Sand is bad news for corals


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Sand is bad news for corals, according to research by two Australian scientists published in the latest issue of the journal Limnology and Oceanography.

The study by David Bellwood and Chris Fulton used remote underwater video cameras to quantify rates of herbivory by coral reef fishes on algal turfs with natural and experimentally reduced sediment loads.

Algal turfs are thick mats of sand and algae that envelope the rocky surfaces where a coral reef would grow, effectively preventing a degraded coral reef from re-establishing itself.

The authors found that the emoval of sediment increased overall fish feeding rates 3.8-fold, and resulted in a decrease in mean algal turf length of 64% within 4 hours. After 4 hours, sediment accumulated in the treatment plots, but only returned to 41% of the original depth.

A total of 20 species actively fed on the sediment removal plots, compared with 12 species in control plots. Of the five numerically abundant herbivorous fish species, all increased feeding by at least 225% in the absence of sediment.

The key to the success of the algae in preventing the re-establishment of coral is the presence of sand, which renders the algae less appealing to the fishes.

The reasons why sand turns off the fishes appetite are unclear. According to Dr. Bellwood, may be that the sediment acts as an antacid and gives the fish indigestion by preventing their stomach acids digesting their food. Or it may simply be that fish, like people, don t appreciate a mouthful of sand and mud.

For more information, see the paper: Bellwood, DR and CJ Fulton (2008) Sediment-mediated suppression of herbivory on coral reefs: decreasing resilience to rising sea-levels and climate change? Limnology and Oceanography 53, pp. 2695"2701.