Coral Reefs in a Changing World

Coral reefs are one of the most diverse and important ecosystems on our planet, providing numerous ecosystem services and serving as a home to a wide range of biodiversity. In my previous article, I explored the significance of coral reefs and how they are constructed. Although my main goal with this blog is to highlight the beauty of nature, I think it´s important to understand the challenges that coral reefs are facing nowadays. Therefore I would like to elaborate on how climate change impacts the health of coral reefs in this blog post. So what happens to coral reefs in a changing world?

Locations of Coral Reefs Around the World

To begin with, here is an overview of the affected ecosystems: Coral reefs are located in oceans all around the globe, primarily in shallow, warm waters near the equator. Some of the largest coral reefs are the Great Barrier Reef on the Australian coast and the Red Sea Coral Reef near Israel and Egypt. Moreover, the largest reef in the western hemisphere is the Mesoamerican Reef along the coasts of Mexico, Belize, and Honduras. The picture below shows the Coral Triangle in the Western Pacific ocean. With 600 different types of reefbuilding corals and around 2000 species of reef fish it is a global hotspot for biodiversity. ^{1 2}

Coral and Algae: A Symbiotic Relationship for Survival and Color

To understand what threathens corals is helpful to take a closer look at their way of life. Corals contain zooxanthellae, a type of algae with which they live in symbiosis. To put it simple, a symbiosis is a relationship offering benefits for both partners. On one hand, the coral protects the algae through its rigid calcium carbonate shell. On the other hand, the algae provide corals with food products from their photosynthesis. In addition, Zooxanthellae are also responsible for the coral’s vibrant colours. It seems like a good trade. ³

How can algae make corals colourful? Zooxanthellae are responsible for the colour of corals because they contain colour pigments. Consequently, the colouration is influenced by the density of zooxanthellae in the coral’s tissues and the conditions under which the coral is growing. These pigments protect the algae itself from excess light and harmful UV radiation. ⁴

The Threat of Coral Bleaching to the Zooxanthellae Symbiosis

This symbiosis between corals and algae is a fragile relationship, depending on the surrounding water´s temperature. When the water gets to warm due to climate change, corals expel zooxanthellae from their tissue. In this process which is referred to as coral bleaching, the coral loses its major source of food and with it its colour. Altough, corals do not always die during coral bleaching, they need cooler water temperature to recover their relation with zooxanthellae over time. ³

The Impact of Ocean Acidification on Coral Reefs

Another phenomenon that is making life difficult for corals is ocean acidification, which is caused by rising carbon dioxide levels in our atmosphere. Measurement for acidity is the pH scale. The lower the pH, the more acidic the water. As a result of high CO2 concentrations in the atmosphere, the oceans absorb carbon dioxide to create an equilibrium. In interaction with water, carbon dioxide forms carbonic acid. Consequently, the pH of the sea water gets lower. ³

As the pH lowers, the availability of carbonate ions decreases. However, carbonate serves as a primary building block for calcium carbonate, implying that corals have problems building up their skeleton. Corals may already dissolve at atmospheric CO2 concentrations of 560 parts per million, according to new research. If emissions are not reduced, this concentration could be reached by the middle of the century. ³

Important to remember from this section is, that rising ocean temperatures and acidification will almost certainly cause “major changes to coral reefs over the next few decades and centuries” (National Geographic, s.a.). ³

Conclusion

To sum up, there are two main problems coral reefs are facing in our changing world. They are threatened by rising ocean temperatures and acidification due to climate change. Corals have an intimate relationship with zooxanthellae, a type of algae that provides food and colour to the coral. As a result of rising water temperatures, the coral expels the algae from their tissue, leading to coral bleaching. Moreover, Ocean acidification reduces the availability of carbonate ions, causing problems for corals building their skeleton. By understanding the challenges faced by coral reefs and taking action to protect them, we can ensure that these incredible ecosystems continue to thrive for new generations.

References:

1. WWF, s.a. Coral Triangle. Available at: https://www.worldwildlife.org/places/coral-triangle [Accessed on 19.03.2023].
2. GCRMN, 2021. Coral Reef Regions. Available in: https://gcrmn.net/regions/ [Accessed on 10th February 2023].
3. National Geographic, s.a. Coral Reefs. Available at: https://education.nationalgeographic.org/resource/coral-reefs [Accessed on 6.02.2023].
4. NOAA (National Oceanic and Atmospheric Administration), 2021. What is coral bleaching and what causes it? Available at: https://oceanservice.noaa.gov/facts/coral_bleaching.html [Accessed on 10.02.2023].