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Staying pristine. Bubbling nitrogen into a ship's ballast tank (bottom) avoids a rusty tank (top) and may also protect against species invasion.

Battling Marine Stowaways

With their plankton offspring hidden in the bellies of cargo ships, many coastal creatures have spread around the globe and badly damaged foreign ecosystems. Scientists are searching for a quick and effective way to kill ballast-water stowaways without placing too heavy an economic burden on the shipping industry. One method of preventing rust in a cargo ship's ballast tanks, large tanks that lie along bottom and outside walls of the ship, has the additional benefit of reducing the number of invasive species transported across oceans, report scientists at the Monterey Bay Research Institute (MBARI) in the January issue of Biological Conservation.

After unloading, cargo ships fill their ballast tanks with water to remain stable. Many plankton species and small coastal swimmers are sucked into the tanks along with the water--only to be discharged in unfamiliar ports. In many regions around the world, these invaders have decimated native species, destroyed fisheries and shellfish farms, and cost governments billions of dollars in control measures.

Mario Tamburri, a MBARI marine biologist, was intrigued by a talk he heard at a conference in Japan: Sumitomo Heavy Industries, a Tokyo ship building company, had designed a device that continuously bubbles nitrogen into the ballast water, stripping out the oxygen, dramatically reducing corrosion, and saving money. Tamburri wondered what impact this technique could have on the invasive species transported in the ballast tanks. So he and his colleagues bubbled nitrogen into lab tanks to recreate the low-oxygen conditions in ballast tanks. After 3 days, the lack of oxygen had suffocated more than 80% of the worm, crab, and mussel larvae they tested. The scientists found similar kill rates when they scoured the literature for additional studies of marine organisms in low-oxygen conditions, leading them to believe that the industrial approach could help begin to prevent further invasions.

Tim Wilkins, a marine biologist with INTERTANKO, an international association of independent tanker owners based in Oslo, Norway, agrees that the new technique shows promise, but points out that stripping water of oxygen doesn't eradicate all species present, such as algal cysts and spores. The International Maritime Organization (IMO), a London-based branch of the United Nations that drafts shipping legislation, is currently debating how strictly to set eradication standards for ballast water treatments, Wilkins says. The nitrogen bubbler wouldn't meet the stringent 95% kill-off standard the IMO is considering. But Wilkins says he'd be interested in seeing how the method performed when coupled with another technique such as filtration or UV radiation.

Related sites
IMO's ballast water information (shipping industry perspective)
National Ballast Water Information Clearinghouse (Scientific perspective)
Mario Tamburri's Web page