 km). The reefs in this northern central section occupy up to 50% of the outer shelf and come within 20 km of the mainland, but there are no shelf-edge ribbon reefs as are found further north. However, there are similar submerged ribbon reef features north of Innisfail. Where the coastal orientation changes from north-south to north-west-south- east, the shelf widens and the reefs occupy the outer 35% of the shelf and do not come within 50 km of the coast. The reefs in this region are irregular patch reefs, generally much smaller than the reefs to both the north and the south. Even the largest reefs are less than 15 km long, and the majority of reefs are 5-10 km long and up to 5 km wide. South of Cairns down to Whitsunday Islands there are no true reef islands made up of reef deposits alone, although there are many high islands in the area, such as Hinchinbrook Island. The reefs in this section tend to be fairly low (not so exposed at low tide) and relatively few have permanent sand cays. There are none with permanent vegetation.
Further south in the central section of the Reef is a number of massive reef complexes with deep narrow intervening channels (see figure 2.43(e)). These extend south-east from 19°S to 20°S where they merge with the Pompey Reef complex. The shelf edge reefs, from 20° to 21°S from the hardline Pompey Complex, up to 250 km offshore, which consists of massive individual reefs up to 100 km- in area with narrow passes separating them, often only a few hundred metres in width. These passes form a dendritic pattern through the reefs and generally coalesce towards the north-east. The channels have recorded depths of around 100 m, some 40 m deeper than the general shelf depths. It is in this area of the Pompey Complex that the Blue Holes are found (see page 46). These features are similar to blue holes found elsewhere in the world, and are erosional features formed by limestone solution during periods of low sea level, rather than being modem growth features.
The southernmost section of the Reef is an isolated group of reefs referred to as the Capricorn and Bunker group, situated about 80 km off the coast from Gladstone. The reefs have little in common morphologically with the hardline or ribbon reefs in the northern part of the Reef, but are more similar to some of the mid-shelf patch reefs. The reefs in this southern section vary in size from 1 to 40 km2 and exhibit a diversity of features. A major feature of the reefs here is the abundance of reef-top sediments and the presence of 15 coral cays, most of which support dense vegetation. The reefs tend to be oriented toward the prevailing winds and have a well-developed high algal rim, especially on the windward margins. Some of the reefs have large, deep lagoons with varying degrees of reef flat development around them.
This variety in reef types reflects the variation in reef-building processes in response to a number of factors. The actual distribution of the reefs is governed to a large extent by the shape of the continental shelf, together with factors such as water depth, suitable substrate, nearshore sediment, and river run-off. Each reef has evolved under a unique set of physical processes that occurred in a specific location. In some cases reefs have reached modem sea level relatively early and have had time to accumulate supra-reef deposits which have been consolidated into reef islands. Elsewhere, some reefs have grown up to modem sea level much later and consequently have no reef deposits above high tide level. These different reef types attract different types of human impacts.
Major activities on the reef are tourism and fishing. The only activities that are specifically prohibited are commercial mining and oil drilling. Tourism is now the fastest growing activity and has been expanding at an estimated 10% per year. Recent technological changes have also meant that the type of tourism has changed in style from a less sophisticated, inexpensive family style tourism to rapid-transit, high-capacity and high-technology vessels which can move hundreds of tourists at a time and compact the 'reef experience' into a relatively short time frame. The other major reef user is the commercial and recreational fishing industry.
The great extent of the reef and the variety of reef forms pose unique coastal management problems to cope with different uses. For example, the popular southern reefs are readily accessible and have a number of reef islands which create different human use patterns and associated impacts, compared to some of the less accessible outer reefs in the central section where there are no reef islands. The management of the Reef became important when there was talk of mining and drilling in the 1960s, on which two royal commissions deliberated. The first commission, in 1970, investigated oil drilling on the Reef. Then in 1973, the Commonwealth passed its Seas and Submerged Lands Act to establish federal jurisdiction below low water mark. In 1975 the Great Barrier Reef Marine Park Act was passed. This Act has a number of key elements, including the establishment of a Great Barrier Reef Marine Park Authority (GBRMPA) and a consultative committee, prohibition of mining and oil drilling and, most importantly, allows for the regulation of activities.
In 1981 the Reef was inscribed on the World Heritage List. The Great Barrier Reef World Heritage Area contains almost the entire Reef (figure 2.42), extending from the tip of Cape York to just north of Fraser Island and from the low water mark to the edge of the shelf. It is 348 700 km2 and comprises the Great Barrier Reef Marine Park (93%), Queensland waters not in the park (2%) and islands (5%). In 1994 a 25-year strategic plan was prepared for the Great Barrier Reef World Heritage Area.
The management of the Reef, its associated Marine Park Act of 1975, and the Great Barrier Reef World Heritage Strategic Plan of 1994 have been quoted (Harvey 1999) as a good Australian example of integrated coastal management. This is discussed further in chapter 4. A key reason for the success has been the use of education rather than enforcement, and also the provision for the regulation of activities where there are clearly competing uses for coastal resources.
