Algae blooms pose new threat to reefs

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Algal blooms are emerging as a new threat to the survival of coral reefs, according to a study to be published in a forthcoming issue of the journal Marine Pollution Bulletin.

Andrew Bauman and co-authors document the decimation of a coral reef in the Gulf of Oman in only three weeks after a harmful algal bloom (HAB) struck the area. 

In October/November 2008, a HAB of the dinoflagellate Cochlodinium polykrikoides covering an area of more than 500 square kilometers engulfed the Gulf of Oman. 

The authors surveyed two sites with fringing reefs in the gulf before and after the HAB, and found that the bloom had killed up to 95% of the hard corals (including all of the cauliflower coral Pocillopora damicornis and the tabletop coral Acropora arabensis at one site). The HABs kill the corals by depriving them of sunlight and oxygen

Worse news was to follow, as the subsequent loss of the reef habitat snowballed into a sharp drop in fish abundance, with an average decrease of 72% recorded at the two sites.  About 83% of the most abundant fish species suffered severe reductions in abundance, or were eliminated altogether. 

This loss was also not a temporary one, as the authors surmise that the changes in fish community structure associated with the HAB event are most likely to have resulted from increased mortality of individuals -  the HAB covered such a wide area that only fishes at the edges of the bloom were able to escape to surrounding unaffected reef.  The presence of dead fishes and the occurrence of fish die-offs in other parts of the gulf provided compelling evidence for this hypothesis.

Coral reefs are already under threat from global warming, ocean acidification, overfishing and pollution, among other anthropogenic effects. The addition of HABs as a new threat is not welcome news, especially since rapid changes in oceanic climate, enhanced coastal eutrophication and increased global distribution of HAB species within ballast water have led to the prediction that large-scale HAB events will increase dramatically in both intensity and distribution and can be expected to have increasingly negative effects on coral reef communities globally.

For more information, see the paper: Bauman AG, Burt JA, Feary DA, Marquis E, Usseglio P. Tropical algal blooms: An emerging threat to coral reef communities?