Can Corals Recover from the Effects of Climate Change?

Reference: Evans, Richard D., et al. “Early recovery dynamics of turbid coral reefs after recurring bleaching events.” Journal of Environmental Management 268 (2020): 110666.

Coral reefs are cherished around the world for their beauty and their ability to provide for a variety of animals. From Australia’s Great Barrier Reef to Mexico’s Palancar Reef, divers, snorkelers, and beach-goers of all kinds are attracted to these underwater habitats. Unfortunately, many coral reefs have been shrinking in size due to coral bleaching.

A diver surveying a coral reef that has extensive bleaching in Hawaii. The white, bleached coral is a stark contrast from the healthy, red-brown coral in the foreground of the photo. Image via Wikimedia Commons.

What are corals, and how do they bleach?

Corals have two main parts: the polyps, which are soft bodied organisms that provide the shape of the coral, and zooxanthellae (zoh-uh-zan-thel-ee), which are algae that live within the tissues of the polyps and provide corals with their color. The zooxanthellae also provide energy to the polyps through photosynthesis. As the polyps grow, they can create a skeleton to give the coral structure. This relationship is essential to the survival of the coral. When corals become stressed, usually due to pollution or increased water temperature, they release their zooxanthellae. Since these algae provide energy for the polyps, the polyps starve, and are at an increased risk of death. In addition, because the zooxanthellae also give the corals their color, the corals then turn white.

Bleached coral does not necessarily mean dead coral, and sometimes parts of a reef that have been bleached can recover if the algae can return to the polyps. This most often occurs when the water conditions that caused the bleaching are reversed, such as water temperatures returning to a normal range after a warming event. Given that corals provide food and shelter that many marine animals rely on to live, and that corals are a huge tourist attraction for the areas in which they are located, scientists are very interested in the environmental conditions under which corals can best recover after bleaching. To get at this problem, researchers examined coral growth off the western Pilbara coast of Australia.

An example of an acroporid coral on a Hawaiian reef. Acroporids differ from other hard corals because in addition to secreting a skeleton at their base, these corals also have a calcified skeleton at the tip of each polyp. Image from NOAA via Wikimedia Commons.

Measuring factors of coral recovery

Researchers focused on the growth of hard corals as a whole, as well as acroporids specifically. Hard corals are those species which secrete a skeleton, and acroporids are a subset of hard corals that are typically first to appear as coral reefs recover from bleaching. The research team measured the number of settled coral recruits (larval corals which have attached to a surface), juvenile corals, and adult corals in several sites along the west Pilbara coast before, during, and after a bleaching event caused by extremely warm temperatures. The bleaching event occurred in 2011, and the study examined coral reef sites from 2009-2018. These sites had differences in presence of herbivores (animals such as fish, crabs, and sea urchins), water depth, algae cover, and turbidity (i.e. the cloudiness of the water). By focusing on how each of these factors helped or hindered coral recovery, this study provides valuable information about the water conditions needed to protect and conserve our existing coral reefs.

An example of coral recruits. These coral larvae are in the water column until they can successfully attach to a surface, and can only grow once this attachment occurs. Image by Narissa P. Snipes via Wikimedia Commons.

Which factors help or hurt coral recovery?

Coral coverage declined considerably during and after the bleaching event. In 2011, coral cover was 45% in west Pilbara, but by 2014, coral cover reached its lowest point at just 5%. Nonetheless, reefs did show slow signs of recovery in the following years, eventually increasing cover to just over 10% by 2018, seven years after the main bleaching event.

Acroporid coral species were the first to bounce back, as researchers predicted based on the findings of previous coral recovery studies. No relationship was found between levels of larval coral recruitment and adult coral recovery. Juvenile coral growth, however, was strongly associated with adult coral growth and recovery. Thus, if conditions allow coral to grow into juveniles, there is an increased chance of adult coral recovery.

Considering the environmental factors studied, water turbidity had the most detrimental impact on adult coral recovery. The cloudier the water, the less coral recovery occurred. This makes sense, since the corals rely on light to gain energy from their photosynthetic zooxanthellae, and cloudy water means less light can actually make it to the corals. Depth had a negative effect on coral recovery for a very similar reason: the deeper you are in the water, the less light reaches you. Macroalgae also negatively affected corals. Algae can grow very fast and can take over an area once coral cover has decreased, making that space unavailable for coral recovery. Macroalgae can also grow very tall or wide and shade growing coral, also causing diminished growth due to lack of sunlight. Given this relationship with macroalgae, it is no surprise that corals have higher recovery when herbivores, which actively eat algae, are present in an area.

An intertidal coral reef that has been taken over by macroalgae in Taiwan. The reef is covered by water during high tide, but the algae overgrowth prevents coral from getting enough sunlight to grow. Image by Wei, Wan-Chen via Wikiemedia Commons.

Why should we preserve corals?

Coral reefs are incredibly important habitats, and provide many benefits for the animals and communities that interact with them. With the growing threat of climate change, coral reefs around the world have increasingly become subject to bleaching. Coral reefs aid coastal cities by protecting against storm surges, tourism and fishing rely on coral reef species, and the ocean as a whole requires resources that reefs provide. If all corals were to bleach and die, our marine ecosystems would be forever impacted, and a valuable resource would be lost. By determining the circumstances under which corals can best recover after bleaching, we may be better able to preserve these extremely critical environments for years to come.

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