Coral Reef Bleaching: Human Threats to Coral Reefs

Coral reefs are often regarded as the most complex and diverse assemblages of fauna and flora in the oceans. They are important for numerous reasons; they provide a livelihood to millions of people, are a major source of food to humans, and they have a significant role in the survival of the rest of the ecosystem. However, despite occupying the oceans for 500 million years, their position is being threatened largely by a host of human-influenced activities. Coral bleaching is the main threat to coral reefs, caused when the symbiotic zooxanthellae living within the corals become expelled. Climate change is regarded as being largely to blame for the increase in bleaching, as it is thought to lead to warmer oceans, and more extreme environmental conditions. Other human factors such as overfishing, pollution, sedimentation and weapons testing are also thought to be having detrimental effects on the reefs. If steps are not made to protect the reefs, their distribution and the diversity of coral organisms will be severely reduced. I suggest further studies into coral resilience involving different species of zooxanthellae with varying resistances to environmental stresses, in order to determine if recolonisation of bleached corals can readily occur. Other suggested studies will look into changes in management practices to aid in the preservation of coral reefs.


Keywords: coral reef; bleaching, anthropogenic; zooxanthellae; resilience; management

Coral-Zooxanthellae Symbiosis - Costs

Despite the clear advantages obtained by the symbiosis, there are several costs induced from it. The coral sustains costs due to investment of resources in a system to expel algae if numbers exceed the optimal level or if environmental conditions are not favourable. Also the excretion of perialgal vesicles containing nutrients destined for the host cells uses resources. For the zooxanthellae, regulation of growth rate occurs as a result of the symbiosis as growth rate is slower when residing within coral than when in the coccoid stage. Expulsion from the host is also costly, as well as the costs involved in transport of carbon to the host, limiting its own usage. In order for the symbiosis to be a mutually beneficial one, the benefits must outweigh the costs for both members. If this is not the case, the symbiosis is likely to collapse, with the zooxanthellae being expelled or leaving the coral. A host of environmental factors act on members of the symbiosis, with extreme conditions potentially leading to an unbeneficial symbiosis and loss of zooxanthellae from the corals.

Coral-Zooxanthellae Symbiosis – Environment: Light & Water Depth

The productivity of the zooxanthellae is also largely dependent on environmental factors including, significantly, the amount of light penetrating the corals and reaching the dinoflagellates. Productivity is proportional to increasing light intensity, although it peaks when productivity becomes independent of light intensity. This explains why corals only inhabit relatively shallow coastal regions – light intensity in deep water is not great enough to permit photosynthesis within the zooxanthellae and in turn the corals themselves cannot be supported. However the depth that corals can survive is also dependent on the clarity of the water. Corals are found living at a depth of 100m in reefs in Jamaica for example, but only 12m in certain reefs in Puerto Rico. Different species of zooxanthellae are however adapted differently to cope with light intensities in varying habitats, for example zooxanthellae that specialise in slightly deeper water have more photosynthetic pigments in order to absorb more light for photosynthesis.

Coral-Zooxanthellae Symbiosis: Conditions for Optimal Productivity

Temperature is also a key environmental factor influencing the productivity of zooxanthellae, as fluctuating temperatures affect their metabolic rate. This is mainly due to the effects temperature has on both the rate of photosynthesis and respiration. On studies of A. palmata corals in Jamaica, it was found that growth rates were proportional to the increase in sea surface temperature (SST). Therefore temperature also affects the corals and temperatures outside the tolerant range of a species of zooxanthellae may cause loss of the algae from the coral, leading to possible coral death. As with light, different species may be physiologically adapted to cope with varying temperature ranges, depending on their habitats.

In order to achieve a stable symbiosis between coral and zooxanthellae, the density of zooxanthellae within the coral must remain fairly constant. Usually this process is self-regulating as zooxanthellae are expelled if their reproduction rate exceeds that of the coral, and vice versa. Environmental cues are central in determining a stable symbiosis.

Threats to Coral Reefs - Human vs Natural Causes?

There are many threats to coral health, both natural and anthropogenic. But it is thought that human activity poses a much greater threat than natural causes. Factors such as fishing, sedimentation and pollution all have an impact on coral health. However here I will look in detail at coral bleaching, perhaps the biggest threat to coral health, a process generally regarded as being influenced by human behaviour. I will also discuss in less detail the other main human threats to corals.

Coral Reef Bleaching - What is Coral Reef Bleaching?

Coral bleaching is the term given to the event where the zooxanthellae residing within the corals die or leave the cells, either by expulsion or by their own accord. The coral appears bleached because of the loss of pigmented zooxanthellae. The corals themselves have no colour as the underlying calcium carbonate framework beneath and surrounding the polyps is a white colour, and the polyps on the surface layer are translucent. Bleaching can also occur when the zooxanthellae simply lose their pigment but remain in the host cells. Figure 4 shows a coral in the process of bleaching; the central polyps have lost their zooxanthellae but some outer polyps are still healthy, with a yellow-green colouration. The phenomenon occurs at times when the corals are under unsustainable environmental stress. Environmental factors include extreme temperatures, UV radiation, aerial exposure, nutrient imbalance, sedimentation and chemical contamination.

Coral Bleaching - A Major Threat to Coral Reef Health

Coral bleaching is often regarded as one of he greatest threats to the health of coral reefs. Due to the relatively fragile nature of the coral-zooxanthellae symbiosis, a small change in environmental conditions can cause the symbiosis to collapse and the zooxanthellae to be expelled, leading to bleaching. Although corals can usually recover over a period of years, by re-forming a symbiosis with zooxanthellae, in extreme cases the coral polyps will die which can have more serious knock-on effects for the whole ecosystem. Coral bleaching events have been occurring since records began, but what is worrying is that bleaching events seem to be occurring at a more frequent rate than in the past. The highest SSTs ever recorded were in 1998, and with this the most widespread coral bleaching events recorded. The rising SSTs over the last fifty years correlate with the amount of bleaching events being seen around the world, and in turn subsequent death to the coral. Figure 5 shows recent bleaching events, the majority took place in the West Atlantic and East Pacific.