Coral reefs

1  Coral reefs

1 Introduction

It has been estimated that tropical coral reefs occupy 284,300 sq km of the planet’s surface.1 This is less than 0.1 per cent of the total surface area.2 Despite this relative scarcity, in practical and economic terms, the contribution of these reefs is great. Ecosystem values range from the provision of protein, coastal protection from wave energy, land formation where coral rubble and sand has helped to raise land above sea level enabling human habitation, and acting as a natural wonder, attracting tourists from around the globe.3 To those in need, reefs may also bring relief from medical conditions, as scientists explore new organic chemicals present in the coral reef ecosystem.4 Indeed, estimates suggest that coral reefs contribute the equivalent of US$375 billion per year as part of a global total ecosystem value of US$33,268 billion per year.5 Therefore, based on these estimates, coral reefs contribute 1.13 per cent of the annual total.

Complex natural processes around tropical coral reefs govern the ecosystem, involving predation, climate and erosion. Over time this has meant that the individual coral polyps, the reefs they help build through the deposition of calcium carbonate, and the diversity of life thriving in the myriad niches on offer in the ecosystem, have ebbed and flowed in abundance and geographic distribution.6 But natural processes are increasingly being disturbed by anthropogenic interference and pressure. Uncontrolled fishing has directly and indirectly harmed coral reef ecosystems.7 Growing coastal populations (both permanent and transient), together with the attendant development of urban areas, tourist accommodation and agricultural production, have increased the scale of pollution and sedimentation with which coral reefs must contend.8 On a wider scale it is estimated that rising water temperatures destroyed 16 per cent of coral reefs worldwide in 1998.9 Indeed, it is predicted that anthropogenically fuelled climate change will cause the greatest mortality of corals in the coming years through increased coral bleaching, particularly where coral reef ecosystems are also subject to other anthropogenic stresses.10 The vast economic benefits of tourism come at an additional price as some divers and boats cause physical damage to reefs, while the fish stocks are utilised further to feed visitors and produce curios.11

Much scientific energy and effort has now been put into understanding coral reefs and resolving these problems. However, as society comes to better understand these ecosystems and their significance, so it becomes more apparent how urgent it is to take steps to conserve them. What is more, as Mark Spalding observes:

One of the saddest facts about the demise of reefs is that it is utterly nonsensical. Protecting and managing reefs is not just for the good of the fishes, in every case it also leads to economic and social benefits for local communities.12

2 Scope of the study: hermatypic corals and tropical coral reefs

This book considers the progress to date under international law for promoting the conservation of tropical, shallow-water coral reefs. An ecological limitation has therefore been placed on the scope of the research in that deep-water coral reef habitats are not going to be covered. A brief account of the biology of warm-water corals and the reefs these creatures form, highlighting the differences with their cold-water cousins, will help to clarify the implications of this limitation.13

Corals, as a biological order, are found throughout the earth’s oceans from the tropics to the polar regions. Belonging to the same phylum as jellyfish (Cnidaria) and the same class as anemones (Anthozoa), there is significant variety amongst coral species. Corals can be found both in the cold waters of the oceanic depths and in the shallows of warm tropical seas. Individual corals found in both cold and warm water are capable of depositing calcium carbonate. By increment this contributes either entirely or predominantly to the formation of carbonate skeletons and reef structures.

Some species of coral capable of surviving in the cold, dark waters of the deep oceans can be encountered on the shallower tropical reefs.14 Nevertheless, there remain significant differences between the two, as listed by Corcoran and Hain.15 For example, there are only six species of reef-building cold-water corals compared to 800 warm-water corals.16 Further, calcification rates are much slower for cold-water corals: 25mm/yr-1 compared to 150mm/yr-1 for warm-water corals.17

That greater ability to calcify is the result of one of the most important distinctions between the two groups of corals. The fixing of calcium carbonate from marine water requires the coral polyp to produce large amounts of energy to drive the process. Cold-water corals derive this energy from capturing and consuming zooplankton and other dissolved organic matter carried on the ocean currents.18 Tropical waters, however, are more barren in terms of readily available food for the corals to capture. Consequently, these warm-water corals have developed a symbiotic relationship with tiny plants called ‘zooxanthellae’, which live in the coral’s cells. The noted shortfall of food is then compensated for by the transfer of energy-rich organic compounds (e.g. sugars, carbohydrates, amino acids) from the zooxanthellae. This energy is produced by the zooxanthellae through photosyn-thesis.19 Marine biologists are therefore able to divide corals between those that are host to zooxanthellae and those that are not. They refer to the former as hermatypic corals and to the latter as ahermatypic. Cold-water corals are all ahermatypic whilst warm-water corals are hermatypic.20

