Ecology of Coral Reefs Review

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Table of Contents

Introduction
Ecology of Coral Reefs
Formation of Coral Reefs
Threats to Coral Reefs
Summary and Conclusion
References

Introduction

Earth looks like a blue ball from the moon because over 70% of the Earth’s surface is covered with water (The National Energy Foundation, 2001). If we look into the oceans and the organisms living in it, it can be said that thousands of meters below the surface of the ocean is in complete darkness and is sparsely populated. However, there are also places in the ocean such as the seafloor slopes up toward the continental shelf and the oceanic islands where the marine life is concentrated due to the availability of sunlight and nutrients. In some hard bottom areas where conditions are right, the seafloor is colonized by a variety of tiny colonial invertebrates known as corals. It is the most beautiful part of the world and is developed over a million of years. The tiny organisms have created these enormous underwater structures that provide a dwelling place for a number of marine organisms.

As of now the scientific community has identified this highly complex interdependent system of coral reefs supports over one million species. However, they also suggest that there are potentially millions more yet to be identified. There is great significance of reefs in addition to their importance for biodiversity. Coral reef ecosystems are important for performing vital ecosystem services such as supporting major fishery resources, they are also important for educational, social, recreational, cultural, and medicinal opportunities. Additionally, they are a great source of income and generate economic benefits for millions of people, particularly through coastal tourism (Waddell, 2005). For instance, the economic benefits from the Saipan’s coral reefs come from the five major sources i.e. tourism, recreation, fisheries, coastal protection, amenity and research that accounts for nearly $ 61 million annually (See Figure 1) (Beukering et al. 2006). This value is just from one coral reefs in Saipan. The total world economy of coral reefs is huge.

Figure 1: Total economic value of Saipan’s coral reefs

Today, these ecologically significant structures and the organisms that it supports are in great threat. The data generated by the scientific community show decline in the area of coral reefs particularly due to the anthropogenic factors. This paper discusses the ecological importance of coral reefs and the impact of various human activities that is causing destruction of these scenic wonders. Therefore, the economy form the reefs are also at threat.

Ecology of Coral Reefs

Coral reefs occupy less than 1% of the ocean floor but are among the most biologically diverse ecosystems in the world (See Figure 2). The main reason for the limited distribution of coral reefs is due to the narrow physiological tolerances of hermatypic, or reef-building, corals. It is estimated that most of all coral reefs are found throughout tropical and subtropical oceans between 30ºS and 30ºN latitude (Huston, 1985; Grigg and Epp, 1989). Tropical corals do not grow at depths of over 50 m (165 ft). The coral reefs are the home for at least 25% of all marine species. Scientific community estimates that almost 32 of the 33 animal phyla are found on coral reefs where as only nine are in tropical rain forests (UNEP 2003).

Figure 2: The red colored areas represent the world’s coral reefs and the intensity of the green color indicates the national per capita GDP.

In addition to the temperature and the sunlight, their distribution is also influenced by nutrient availability, salinity, substrate, sediment type, and exposure to wave action (Wolanski et al., 2003). It is estimated the ideal seawater temperatures in coral ecosystems generally range between 18°–29°C (Barnes and Hughes, 1999). However there are some exceptions to this as there are some corals that seem to have adapted to tolerate slightly higher temperatures for short periods of time. It is also found that most of the organisms living in coral reef ecosystems are photosynthetic and require good sunlight for their survival (Waddell, 2005). Additionally, these ecosystems do not need much of nutrients in the water for survival. The presence of high nutrient levels such as those coming from the runoff from agricultural areas can harm the reef by encouraging the growth of algae (ARC Centre of Excellence for Coral Reef Studies, 2007).

Formation of Coral Reefs

The beautiful structures or the building blocks of coral reefs are composed of calcium carbonate (limestone). Corals consist of a group of small, tropical marine animals that attach themselves to the seabed and form extensive reefs. These reefs are made up of the calcium-carbonate skeletons of dead coral animals. Additionally, there are several other organisms living in the reef community that contribute their skeletal calcium carbonate that add on to the reefs (Advameg Inc. 2008).

