What is Coral Bleaching?
Coral bleaching is the effect of coral turning white after getting rid of algae (zooxanthellae) living in their tissues. It is usually a response to stress in their environment, that unfortunately leaves them vulnerable. Although not always fatal, corals can only survive to a certain extent. If the stress factors persist, a bleached coral will eventually die.
For a real-life example, let’s look at some of the causes of coral bleaching in the Great Barrier Reef.
- Rising Water Temperatures: Warmer water places greater heat stress on coral, which can only tolerate temperatures above 18°C (64°F) and below 40°C (104°F); the ideal range for them is from 23° to 29°C (84°F).
- Ocean Acidification: Higher acidity in the water prevents corals from effective calcification, meaning their structures tend to be smaller, weaker, and therefore in poorer health.
- Poor Water Quality: Terrestrial waste run-off significantly affects corals. Experiments have shown that nitrogen pollution from fertilizer or sewage runoff causes and aggravates coral bleaching by disrupting the relationship between corals and algae.
- Coastal Development: As cities grow, so do their impacts on the Great Barrier Reef. Activities like mining, agriculture, shipping, and tourism, all contribute to bleaching. Factories disposing of waste into the water contribute to poor water quality, while boats travelling in shallow waters often scrape corals, damaging them.
Effects on the ecosystem
Bleached corals face reduced growth rates, reduced reproductive capability, increased disease vulnerability, and higher mortality rates. About 60% of the corals in the Great Barrier Reef are bleached, and scientists believe that most of them will not survive (Great Barrier Reef suffers biggest bleaching event yet).
Many species, such as fish, arthropods, and cephalopods, depend on corals for food, shelter and protection. If coral reef systems decline due to bleaching, many of these species will disappear, severely impacting genetic and species diversity. Additionally, coral bleaching can lead to fish behavioural changes. For instance, butterflyfish aggression over food has been observed to be severely reduced after corals bleach. Despite no apparent changes in the species’ abundance, scientists believe this change may reflect reduced food sources forcing individuals to roam farther; the butterflyfish may later experience population decline (Coral bleaching starves fish).
Effects on us
Bleached corals quickly lose their original features, turning into ugly skeletal-white structures! Coral reef tourism is a big industry in coastal countries, like Australia; tourists visiting the Great Barrier Reef generate $6.4 billion a year for local businesses. As corals bleach and lose their aesthetic value, fewer tourists go, leading to an economic decline (Great Barrier Reef tourism income).
Over 500 million people worldwide, including Australia, depend on fish for food and their livelihoods. When fish populations decrease due to coral bleaching, the global fish industry is impacted and more vulnerable to additional stress factors. For example, fisheries and aquaculture production value around the world declined by 12% from 2019 to 2020. Although the decline may be attributed to the impacts of COVID-19, bleaching corals are only one more contributing stressor. (Australian fisheries and aquaculture outlook 2020)
Coral reefs are a valuable source of medical compounds. Antiviral drugs Ara-A (vidarabine), AZT (zidovudine) and the anticancer drug Ara-C (cytarabine) are some of the earliest modern medicines extracted from coral reefs (Life-Saving Products from Coral Reefs). With bleached or dead reefs, scientists would lose valuable sources for powerful drugs that could treat many illnesses.
What can you do about coral bleaching?
- Avoid directly touching corals!
Oil from your skin disturbs the fragile mucous membranes that protect the corals from disease, which can lead to coral bleaching and even the death of an entire colony!
- Avoid leaving any garbage during your visit to corals!
Garbage like plastic bags, plastic water bottles can cover on top of corals and block the sunlight they need for photosynthesis. They can also entangle them and kill reef organisms and break or damage parts of the corals.
- Avoid buying reef-inhabiting fish!
If you are determined to have a fish as a pet, make sure to ask the staff at the pet store where the fish you want is from, or do your own research. Fish catches to meet the demand for reef fish pets have caused significant biodiversity and population loss from coral reefs worldwide.