Mounting loss of seagrass in the world's oceans, vital for the survival of endangered marine life, commercial fisheries and the fight against climate change, reveals a major crisis in coastal ecosystems, a report says.

A global study of seagrass, which can absorb large amounts of planet-warming carbon dioxide, found that 29 per cent of the world's known seagrass had disappeared since 1879 and the losses were accelerating.

Seagrasses are flowering plants found in shallow waters. They were vanishing at the rate of about 110 square kilometres a year since 1980, said the study to be published in the US journal Proceedings of the National Academy of Sciences.

The study by Australian and American scientists found seagrass meadows were "among the most threatened ecosystems on earth" due to population growth, development, climate change and and ecological degradation.

It said there were only about 177,000 square kilometres left globally.

"Seagrass meadows are negatively affected by impacts accruing from the billion or more people who live within 50km of them," said the report received by Reuters yesterday.

The study said the loss of seagrass was comparable to losses in coral reefs, tropical rainforests and mangroves.

"Seagrasses are sentinels of change" and the loss of seagrass was an indicator of a deteriorating global marine ecosystem. "Mounting seagrass loss reveals a major global environmental crisis in coastal ecosystems," it said.

It is estimated that 70 per cent of all marine life in the ocean is directly dependent upon seagrass, according to US-based Seagrass Recovery (www.seagrassrecovery.com).

Seagrasses are the only flowering plants that can live entirely in water. They are most closely related to lilies and are very different to seaweeds, which are algae.