Friday 2 November 2012

Coral decline on the Great Barrier Reef.

Coral Reefs are generally considered to be among the most important of marine ecosystems, providing protection to storm battered tropical shores and forming biological hotspots. However in the late twentieth and early twenty first centuries reef ecosystems have undergone a decline around much of the world, something generally assumed to be a result of human activity.

In a paper published in the Proceedings of the National Academy of Sciences  on 1 October 2012, Glenn De’athKatharina Fabricius and Hugh Sweatman of the Australian Institute of Marine Science and Marji Puotinen of the School of Earth and Environmental Sciences at the University of Wollongong, look at Coral growth and dieback at 214 monitored sites of the Great Barrier Reef in Australia over a 27 year period, from 1985 to 2012.

Map of the Great Barrier Reef showing mean average Coral coverage over the period 1985-2012 as a percentage (background colouring) and overall increase or decrease at each monitored site during this period (circles). De'ath et al. (2012).

De'ath et al. found that during the period 1985-2012 the Great Barrier Reef suffered an overall loss in Coral cover from 28.0% to 13.8%; a loss of 50.7% of initial cover, but that this varied strongly at a local scale, with 67.8% of the study sites suffering an overall loss in coral coverage, and 32.2% of the sites having an overall gain in coverage during the study period.

They then looked at the cause of dieback over the effected areas, and found that 48% of the losses were caused by tropical cyclones, 42% by predation by the Crown of Thorns Starfish (Acanthaster planci) and 10% by bleaching events (death of symbiotic algae due to excessive sea temperature). 

Tropical cyclones and bleaching events are both associated with global warming, an important issue but one beyond the immediate reach of local wildlife managers, however the Crown of Thorns Starfish could potentially be tackled at a local level, with the potential to have a genuine impact on the decline in Coral on the Great Barrier Reef. De'ath et al. also note that Coral left to its own devices expands its coverage at a rate of 2.85% per year, from which they calculate that without the Crown of Thorns Starfish, Coral coverage on the Reef would actually of expanded by 0.89 per year over the study period (though this figure cannot safely be applied to future years, when the effects of global warming could potentially be worse).

The Crown of Thorns Starfish (Acanthaster planci). Marine Science Today/NOAA/David Burdick.

Increases in the population of the Crown of Thorns Starfish are believed to be linked to increasing nutrient runoff from agricultural land into rivers and estuaries and eventually to seawater. The larvae of the Starfish has been shown to have a better survival rate in water with increased nutrient levels, and increased nutrients reaching the Barrier Reef favors the growth of algae there, which is the main food of the Starfish. Raised nutrient levels do not directly harm the corals, but they do promote the growth of algae, which in turn supports higher population levels in the Starfish, which eat the algae and the Coral.

Thus De'ath et al believe that controlling levels of agricultural runoff reaching the waters of the Reef is one of the most important measures to be taken in order to protect the Great Barrier Reef.

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