The reefs constructed by ahermatypic corals in the dark, cold waters of the ocean depths attract fish and invertebrates, resulting in localised abundance. For those with the technology and finances to fish at such depths, they are attractive locations. These reefs are, however, very fragile and susceptible to destruction by trawlers and climate change. Recovery of these habitats is then difficult given the timescales over which calcification is possible. Whilst their presence was known to trawler captains and a few scientists in the mid-nineteenth and early twentieth century,21 it has only been since the turn of this century that concern for, research into and action to protect these ecosystems has increased.22

Regrettably, coverage of how international law is lending assistance to such scientific endeavour and protection is not possible in this book. The location of these habitats in parts of the high seas where no one state exercises jurisdiction, creates distinct legal problems in comparison to tropical coral reefs, which are not found in these maritime areas. Space and time constraints mean that this book cannot deal with those legal issues as well. There has therefore been a need to prioritise tropical, warm, shallow-water coral reefs and defer an investigation into cold-water reefs.

Prioritising tropical coral reefs for treatment at this time is justified if a little thought is devoted to the distribution of these habitats. There is a mounting sense around the world that tropical coral reefs will be the first major ecosystem to be destroyed because of climate change unless states agree on effective action against this phenomenon and ensure the health of corals so that they can cope with the inevitable changes that are already taking place.23 Any such loss would have a significant impact on many of the poorest nations. The abundance of life on coral reefs is staggering, given the poor nutrient levels of tropical oceans in general.24 The gross primary production of nutrients in open tropical oceans is estimated as between 18–50g C/m²/yr.25 In contrast, tropical coral reefs are one of the most productive of all marine ecosystems, with gross primary productivity estimated at between 1–5kg C/m²/yr.26 It is this productivity that ensures the survival of many local people.27 What is more, this productivity is practicably accessible for people with limited resources. This is significant given that the people who live in close proximity to tropical reefs, and rely on their economic and life support functions, are predominantly from poorer developing countries.28

3 Reef building and reef distribution

Given this book’s prioritisation of warm-water coral reefs, Mark Spalding’s definition for these habitats can be adopted. Thus when discussing coral reefs in this book, reference is being made to: ‘a physical structure which has been built up, and continues to grow, over decadal time scales, as a result of the accumulation of calcium carbonate laid down by hermatypic corals and other organisms.’29

The most important of the organisms Spalding refers to are coralline algae and the corals themselves. However, whilst coralline algae play an important role in cementing the reef structure together, corals are the principal biological medium through which calcification occurs.30 Corals, utilising the energy supplied through their symbiotic relationship with zooxanthellae, secrete calcium carbonate created by drawing upon the calcium and carbon dioxide held in solution in the ocean. It has been estimated that in so doing, corals remove about 700 billion kg of carbon per year.31

The calcium carbonate skeletons of corals grow at different rates depending upon species, the continued presence of zooxanthellae, age and location. For example, branching corals grow relatively quickly (15cm per year) compared with others types, such as brain corals.32 Such growth is balanced by erosion, both in the form of bio-erosion – which is the action of various organisms degrading the calcareous substrates – and through other forces of nature, for example wave action or storm damage.33 The rubble and sand produced by such erosion either fall into fissures in the coral reef (where they may be cemented into the structure by calcium carbonate produced by algae) or are washed inshore to form beaches and other important coastal habitats. Consequently, the net growth or retreat of coral reefs is very slow and takes place over geological timescales.34

Tropical coral reefs are not distributed evenly or found throughout the oceans (see Figure 1). They predominate in coastal tropical areas, i.e. between latitudes 25°S and 25°N and in two main swathes: (a) the Caribbean and (b) the Western Pacific and Indian Ocean. The factors that limit reef building by corals and therefore determine the distribution patterns of coral reefs are as follows.

