Invasive Species "Stowaways" May Lose Rides on Ships

Bijal P. Trivedi
National Geographic Today
January 9, 2002

A new method to reduce rust and corrosion in the ballast tanks of ocean vessels may also be key to preventing the spread of foreign aquatic species that stow away in ballast water.

Global shipping moves about 80 percent of the worlds' commodities and is an essential part of world trade. But when ships stop at ports around the globe, they deliver not only their cargo but also a menagerie of aquatic hitchhikers that lurk in the ballast tanks—enclosures in ship hulls that are filled with water to counterbalance a cargo load.

Every day thousands of ships suck up thousands of gallons of water in one port and later release it in another—sometimes thousands of miles away. The result: Hundreds of non-indigenous species are introduced to different habitats each day. In many cases the introduction of these "alien species" wreaks environmental havoc, which collectively has caused hundreds of millions of dollars in damage.

In 1986, for example, a ship that emptied ballast water from the Black Sea into Lake Erie released a tiny stowaway, the zebra mussel—now the poster child of invasive species. The dime-size mussel has reproduced uncontrollably, spreading to the Hudson and Mississippi Rivers and even the Gulf of Mexico, forming thick carpets of mussels that clog pipes and waterways.

Solutions to the foreign-species transfer problem have included using various filtration systems, heat treatments, and biocides for the ballast water. Most of these approaches have been dismissed as too expensive to install or because biocides could be harmful to the environment.

Now, engineers at Sumitomo Heavy Industries in Japan have developed a solution that should simultaneously benefit the environment and please the shipping industry: remove the oxygen from the ballast water.

"Simple and Elegant Solution"

The system developed by Sumitomo's engineers pumps nitrogen gas through the ballast tanks to purge the seawater of oxygen.

Reducing oxygen in ballast water to a low level can kill as much as 80 percent of the organisms in the water, according to the researchers. At the same time, a low oxygen level reduces the amount of rusting.

"It's a simple and elegant solution," said marine ecologist Mario Tamburri of the Monterey Bay Aquarium Research Institute in Moss Landing, California, who collaborated with Sumitomo in part of the research.

Rusting of ballast tanks as a result of exposure to seawater is a big problem for the shipping industry. Painting is the only way to prevent corrosion, but this is expensive and time consuming. During the approximately 25-year life of a shipping vessel, painting and maintaining the ballast tanks generally costs more than U.S. $10 million.

Low oxygen levels in the ballast water reduce rusting and corrosion by 90 percent.

Tamburri said that deoxygenation would save a ship owner an average of $70,000 a year, theoretically, at least, extending the life of the vessel. He estimates the system would cost about $9 million over the life of the ship.

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