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Pumice rafts bring life to tropical reefs

7 August 2012, by Adele Rackley

Rafts of pumice thrown out by volcanic eruptions probably helped populate the world's coral reefs and continue to be important for their survival, according to new research.

Pumice raft in the ocean

The Tongan pumice raft on its eight-month journey to Australia's east coast.

Researchers led by Queensland University of Technology have made the first-ever systematic record of a raft's journey across tropical seas. They found that light-weight, frothy volcanic rock called pumice, thrown out by Tongan eruptions in 2006, transported around 80 species of marine creatures to Australia's Great Barrier Reef.

Their results confirm the importance of pumice rafts for transporting marine life around the oceans, and suggest they are likely helping reefs recover from natural and man-made damage.

'This is good news because we know the reef is being replenished as a result of volcanic activity in the south-west Pacific and volcanic activity is frequent, with eruptions in the area occurring every five to 10 years,' said Dr Scott Bryan, formerly from Kingston University, and lead author of the research which is published in PloS ONE.

Pumice rafts have sailed the Earth's oceans for millions of years, and they are particularly common in the Pacific. Because its surface is pitted with burst air bubbles, pumice offers plenty of space for marine organisms to cling to and shelter from predators.

It's also very resistant to chemical and physical weathering, so it can travel long distances relatively unaffected by changes in ocean temperature or climate. So these rafts dramatically increase the range of marine organisms that would otherwise be isolated in shallow marine and coastal areas.

To see just how useful a role these rafts play in marine transport, the research team mapped the journey and biological cargo of the pumice thrown out by volcanic eruptions in Tonga in 2006. They used observations of pumice stranded en-route, and models of wind and ocean currents.

Marine life retireved from the pumice raft

A collection of marine species retrieved from the pumice.

They found that just six weeks after the eruption the raft had picked up its first hitch-hikers: goose barnacles. By the time it began to arrive along Australia's eastern coast, after an eight-month, 5000+km journey, it was carrying over 80 different species including corals, anemones, barnacles, mollusks and crabs.

The raft's resilience and its speed – roughly twice the average ocean currents – make a significant difference to the geographical reach of marine species which would otherwise only survive for a couple of weeks in deep water in their larval stage.

Bryan believes rafts are so important that this type of event was likely responsible for the colonisation of the reef in the first place, and modern volcanic events continue to bring colonisers to replenish reefs that have been damaged by storms or human activities.

There is a small downside. 'Marine pests, for example some species of sponge or mussel, can also be carried along on the pumice,' Bryan notes.

So policy-makers concerned with the management of sensitive marine environments need to be aware that rafts could bring invasive species to reefs.

Pumice has been transporting life around Earth's oceans for a long time so perhaps there's no immediate concern; but as reported recently in the Guardian, the US is facing a new environmental threat from last year's devastating Japanese tsunami, with fears that a section of dock washed up in Oregon could be a vehicle for invasive plant and animal species.