3.1 Temperature

Whilst a few corals can survive at lower temperatures, reef building in shallow, tropical marine areas only occurs where the temperature of the water ranges from a minimum of 18°C to a maximum of 30°C.35 This limiting factor explains why coral reefs lie within the 20°C isotherm (i.e. within the boundaries of the tropical biogeographical zone). It also explains, inter alia, why shallow reefs are not found on the west coasts of Africa and Central/Southern America, as these coastal areas are cooled by the action of northerly currents and upwelling of cold waters from deeper waters.


Figure 1 The distribution of coral reefs around the world

Key: Coral reefs are indicated as black areas.

This diagram has been adapted from a map produced by UNEP-WCMC using tools available at

3.2 Light and depth

The availability of light is of paramount importance to the development of coral reefs. Whilst corals can survive for short periods of time without zooxanthellae, it is this symbiotic relationship that is the main source of energy for calcification.36 Insufficient light has the effect of reducing energy supply from the zooxanthellae and accordingly inhibits the ability of corals to secrete calcium carbonate and thus build reefs. Given that light decreases with depth, reef formation is correspondingly limited. Reef building undertaken by warm-water corals therefore flourishes in water depths of less than 25m,37 and ceases altogether beyond 100m.38 Reduction in the intensity of light by sedimentation in the water (turbidity) will logically bring the limits of such reef building closer to the surface.

3.3 Sedimentation

Sedimentation can prevent reef formation in two ways. First, coral reproduction through the production and release of coral larvae depends upon the larvae being able to settle upon solid substrata in order to fix themselves to a firm foundation – something which will not be present if sedimentation covers the sea floor with a fine mud.39 Further, once a colony has been established, subsequent sedimentation may cause corals to become smothered. Corals have a natural mechanism for removing small amounts of sediment (as mucus can be secreted to carry it away) but this response cannot cope with larger quantities since this clogs the corals’ feeding structures.40

3.4 Salinity

Being marine animals, corals require salinity levels that do not differ far from the norm.41 It is for this reason that reefs do not form where rivers discharge fresh water into the ocean. Consequently, reefs do not form on the coast of South America where the Orinoco and Amazon flow into the sea, or on the west coast of Africa where the Congo and Niger discharge fresh water and sediments. On a smaller scale, small breaks in fringing coral reefs can be observed in the tropics where streams or smaller rivers flow into the sea and thus lower salinity levels.

3.5 Low tide levels and exposure to air

The last limit to coral reef development is that of exposure to air, which is in turn linked to the level of the lowest tide. Stark illustration of this factor was provided when reefs around Sumatra and the Andaman Islands were pushed up out of the ocean and died following the earthquake that triggered the tsunami in December 2004.42 That said, corals can withstand short periods of time exposed to the air (1–2 hours) since the previously mentioned mucus mechanism can also provide protection at such times against drying and therefore dying.43

4 Coral reef ecosystems: a marine oasis

Life on earth does not exist in isolation – species interact with each other and their physical environment in order to survive. The term ‘ecosystem’ is used to describe the interactions between biotic (living) and abiotic (non-living) components.44 The term ‘coral reef ecosystem’ in this study is therefore defined by: (a) the community of organisms interacting with and (directly or indirectly) dependent upon each other and the reef environment and (b) the coral reef in which they live. In this way, threats to coral reef ecosystems include threats to the reef itself as well as the corals and other reef inhabitants.

Where conditions are suitable, coral reefs form the foundations for what is one of the most diverse ecosystems on the planet. It is an often repeated cliché that coral reef ecosystems are the rainforest of the sea, yet statistics support such a comparison.45 There are an estimated 4,000 species of coral reef fish worldwide (one quarter of all marine fish species), which is comparable to the variety of birds found in rainforests.46 The total number of species of animals and plants found on coral reefs is estimated at between 600,000 and 9 million.47

Of course, diversity within coral reef ecosystems varies throughout the oceans.48 Austin observes that records of coral diversity show a pattern of concentration centred on South-East Asia (particularly the triangle bounded by Indonesia, the Philippines and northern Australia), in contrast to much lower levels of diversity in the Atlantic and Caribbean.49 The reason for the pattern is principally linked to tectonic and climatic history. Oceans became isolated by the movement of continents and glaciations affected some areas more than others.50

With such an abundance of life found on coral reefs, many species have had to adapt to living within small niches; both physical niches and through focusing effort towards specialised diets. Further, the interactions between the resident species are highly complex. A few of these relationships merit discussion so as to inform this study’s later discussion on human impacts on reef ecosystems.

