Throughout earth’s life, Coral reefs have been one of the most important ecosystems of all. It has been said that they are the most productive and diverse communities of the sea. This is because of the massive area they have and the great amount of habitats they host. Some can be thousands of km long, and hundreds of m deep. In fact the Great Barrier Reef (GBR), the greatest of all reefs, stretches for 2000 km across the coast of Australia, in the Pacific Ocean and covers an area of 48000 km2.
The abundance of various animals and sea plants within the communities of coral reefs, have been a food provider for people from an early stage of life till today. In addition, their positioning near the coast protects the shores from wave action. This is because as waves approach the shores, coral reefs act as underwater barriers that absorb wave energy, leaving little energy for the wave to continue and eventually reach the coast line with little impact. These abilities of the coral reefs result in the destruction of the corals and the loss of habitats. To appreciate the importance of these destructions we have to consider the amount of time that the corals need to restructure and the way they do it.
The type of coral reefs that is to be developed depends on factors such as the availability of the substratum, the light levels, depth, salinity and wave action. They usually select sites with rocky substratum and high light levels. Once the upwards development reaches the sea level, at the low tide period, corals start growing sideways until there is not enough light or substratum to support life. The upwards growth of the coral reefs stops because of their intolerance to desiccation and heavy wave action.
Three types of reefs have been described.
- Patch reef: the patch reefs are small and grow in areas with shallow water. These areas are called lagoons and are surrounded by sand dunes.
- Barrier reef: this type of reef runs parallel to a shore. Between the reef and the shore there is a deep lagoon. Barrier reefs have developed through periods of time when the sea level was high. They surround many of the tropical islands, thus protecting them from the strong wave activity
- Atoll reef: an atoll is a ring form coral reef that surrounds a lagoon. They can have a diameter of up to 10 km. There is no sure theory about how these reefs have developed, but the most convincing and yet standing theory is the one of Darwin. Darwin suggested that a volcanic island is firstly colonised by corals. Later on, the island subsides to the sea level where a barrier reef is formed as a continuation of growth of the coral colony. As the sea levels rise again, or further subsiding occurs, a lagoon is formed which is surrounded by a ring of coral reef.
The fastest net growth rate that has been recorded is 20m over a period of 1000 years. Today net growth rate of 3 m per 1000 years is said to be the mean.
Coral reefs are made up of polyps. In most of the phylum Cnidaria and especially the Anthozoa class, polyp is the only life phase present. The polyps are attached to a substratum but do not take up any nutrients from it. They are filter feeders and posses tentacles for that. They mainly depend on a symbiosis relationship with photosynthetic microalgae. This microalgae, usually zooxanthelae, provides the polyps with almost 50% of its energy requirements. In return zooxanthelae gets a safe place to inhabit and photosynthesise. Although zooxanthelae are hosted by the polyps, under periods of stress, like high temperature and excessive UV light, this symbiosis may be interrupted and zooxanthelae will be expelled from the coral. This action is termed as bleaching, and may be very important as it may determine the corals faith. Sometimes bleaching is used for adaptation reasons. If the environmental conditions change and zooxanthellae cannot survive in the new conditions, it is then expelled and another microalgae will be incorporated.
In a healthy environment with no bleaching occurring, polyps reproduce both sexually and asexually. Sexual reproduction occurs when eggs and sperm are released into the environment. This is where fertilisation occurs. Once the zygote has developed into a plantonic larva, it then has the ability to drift over long distances and settle in an area far away from the parental polyps, so it develops. There are two ways of asexual reproduction and both are referred to as budding. This is the intra-tentacular and the extra-tentacular budding. As the names poses, in intra-tentacular budding, the offspring buds off directly from the parental polyp, whereas in extra-tentacular budding, the offspring buds off near the parental polyp, through the network of the colony.
Although coral reefs have a stone-like structure, they are not very strong when it comes to diseases and natural disasters. As mentioned above, bleaching can be dangerous for the corals. Once bleached, a coral may be recolonised by a species of filamentous green algae. These algae will spread and eventually the reef will be appropriated by it. In this way corals lose control of the reef and therefore their growth is inhibited. In this situation, animals leave the area due to the loss of their habitat so eventually the reef loses its biodiversity.
