A huge iceberg that has run aground in an Antarctic bay is providing the opportunity to observe a real-life science experiment showing how vulnerable marine ecosystems can be to changes in ice cover, researchers say.

The iceberg became "beached" 3 years ago, and the bay - normally kept ice-free by winds off the Antarctic continent for most of the year - has seen a dramatic increase in ice cover.

Sea ice several meters thick has covered the bay all through the year.

The result, say scientists from the University of New South Wales in Australia, is that almost all seaweed living on the bay's sea floor has died or suffered discoloration or bleaching because of the lack of sunlight reaching it.

"Understanding the ecological effect of changes in sea-ice is vital for understanding the future impacts of climate change, but it is impossible to manipulate sea-ice at the scale we need to conduct experiments," says Graeme Clark, co-author of a study appearing this month in Polar Biology.

"Luckily, the grounding of an iceberg in Commonwealth Bay in 2010 provided us with a perfect natural experiment to carry out research on this important issue."

At the end of 2013 Clark and his colleague Ezequiel Marzinelli drilled a hole in the sea ice covering the bay to lower cameras into the water to survey the conditions on the seabed.

They compared the images they gathered with underwater footage taken of the same region of the seabed in 2008.

"After three years of sea-ice cover, the forests of healthy seaweeds that previously dominated the seabed were in a severe state of decline," says research team leader Emma Johnston of the university's School of Biological, Earth and Environmental Sciences.

Around three-quarters of the seaweed cover has decomposed, the researchers say, while the rest is showing signs of poor health.

The ice cover isn't bad news for the entire ecosystem, Johnston says, pointing out that "darkness-adapted vertebrates such as brittle stars and fan worms were starting to colonize the bay."

The study in Commonwealth Bay has provided the first opportunity to observe a swing from a seaweed-dominated ecosystem over to an invertebrate-dominated one where the start date of the shift was known, the researchers say.

"Continued sampling at the site would allow the rate at which this transition is occurring to be worked out," says Marzinelli.