5 Interaction, predation and grazing upon corals and algae

Corals are in constant competition with each other to dominate space on the reef and receive light. For example, as already noted, some corals grow faster than others, such as the branching corals. Such accelerated growth helps these species to outrun others into the prime positions. That said, the continued existence of the slower-growing massive corals indicates that these species have developed responses enabling them to compete with their faster-growing relatives. One such method is the extension of filaments from the gastro-vascular cavities that are capable of killing tissues of competing coral species in close proximity.51

These competitive interactions between coral species are further complicated by the impact of other species. For example, corals are in competition with other invertebrates, especially algae. Algae are of particular importance to the coral reef ecosystem.52 Red coralline algae secrete calcium carbonate and, as they are spread out over the reef in a thin layer, cement together various pieces of calcium carbonate into the coral reef structure.53 In so doing, the entire reef is strengthened and reinforced. However, if left unchecked, algae can advance over much of a reef, smothering coral polyps and preventing their growth and dispersal.54 This state of affairs is primarily avoided through intensive grazing on the algae by fish and sea urchins who, it is estimated, jointly remove in excess of half of the algal cover on a reef.55 Removing such species can therefore have disastrous consequences for the health of a reef ecosystem.

6 How do coral reef ecosystems help humankind?

For many millennia coral reefs and their ecosystems have been supporting human life in physical, economic and nutritional terms. In more recent times, scientific research and the growth of the tourist industry have also developed around these habitats. Each will be looked at in more detail.

6.1 Fisheries and food production

Coral reef ecosystems have provided crucial protein to generations of humans dating back at least 30,000 years.56 More recently, it has been estimated that fish catch from reefs is 6 million metric tonnes.57 On top of this, an estimated 9 million metric tonnes of shellfish and other molluscs are taken per year in and around coral reefs.58 Charles Sheppard and others estimate the annual value of the fishery to be US$5 billion.59 These figures may be on the conservative side since they do not reflect the additional harvesting of resources through subsistence fishing by local fishers.

The nature of this fishery is quite particular. Most fishing activity around reefs is undertaken by local people, using traditional methods, to support local needs.60 Catch is often multi-species (e.g. groupers, jacks, snappers, puffer fish), partly because of the diversity of life found in coral reef ecosystems, but also because pressures from large local populations mean that sources of protein must be maximised.61 That said, occasionally single species can support dedicated industries, as is the case with spiny lobsters and sea cucumbers.62

Coral reef ecosystems are not, however, harvested purely as a food source. Catching fish, removing pieces of live coral rock and harvesting other ecosystem inhabitants for the aquarium trade is far more lucrative. In 2000, Spalding recorded that 1kg of live fish caught in one island state was valued at US$500 to the aquarium trade, whilst the same kilo would have been worth only US$6 as food.63 Properly managed, such trade can be highly lucrative and sustainable.64 If the revenue produced from trade in other reef products (such as pearls and coral-based jewellery) is factored in,65 clearly coral reef fisheries are extremely beneficial to humans.

6.2 Genetic resources and bioprospecting

Natural ecosystems are a valuable resource to medical and scientific research. Given that knowledge on coral reef ecosystems only began to develop in the latter half of the last century, the full potential of these ecosystems to science and medicine is only now beginning to be recognised. As Spalding notes, many reef inhabitants have had to develop diverse forms of defence within the complexities of the ecosystem against a broad range of predators, and this has driven the development of biochemical compounds in numerous and potentially valuable directions.66 Of particular interest to the scientific community are possible alternatives to established (and now weaker) antibiotics that can be derived from toxins found in coral reef inhabitants such as puffer fish.67

Such bioprospecting is controversial. Whilst a need to encourage research and development exists, a balance is called for to ensure source countries receive a fair return from exploitation of their natural resources. This is especially the case where research and development is conducted by drug companies based in other states.68 Further, it is difficult to cultivate these useful marine organisms in captivity, so pressures on naturally occurring stocks to supply potentially large demands raise concerns over sustainability.69