Another way by which the corals lose their biodiversity and may die is with diseases caused by pathogenic bacteria or parasites. An example of such a disease is the Black Band disease. This disease affects the Caribbean’s “brain” coral and is caused by an invasion of the cynobacteria Phormidium corallyticum into the corals tissue. Once inside, it produces a black band that spreads across the whole of the coral at a rate of 1 cm per day. The cyanobacterium produces a toxic substance called anoxia that kills the living coral.
Besides these biological endangers the coral reefs may have, they are indeed vulnerable to physical phenomena, like storms, cyclones and hurricanes. Oceanic storms, cyclones and hurricanes produce strong waves and lots of them. These waves, as described earlier, destroy the coral reefs. One specific genus of coral is well adapted to such kind of conditions because of their high rate of growth. This is the Acropora genus which can be found in the cyclone belt (this is 10-25 degrees north and south of the equator).
Last but not least, the Crown of thorns starfish (Acanthaster planci). This animal feeds on living corals and in abundance they may kill very large areas of corals. 12 to 60 million are the number of eggs that an adult female can produce during a single spawning season. Once fertilised, the larvae lives for 9 to 23 days and then settles. Once settled, the juvenile life is 4-5 months and after that it starts eating the living corals. They invert their stomachs on top of the corals, secrete enzymes to break down the corals tissue into simple compounds, and the nutrients are absorbed.
The catastrophic outbreaks of this species are not understood. Many theories have been implied but none can be applied to all cases. It is thought that not all cases have the same cause. The most believed theory describing the outbreaks is that overfishing reduces the number of predators that feed on the Crown of thorns starfish juveniles, therefore increasing in numbers when these reach the adult stage. These outbreaks happen naturally but not to the extent that they occur nowadays. It is believed that this phenomenon is encouraged by human activities.
Human activities also include water pollution, eutrophication and fishing and they all have an effect in their own way. When untreated sewage or industrial waste is dumped in the sea, it promotes algae growth. Algae take over the reefs and so corals lose control. By means of fishing, the habitats are destroyed, thus also the ecosystem. Because of the reduction in numbers of fish and that not enough time is left replenish through reproduction, after a period of time a reduction in the mean biomass of the fish will be observed. Fishing methods like reef drive netting and trapping, explosions and chemicals have a great impact on the structure and function of the whole reef ecosystem.
Bearing in mind the numerous dangers that coral reefs have to encounter every day,the slow rate of growth, and the importance of them to people, the Australian Government has found a way to preserve them. The GBR that lies just outside their continent is very important to the government and people of Australia for economical, ethical, and scientific reasons.
Because of the great area it takes up, it may host a lot of diverse habitats and therefore a huge variety of plant and animal species. It has been estimated that almost one third of the world’s marine fish species can be found within the GBR. But how are scientists able to get the most out of this barrier reef? It took a lot a planning and rescheduling but finally in 2004 they managed to come across with the greatest plan they could, and it is still running today and doing very well.
Scientists divided the whole area into sectors. Each sector according to its importance is categorised into a zone. There are seven different zones and each one differs from the others in terms of accessibility, activities permitted and many other things (as shown in figure 1). The following zones have been proposed by the scientists: general use zone, habitat protection zone, estuarine conservation zone, conservation park zone, buffer zone, scientific research zone, marine national park zone, preservation zone.
Due to this marine conversation strategy they applied on the GBR, especially the Marine National Park Zone, or ‘no take’ zone as its known locally, and the preservation zones or else known as ‘no entry’, a lot of good has come to the now healthier GBR. Scientific analysis and comparison of current data with data taken from the same areas before the zone planning took place show a difference in the number of population and size of specific target species of fish.
