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Seaweed may help protect coral reefs

September 8, 1998
Webposted at 11:20 AM EDT

By Environmental News Network staff

Australian scientists have reported a surprising increase in coral bleaching in areas where the normally abundant canopy of seaweed was removed.

(ENN) -- Scientists with the Australian Institute of Marine Science have recently completed an 18-month study of coral bleaching with some surprising results.

Many inshore reefs of the Great Barrier Reef region have abundant beds of large brown seaweed or macroalgae on the reef flat, often dominated by species of Sargassum. The abundance of the seaweed could be a result or even a cause of reef degradation; scientists aren't quite sure which. The increased runoff from land as a result of floods could make reef waters less suitable for corals and more suitable for algae. Or these conditions may allow algae to outcompete corals, causing reef decline.

In order to test whether the seaweed beds had any effect on coral reefs, the AIMS scientists established and maintained several large seaplots at Cannon Bay and Goold Reef. In some seaplots, the Sargassum was removed for about 18 months and in others it was left in place, forming a thick canopy often 1-2 meters thick, with 100 percent cover. Despite this Sargassum canopy, the plots had quite high cover of live corals (up to 50 percent).

In the middle of February, more than a month after major flooding, scientists found extensive bleaching of corals at the two sites. The likely causes of the coral bleaching include low salinity, high temperature and high ultra violet light intensity. Both the Sargassum canopy and removal plots were assessed and the condition of the coral measured.

At both reefs, the average percentage of corals bleached was significantly higher in plots that had had the Sargassum canopy removed than in plots with an intact canopy of the macroalgae. Overall, 19.6 percent of corals were bleached under "normal" conditions for these reefs, but 36.4 percent were bleached when the Sargassum canopy had been removed.

AIMS scientists have concluded that it seems likely that the seaweed canopy reduces damage to the corals by decreasing exposure to high temperatures, high UV light intensities, or perhaps by reducing mixing of low-salinity waters. Evidence is available for similar effects of algal canopies from temperate areas: Such canopies can dramatically reduce thermal stress and water movement. The significance of this result is considerable, since it raises the possibility that algal canopies could actually provide protection to corals, instead of, or as well as, competing with them.

"Although the results by no means disprove the possibilities that corals are inhibited by macroalgae, they certainly provide further evidence that abundant macroalgae should not be assumed to be detrimental to inshore reefs without much more information," said Dr. Laurence McCook, chief AIMS researcher on the project.

Copyright 1998, Environmental News Network, All Rights Reserved

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