6.3 Coastal protection and land formation

The iconic view of a tropical coast involves beaches of pale sand, with surf breaking in the middle distance. In fact, such views may well owe a debt to the crucial role that coral reefs play in protecting many islands from the force of waves as well as their role in land formation. Corals thrive in moderate wave action. The barrier that forms as the corals and other calcifying organisms lay down calcium carbonate, shields the land by breaking the power and action of those waves.70 Further, the nature of calcification by algae, and the breaking down of coral into small particles by reef fish, is the main source of the sand that washes into the calm waters behind a reef, eventually forming beaches.71

Even in the wake of storms where break-up of the reef structure can occur, the rubble and sand created are often forced up onto the land. This build-up creates higher ground, which over time becomes the substrata upon which vegetation grows and humans can survive. This process of land formation is the very foundation of many small island states.72

6.4 Tourism

According to the World Tourism Organisation, 898 million people travelled to a foreign country in 2007; an increase of around 100 million over two years.73 This promised an increase in tourism revenues of over US$740 million.74 Whilst the economic downturn has had an impact upon the numbers travelling in the following two years,75 tourism remains a significant revenue stream. One area of growth in tourism has come from people wanting to snorkel and dive around reefs.

The number of people diving each year can be difficult to determine with precision. Figures based upon the number of registered divers according to certification agencies such as the British Sub-Aqua Club and the Professional Association of Diving Instructors, merely give an indication of the considerable and growing interest in the sport.76 In addition, many dives go unrecorded as dive operators offer one-off (pre-certification) ‘try dives’ so as to introduce people to the sport. What can be observed is that reef-based tourism, as a result of the growth in interest in diving and the increased affordability of international flights, is expanding and extremely lucrative. In a ten-year period from 1985 to 1995, the number of people visiting the Great Barrier Reef in Australia grew from 1.1 million to over 10 million, whilst the value of this tourism to the same area was estimated at US$700 million in 1997.77 Elsewhere, capacity for tourists in the Sinai Peninsula, Egypt, grew from 1,030 beds in 1988, to over 15,000 by 1998. The Egyptian government has set a ceiling to this capacity at 160,000 beds and this is expected to be reached by 2017.78 Such expansion is linked to the reefs that fringe the shores of the Red Sea and Gulf of Aqaba, attracting many divers from around the world and in particular from Europe.79

Clearly, coral reef ecosystems form a strong basis for tourist developments whether as a destination for divers, or simply for travellers seeking sandy beaches and warm waters.

7 Human impacts

Human impacts upon coral reef ecosystems are varied. They relate to pollution, sedimentation, fishing, climate change and non-fishing-related physical damage. Understanding the nature of these threats helps to focus conservation strategies to tackle each problem.

7.1 Pollution, sedimentation and nutrification

Increased pollution, sedimentation and nutrification attributable to human activity have four negative consequences for corals, reef building and the ecosystem: they may (a) impair photosynthesis, (b) tip the careful competitive balance within the ecosystem against corals, (c) smother coral polyps and (d) harm the reproductive system of corals.

Reference has already been made to the significant role of photosynthesis by zooxanthellae for satisfying the energy requirements of coral polyps. Because of this, light levels as depth increases limit coral density and reef formation. Further, naturally occurring sediment at freshwater outlets reduces light levels and again contributes to the absence of reefs at such points. These are natural limits on the ability of corals to gain energy from the photosynthetic process. However, anthropogenic increases in sediment and pollution can have a similar effect.

Discharging sewage into marine waters is practised around the world. This increases both the level of nutrients found in the water, and particle suspension when the sewage breaks down.80 The latter increases sediment levels in the water in terms of density, whilst the former results in algal blooms and increases in phytoplankton in the water.81 Both inhibit the penetration of light and therefore the potential for photosynthesis.

Increases in nutrients from sewage discharge and agricultural practices in the watershed, can also alter ecosystem community structures. Corals can only maintain a competitive edge over algae when nutrient levels are low.82 As a result, the introduction of more nutrients by humankind’s action tips the balance in favour of fleshy algae. Under such favourable conditions, algae will overgrow and kill the coral, as well as repelling the dispersal and establishment of coral larvae in new areas.83

Reproduction, dispersal and recovery of corals are also hampered by increased pollution and sedimentation. Coral larvae prefer to settle on solid substrata in order to become established.84

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