Over the past century, processes including an increased global level of industrialization and a boom in fossil fuel consumption has led to changes in atmospheric chemistry. Greater levels of CO2 caused by these processes have meant the warming of the Earth’s atmosphere, leading to increasing global temperatures. The increasing levels of CO2 also present further changes in the marine environment through absorption to seawater. One of the major groups of organisms effected by these changes in chemistry are calcifying organisms, in particular corals.

Corals (Phylum Cnidaria) are colonial organisms which contain symbiotic zooxanthelle. They are key players in the formation of reef structures in tropical and cold water environments and furthermore, provide a diverse habitat for many invertebrate and vertebrate organisms. Creating a skeletal structure from calcite and aragonite fibers, means that changes in oceanic conditions present problems various problems for coral species. Different ecological responses have been displayed by corals to changing environmental conditions over recent decades. Highly sensitive and specialized to their habitat, even small changes with regards to temperature and pH have the ability to cause catastrophic effects within coral reef strucutre. These effects stretch beyond the coral themselves and on to the whole ecosystem.

Tropical coral reef at Palmyra Atoll. Credit: Jim Maragos/U.S. Fish and Wildlife Service

Effects of temperature change

The preferred temperature of a coral and its range is dependent on its geographical location, tropical corals have a warmer preferred temperature and cold-water corals prefer colder temperatures. Due to climate change the average global temperature has risen by roughly 1°C and is expected to rise to almost 2°C by the end of the year 2100. Meaning therefore, that the temperature of seawater has and will continue to increase over the coming decades. This is bad news for corals as they suffer greatly due to their failure to adjust to temperature ranges outside of their normal temperature boundaries. The main consequence of poor adaptation to changing temperatures is coral bleaching.

What is coral bleaching?

Coral beaching occurs when photosynthetic symbionts (zooxanthellae) are intolerant to changes in thermal conditions within the environment. This causes them to be expelled from the coral body. Zooxanthellae are essential in the process of gaining nutrition photosynthetically for the coral, in return the coral provides them with a place to inhabit, existing mutualistically. As zooxanthellae are photosynthetic, they provide the corals with vibrant colors. A colorful reef is typical of of a healthy reef. When corals have bleached they appear colorless. Due to lacking any symbiotic organisms the coral has a reduced overall productivity and can result in the death of a coral. This however does not rule the coral out for good, they are often re-inhabited after a while by more tolerant zooxanthellae.

Bleached coral colonies were observed during towed-diver surveys around Howland Island during the 2010 Pacific RAMP expedition. Credit: NOAA photos.

Effects of changing pH

Carbon dioxide is a soluble acidic gas. Therefore, as it is absorbed into the oceanic sink via seawater, it can reduce the overall pH of the oceans. This process is called ocean acidification. This rise in acidity levels causes greater stress on corals, which rely on calcium to provide support in their skeletogenesis process. Compared with pre-industrial times, it is expected we will see a decrease in levels of calcification in corals by 10-50% by the year 2050. By this time it would mean that corals would be under-saturated with aragonite. It should be noted that these changes only represent a change in pH of roughly -0.6 from a pre-industrial era until the end of the 21st century which doesn’t seem significant. Within the marine environment however it indicates a vast change in conditions. These rapid changes combined with a low adaptability may result in coral bleaching events, reducing the overall productivity of the coral and having further implications for the whole reef. With a greater concentration of carbon dioxide dissolving into seawater, it means that there would be a larger uptake of the calcium ions present. This means a reduced amount available for usage by calcifying organisms, overall leading to a weakened structure of the coral. Ocean acidification therefore, reduces the rate of growth in corals and also weakens the structure, potentially resulting in death of the coral.

 

Due to the high level of sensitivity in response to environment conditions, the main effects of climate change only have effects detrimental to the health of corals. This also has further implications within the environment due to the high diversity of life sustained by coral reefs. Unless climate change is tackled on a global scale the chance for coral reefs to remain as healthy, species rich, diverse habitats is not bright.

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