One of the most important organisms that contribute to the structure is the coralline algae. In fact these organisms are important contributors to the structure of the reef especially in those parts that experience the greatest forces by waves. In addition to the protection from the waves these algae also contribute to reef-building by dumping limestone as sheets over the surface of the reef. Other significant organisms that contribute to the structure are the stony corals, colonial cnidarians as their secretions as an exoskeleton of calcium carbonate adds to the strength. The accumulation of skeletal material, broken and piled up by sea waves and bioeroders, results in the formation of massive calcareous formation that supports the living corals and a variety other organisms.

Sunlight plays an important role in the photosynthesis under water. As mentioned earlier the reef-building or hermatypic corals are only found in the photic zone which is above 50 m depth. Though the coral polyps do not photosynthesize, they have a symbiotic relationship with single-celled algae called zooxanthellae. These algal cells inside the tissues of the coral polyps carry out photosynthesis and produce excess organic nutrients that are further utilized by the coral polyps for their metabolism. As a result of this symbiotic relationship, coral reefs grow much faster in clear water where sunlight penetrates easily.

It is estimated that in the absence of zooxanthellae, the coral growth would be very slow for the corals to form remarkable reef structures. Further, studies also prove that corals can get up to 90% of their nutrients from their zooxanthellae symbionts (Marshall and Schuttenberg, 2006). Even though coral reefs are present in the nutrient deficient tropical waters, it supports an extraordinary biodiversity. In fact the uniqueness of this ecosystem is that there is efficient nutrient cycling and recycling between corals, zooxanthellae, and other reef organisms.

Cyanobacteria help in nitrogen fixation in these ecosystems and also provide soluble nitrates for the coral reef. Corals absorb nutrients such as the inorganic nitrogen and phosphorus, directly from the water. These corals feed upon zooplankton that is brought in by the water motion (Peter and Huber, 2000). Interdependence of coral reefs with the surrounding seagrass meadows and mangrove forests can easily be noted as the coral reefs depend on the nutrients supply from the dead plants and animals brought in by the waves and in turn these ecosystems seek protection from violent storms. Besides, they also produce sediment for the mangroves and seagrass to have a good rooting system.

Threats to Coral Reefs

According to a study conducted by Wood (1998) there are several groups of extinct and extant organisms that have combined over 3.5 billion yr (Ga) to form the modern day reefs. In fact, the study points out that most of these communities over the years grew under ecological and environmental conditions that are greatly different from those of the present day coral reefs. Additionally, there is also drastic change in the global distribution of reefs over the geological time.

Anthropogenic activities are a major threat to these unique and most beautiful ecosystems. The problems due to climatic changes and global warming are also major problems. Human beings are involved in coral mining, polluting these ecosystems with organic and non-organic/chemicals, over-fishing, and also by the construction of canals and access ways into islands and bays. Excessive tourism and commercialization of these ecosystems for selfish desires also create problems. Eutrofication or excessive addition of nutrients such as phosphates and nitrates disrupt the delicate balance of the reef communities (globalcoral.org N.D.).

Recent decades has witnessed the mass bleaching of coral which is commonly termed as the ‘coral reef crisis’ (Bellwood et al., 2004). This crisis has caused over 90 per cent coral mortality to almost 16 per cent of the world’s coral reefs in just 1997-98 (Wilkinson, 2002). Though the treat was recognized in the initial stages and halted the destructive procedure, the repair of the already caused damage may take decades (Wilkinson, 2004). Today the need for a management response is well established (Buddemeier et al. 2004). Studies also have projected that the future increase in sea temperature may also exceed the bleaching thresholds making it even more difficult to the survival of corals (WWF, 2004). Coral bleaching is influenced by two main factors, i.e. the sea temperature and the resilience of coral reef ecosystems (West and Salm, 2003). High nitrate levels are especially toxic to corals and presence of phosphates slow down the growth of coral skeleton. Besides, the presence of infectious agents as a result of poor water quality encourages the spread of infectious diseases among the corals (Nowak, 2004).

