By Ella Davies
Earth News reporter
Coral doesn't survive for long under a green cloud
Harmful algal blooms have the potential to lay waste to coral reefs.
Scientists studying coral reefs in the Gulf of Oman have issued the warning after being shocked by the impact of one large-scale bloom, which destroyed a coral reef in just three weeks.
Around 95% of the hard coral beneath the algae died off and 70% fewer fishes were observed in the area.
The rapidly growing patches of microscopic marine plants starve coral of sunlight and oxygen.
The toxic algae starves coral of light and oxygen
Coral reefs are increasingly under threat from environmental stress in the form of climate change, coastal development, overfishing, and pollution.
Climate change is suspected of causing a number of coral bleaching events, as rising sea temperatures stress coral communities.
But the latest study, published in the journal Marine Pollution Bulletin, suggests that algal blooms could pose another significant threat.
Researchers from the United Nations University Institute for Water, Environment and Health undertook studies of coral reef environments at two locations in the Gulf of Oman.
After their initial study, a large-scale algae bloom measuring over 500 square kilometres occurred in the area.
When the researchers returned three weeks later they found the coral beneath the bloom had been almost completely destroyed.
In one area, cauliflower (Pocillopora damicornis) and table top (Acropora arabensis) corals died off completely.
The sudden loss of the reef habitat had a knock on effect for the fish communities living there.
The researchers found an overall reduction of 70%, with 83% of the most abundant species severely reduced or completely eliminated from the survey site near Dibba.
The reef was rich in life before the algal bloom
Gulf parrotfish (Scarus persicus), pearly goatfish (Parupeneus margaritatus) and blackspot snapper (Lutjanus ehrenbergii) were among the affected species.
Analysis showed that hard corals were particularly vulnerable to the presence of the algae Cochlodinium polykrikoides.
"We were surprised at the extent and speed at which changes to the coral reef communities were affected," says marine ecologist Andrew Bauman.
In recent times, the increased occurrence of rapidly growing areas or 'blooms' of algae have been attributed to human activities.
Eutrophication, excess nutrients in coastal areas caused by run-off from agricultural fertilisers and human sewage, is often cited as the trigger for these phenomena.
Certain species of algae, referred to by scientists as harmful algal blooms, have adverse effects on marine ecosystems as they clog fish gills, reduce water quality and starve other species of oxygen.
The presence of large patches of algae close to the water's surface reduces the sunlight accessible to underwater plants for photosynthesis.
Although classed as animals, corals depend on a symbiotic relationship with microscopic marine plants (zooxanthellae) in their tissues for energy and oxygen.
When put under stress by changes in their environment, corals are known to expel their zooxanthellae.
The photosynthetic algae give corals their colour, so after this expulsion only the white 'skeleton' remains. This is known as coral bleaching.
If the symbiotic algae does not return the coral dies, with fatal consequences for the fishes dependant upon it for food and shelter.