One of the species that shows such change is the coral trout, Plectropomus leopardus. The difference in biomass of the coral trout is not because other sites are overfished, but because of an increase in numbers and size of the fish found in the no take zones. Since fishing is prohibited in these areas, there is no reduction in numbers. As an adult fish increases in size, due to the increased lifespan, it may produce more offspring. In the same way, more offspring survive through juvenile life and then even more offspring are produced from them once the adult stage is reached. This is called a cascade effect. A difference in this species has been also observed between no take and no entry zones.
A similar pattern to the trout fish is also shown for the white tip (Triaenodon obesus) and the grey reef sharks (Carcharhinus amblyrhynchos). The numbers of the two species were approximately 4 and 8 times greater, respectively in the no entry zones than before the zonations occured. The two examples mentioned provide evidence that there may be some zonal regulation compliance problem since no fish is allowed out of any of them. But still there may be a biological reason that smolders, as the chance that permited activities in the no take zones makes the animals to move to a less human active area which are the no entry zones.
A great ecological advantage that zoning had on the GBR is the decrease in outbreakes of the crown of thorn starfish in no take zones. As mentioned earlier, crown of thorn starfish feeds on corals thus destroying them. Is has been the major cause of coral death in th GBR, but the frequence of the outbreakes is now reduced by a factor of 3.75, when comparing open to fishing and no take areas. This has an imediate effect on the growth and biodiverity of corals present in the GBR. The reason of this reduction is not known but it is assumed that through a cascade effect, theabundance of coral trout promotes the abundunce of invertibrate predators of the starfish juveniles.
The concern of the almost extict species has not sliped away from the scientist’s minds. Indeed the dugong (Dugong dugong) and the marine turtles that inhabit GBR have been monitored and their abundunce has been studied. Because they are only found in the GBR, the small numbers of offspirng reproduced, and the mobility over areas greater than those of no take zones, these species are considered to be at a serious risk. Although the 2004 rezoning plan protects almost all habitats of high and low conservation value for the dugongs, it has been suggested by aerial suveys that the numbers of the species are so small that even with no human activity it is difficult for them to recover in numbers.
This increase in biomass, habitat and generally all biodiversity, gives Australias government and people a really good reason to keep the GBR healthy. This is becase of its great economical value. The government has an increasing annual income of A$5.5 billion, which is the equivalent of UK£3.44 billion, from use values only. The zoning plan and the whole of the Marine Park provides a full time job to 53000 people. The income from tourism of the GBR is estimated to be 36 times greater than commertial fishing and so on. For these reasons local communities enhance the zonation of the GBR and are also informed about it through television, radio, newspaper and any other possible way.
Besides the great effort that scientists have put into the zonal planing, the amount of no take zones seems to be insufficient, so that fish abundunce reaches an undepletiable level. Although there are some things that may be done to improve the biodiversity of the GBR. One of these is the monitoring of compliance. This will provide the scientist with a data base of target species and then they will be able to improve the manegment on the zoning. But overall, the GBR Marine Park zoning plan has a great impact on all aspects of life. From increasing marine biodiversity and protecting almost extinct species, to the great economic value it has both for the government but mainly for the locals.
References :
Kaiser J.K. et al. (2005). Coral Reefs. In: Marine Ecology (340-67)
Jenkins, S. (unpubl.) Organismal Diversity. Lecture course notes. School of Ocean Science, University of Wales, Bangor.
Australian Government. (2010) Great Barrier Reef Marine Park Authority. http://www.gbrmpa.gov.au/corp_site
Proceedings of the National Academy of Science of the United States of America. (2010). Adaptive management of the Great Barrier Reef: A globally significant demonstration of the benefits of networks of marine reserves. http://www.pnas.org/content/107/43/18278.full
Photograph refference:
Figure 1. edited from a map form http://www.gbrmpa.gov.au/corp_site/management/zoning/zoning_maps.html
Photograph of the GBR. http://4.bp.blogspot.com/_s5XfwPSThU4/TE6AeXEi_LI/AAAAAAAABPE/YpaEm-TtelI/s1600/Great+Barrier+Reef.jpg
Black Band Disease. http://www.gather.com/viewArticle.action?articleId=281474977137941
Map of the GBR. http://www.divetheworldliveaboards.com/map-great-barrier-reef.php