Are Harmful Algal Blooms a New Concern For Coral Reefs?

Bauman, A.G., J.A. Burt, D.A. Feary, E. Marquis, and P. Usseglio. 2010. Tropical harmful algal blooms: An emerging threat to coral reef communities? Marine Pollution Bulletin 60: 2117 – 2122.

Coral Reefs: A Community Already In Danger.

Coral reefs have survived on Earth for hundreds of millions of years, and they support abundant sea life throughout their vast structures. Coral reefs are homes for marine life that are similar to high-rise apartments. The coral polyp creates a shell around itself, and algae grow inside that shell in perfect harmony. The algae photosynthesize using sunlight and provide food to the coral, allowing it to expand and grow into larger, denser colonies. These large colonies turn into bustling underwater cities, as fish and other sea creatures use the reefs as a source of food and shelter.

However, corals depend on clean, clear, shallow waters to survive. When stressed by warming temperatures and rising sea levels, corals lose their algae ‘roommates’, which are also their food source. According to NASA, an estimated 60% of the world’s coral reefs are seriously threatened, which impacts not only the aquatic communities that rely on them, but also the more than 30 million people that depend on coral reefs for their livelihoods. Unfortunately, human activities such as pollution, over-fishing, and development, continue to push the fragile coral reef ecosystem to the brink.

Healthy coral reef community teeming with sea life and fish communities. Photo Credit: NASA,

Harmful Algal Blooms … and Coral Reefs?

Harmful algal blooms can cause numerous problems in the environment. The US EPA describes algae blooms as overgrowths of algae in the water, which can be caused when nutrient pollution (nitrogen and phosphorus) in the water fuels algal growth. Harmful algal blooms can be harmful to aquatic life, as some produce toxins that can make animals sick, while others can die off in large quantities and deplete oxygen in the water when they decompose. Climate change and increasing human impacts on the environment, such as nutrient pollution in coastal waters, increase the likelihood for harmful algal blooms to occur, prompting some researchers to look into how coral reef communities are handling harmful algal blooms.

Harmful algal bloom. Note the murkiness of the surface water due to overgrowth of algae. Photo Credit: US EPA,

The Bloom in Oman.

In 2008, a research team observed a large-scale harmful algal bloom event that occurred in the Gulf of Oman, which is off the coast of the Saudi Arabian Peninsula. The coral reef community and the fish community were surveyed before and after the bloom at two locations: Dibba and East Musandam. Scuba divers identified fish species, recorded their size, and determined their abundance, while other divers worked to identify coral species and count their abundance as well. The goal was to determine if the harmful algal blooms had a significant effect on the health of coral or fish.

Map of Gulf of Oman (left) and two survey locations: East Musandam and Dibba (right). Photo Credit: Bauman et al. 2010.

The Aftermath.

Coral Reef Communities.

After the bloom, the teams found that the coral reef and fish communities at both locations were negatively affected by the harmful algal bloom. They also found significant declines in coral at one location (Dibba). Before the bloom, coral reefs at Dibba covered 53% of the Gulf’s seafloor. After the bloom, that abundance decreased all the way down to 6%. Some corals species at Dibba were completely eliminated during the harmful algal bloom event!

The researchers think this could have been due to the algae bloom blocking sunlight at the surface from reaching the algae within the coral. Without sunlight for coral’s algae, they cannot photosynthesize and create food for the corals. In addition, low dissolved oxygen from the decaying algae bloom likely stressed the corals, which negatively impacted their health.

Dying coral reef lacking colorful algae within coral, along with sea life and fish community. Photo Credit: NASA,

Fish Communities.

Fish did not fare any better, as both locations saw significant declines in fish communities as a result of the bloom. The total amount of fish at each location decreased significantly, while Dibba saw the diversity of fish species reduce by nearly two-thirds! Dead fish were observed during the harmful algal bloom, which has been observed during previous large-scale events. These findings suggest that mass mortalities and shifts in fish communities may be a result of harmful algal blooms.

How can we save our Coral Reefs!?

Harmful algal blooms can stress and damage extensive, underwater ecosystems that are already facing many other stressors. Harmful algal blooms cloud surface waters, blocking sunlight and preventing the coral’s algae counterpart from growing food for itself and the coral. As the bloom dies and decays, the dissolved oxygen is depleted, causing the coral to suffocate.

Damages to coral reefs have a domino effect on fish communities, as seen by the changes to fish abundance and species diversity before and after the bloom. No homes and no food provided by corals mean no fish, which jeopardizes the livelihood of millions of people that rely on the coral reefs for survival. Therefore, findings from this study provide some of the first evidence on how devastating harmful algal blooms can be to coral reef communities, and how dire the need is to address the impacts humans have on these ecosystems. Our contributions to climate change and nutrient pollution must be an area of further attention if we hope to give coral reefs a chance to adapt and overcome perhaps the biggest challenge they’ve faced in the hundred million years they’ve existed on our planet.


Bauman, A.G., J.A. Burt, D.A. Feary, E. Marquis, and P. Usseglio. 2010. Tropical harmful algal blooms: An emerging threat to coral reef communities? Marine Pollution Bulletin 60: 2117 – 2122.

“Harmful Algal Blooms.” EPA, Environmental Protection Agency, 7 April. 2017,

Sullivant, Rosemary. “Earth’s Coral Reefs in Danger.” NASA, Jet Propulsion Laboratory, 30 Nov. 2007,

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Nick Iraola

Nick has a Master of Science in Marine Science from UNC Wilmington. His master's thesis research pertained to eutrophication and nutrient cycling within an urban blackwater lake in Wilmington, NC. Currently, Nick works for the Cape Fear Public Utility Authority testing drinking and waste water for safe consumption and discharge (respectively!!). Nick also works as a part-time research scientist at UNCW's Center for Marine Science in the Aquatic Ecology Laboratory and the Nutrient Analysis Core Facility. When he's not sciencing, Nick enjoys running, swimming, cooking, sailing, and catching up with friends and family. His favorite candy is Reese's pb cups, because what is there not to like!?

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