What is coral bleaching?
The skeletal structures of hard corals are white but they contain zooxanthellae algae (tiny plants called dinoflagellates that reside in the soft tissues of corals) which make them appear coloured. The zooxanthellae are photosynthetic and their relationship with some corals is an example of endosymbiosis. The coral polyps benefit from the products of photosynthesis and in turn the algae benefit from the waste products (nitrogen, phosphorous and carbon dioxide). If the rate of photosynthesis is too high, corals are able to release some of the zooxanthellae in their tissues, which is known as coral bleaching and it is normal. However, if corals are ‘stressed’, they can expel more zooxanthellae than is necessary which causes them to lose their colour and can have negative effects on the coral.
Coral bleaching as a result of ocean warming
Most evidence suggests that the main cause for corals to become stressed, and therefore bleached, is ocean warming- as a result of global warming. Ocean warming is directly reducing coral cover throughout the world’s oceans because of mass coral bleaching. Correlative field studies have named warmer than normal conditions as being responsible for prompting mass bleaching events. Glynn (1984, 1988) was first to provide significant evidence of the relationship between mass coral bleaching and reef mortality and higher than normal sea temperatures. Glynn (1993) indicated that 70% of the many reports of coral bleaching were also associated with reports of warmer than normal conditions. Goreau (1990), Glynn (1991) and Hayes and Goreau (1991) suggested that future increases in sea temperature, associated with global warming, were likely to push corals beyond their thermal limits. The association of bleaching and higher than normal sea temperatures has become even stronger with an increase of correlative studies from different parts of the world (for example, Goreau et al., 1993; Goreau and Hayes, 1994; Hoegh-Guldberg & Salvat, 1995 and Brown, 1997). These studies show a firm association between warmer than normal conditions (at least 1°C higher than the summer maximum) and the occurrence of coral bleaching.
What are the effects of coral bleaching on coral reef ecosystems?
The main effects of mass bleaching on coral reef ecosystems are losses to coral cover and biological diversity. There are many examples of coral loss on individual reefs and, by combining data from many sources, we can gain a better understanding of how coral cover has changed at regional and global scales over time. A number of these ‘meta-analyses’ have been performed by putting together survey data from scientific literature and unpublished data from governmental and non-governmental monitoring programs. The result found is extensive coral loss, even on some of the world’s most isolated and intensively managed reefs, for example, in the Maldives in the east Indian Ocean, where bleaching has caused coral cover to decline to only about 5%. Coral cover in the Florida Keys had also declined to only ~ 8% in 2006 and there have been comparable losses documented throughout the Caribbean. The Pacific generally has higher coral cover than the Caribbean, however, an unpublished analysis of recent survey data indicates that Pacific coral cover is roughly 30%, probably around half of what it was several decades ago (Mackenzie, 2001).
Coral loss has the greatest effect on fishes that depend on corals for food or shelter and these fishes may face considerable risk of extinction with increasing frequency and severity of bleaching. When corals die, the abundance of reef fish quickly decreases, mainly due to the lack of places for larval fish to settle as they leave the open water and settle on the reef where they will spend their adult lives. An example of this can be seen in a study by Jones et al (2004) in Papua New Guinea, looking at the effects of coral degradation on reef fish assemblages. The study showed a parallel decline in reef fish communities with declines in coral cover; reef fish communities were greatly impacted by coral loss and some corals went locally extinct. The study also showed that fish that were dependent on corals as juvenile recruitment sites were more likely to be affected by coral degradation over the long term. Reefs and fish communities both inside and outside of marine reserves were studied and the trends were found to be the same for both; regardless of protected status within a reserve, fish communities will not thrive without having access to healthy coral communities.
Loss of coral cover leads to a loss of biodiversity by destroying the habitat needed for reef-dependant organisms to survive in. Loss of live coral cover causes widespread mortality among fishes and invertebrates dependant on live coral for food or shelter. These effects may be made worse by declines in structural complexity through gradual erosion of dead coral skeletons, which can occur following extensive bleaching. Combined effects of coral loss and structural collapse result in local extinctions for many organisms and dramatic shifts in species composition.
Another problem for coral reefs as a result of climate change is ocean acidification. Through the burning of fossil fuels, the concentration of carbon dioxide in the atmosphere is rapidly increasing. Approximately a quarter of this CO2 enters the ocean and reacts with water to form carbonic acid, which cause the ocean to become more acidic. The pH of the ocean has already decreased by about 0.1 unit which makes it more difficult and energetically costly for corals to secrete their calcium carbonate skeleton. Experiments have shown that even slight decreases in pH can cause weakening of coral skeletons and slow coral growth, which can reduce the ability of corals to compete with other species and may lead to corals being less resilient to other climate change-related stressors like bleaching and disease or natural disturbances like storms (Bruno & McGinley, 2008).