Global warming is yet another major threat faced by the entire global community. Similarly the increase in temperature increases the sea level. As a result of this the coral need to grow faster to get the required sunlight and continue their activities. Studies have found that as the sea temperature increases, there is high amount of disturbance in the coral. There is good evidence of this as during the 1998 and 2004 El Niño weather phenomena, the sea surface temperatures rose well above normal resulting in the killing of many tropical coral reefs. Global warming is also resulting in the increase of diseases such as the Black band disease and White band coral disease. It is estimated that with every 2 degree Celsius rise in temperature it is improbable that coral will not be able to adapt enough physiologically or genetically to keep up with climate change (Glynn, 1993).

There are also several other related problems to global warming such as acidification cause by the increased emission of carbon dioxide. Increased acidification is a major cause long term concern for coral reefs (Kleypas et al. 2006). Since oceans act as a major carbon dioxide sink, the increased atmospheric CO₂ increases the amount of CO₂ at react with water to form carbonic acid, resulting in ocean acidification. There are several studies that point out that corals experience reduced calcification or enhanced dissolution when exposed to elevated CO₂. It is also found that decreased carbonate ion concentration considerably reduces the ability of corals to produce their calcium carbonate skeletons and disturbs the natural balance between reef building and reef erosion (Kleypas et al. 2006).

Over-fishing for commercial purpose, unregulated tourism, and bleaching due to climatic changes are other major threats to coral reefs. For instance, studies have found that the Southeast Asian coral reefs are at risk from damaging fishing practices. These are some of the most important tourist spots and over-fishing for sports as well as commercial purpose is very common. For instance, in Indonesia coral reefs have been victim to destructive fishing, unregulated tourism, and bleaching due to climatic changes.

According to studies conducted by Hughes et al. (2003) it is said that human activity threatens coral reef ecosystems worldwide through overfishing, marine pollution, and disease. Recent years have witnessed the change in climate and warming of ocean. This causes mass coral bleaching and death of corals (Donner et al. 2005). In future the acidification of oceans due to excessive carbon dioxide can complicate the survival of these beautiful ecosystems further endangering coral reef ecosystems (Kleypas et al. 1999). Further, it is projected that the increase in costal population and continuous rise in sea temperature is bound to have adverse impact on the reefs (Wooldrige et al. 2005).

Summary and Conclusion

Due to the unique complexity and diversity, coral reefs can provide food and livelihood opportunities to millions of people. These have great economic and ecological benefits. It is important to protect these environments and provide congenial conditions for their growth and development. Coral reefs are most vital and hold great significance as they hosts wide range of organisms that may hold the key to future medical advances. Biodiversity also plays an important role in conservation of coral reefs. For instance, studies point out that genetic composition of both the coral animal and its symbiotic zooxanthellae influence resistance to bleaching (Baker, 2004) and also help in recovery from bleaching (McClanahan et al. 2004). It is also essential that all development in these regions should be sustainable and should not impose any harm to the coral reefs. As of now it is estimated that about one third of the world’s coral reefs have been destroyed or highly degraded.

In order to find answers for these problems, researchers are currently working to determine the degree various factors impact the reef systems. However, there is still better scope for researchers in this field. Priority is to combine and disseminate existing knowledge of biodiversity, promote institutional cooperation and introduce and enforce policies that will help to minimize, if not eliminate, risks posed by human activities coral reefs. This will not only help the present generation but also the future generation. Therefore, it is our duty to create awareness among the public to protect the coral reefs. Additionally, the development and enforcement of new conservation policies based on the principles of sustainable development could bring about sustainability in these ecosystems. These fragile ecosystems are among the most unique ecosystems on this planet that holds unique significance. It is the duty of each and every individual to protect these environments through sustainable development.

References

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