Study Shows Deoxygenating Ballast Water Can Prevent Invasive Species And Ship Corrosion

Non-native organisms are famous for causing environmental problems when, accidentally or intentionally, they are translocated outside their normal range into a new region. “Biological invasions of non-native species are one of the most devastating threats to native communities,” said study co-author Kerstin Wasson of the Elkhorn Slough National Estuarine Research Reserve. Wasson has documented more than 55 non-native invertebrates in Elkhorn Slough, California, and became involved in this project because “we urgently need practical solutions to stem the tide of aquatic invasions.”

Previous research has shown that ballast water from the global shipping industry inadvertently transports enormous numbers of aquatic organisms from one port to another. These non-native species introductions have caused changes in habitat structure, large economic costs due to factors such as biofouling and predation on commercial species, and are thought to have been involved in 70% of native aquatic species extinctions in the last 100 years. Because of these impacts, researchers from all over the world have been developing ways to clear the ballast water of aquatic organisms that may colonize new habitats.

But current solutions—such as intensive filtration, heat treatments, and biocides—are costly, can be dangerous to ship crewmembers, and can have negative effects on the surrounding environment where the treated waters are discharged. Costly treatments in particular are unlikely to be voluntarily employed by the shipping industry. On the other hand, deoxygenation presents the first solution that safely and effectively removes the majority of organisms found in ballast water while also providing an economic benefit for ship owners.

“Deoxygenation was seen as too expensive for controlling invasive species in ballast water but our study shows that the anticorrosion benefit of this technique is a strong economic incentive for the shipping industry,” said Tamburri. “It’s a win-win treatment for solving an environmental problem and reducing ship maintenance costs.”

Tamburri and Wasson summarize research by Japanese scientists and engineers at Sumitomo Heavy Industries, Ltd. who developed a deoxygenation technique to combat the costly corrosion experienced in cargo vessels. The method involves bubbling nitrogen gas into ballast water to remove oxygen, thereby preventing oxidation or rust in the tanks. The shipping industry currently uses expensive paints, which must be maintained over the lifetime of the vessel—typically around 25 years. It is estimated that nearly $100,000 (US) per year can be saved for each new ship that uses this deoxygenation technique rather than paint to prevent corrosion.

Aquatic organisms are also sensitive to oxygen levels, leading Tamburri and Wasson to explore the nitrogen ballast water treatment as a deterrent to non-native species introductions. They constructed laboratory experiments mimicking the shipboard deoxygenation technique and examined the oxygen tolerance of larvae from three known nuisance invasive species now found in U.S. waters—an Australian tubeworm, European green crab, and European zebra mussel. The resulting low oxygen environment was toxic to essentially all of the larvae after only two to three days while major ocean crossings by cargo vessels typically take weeks.

Tamburri also reviewed the scientific literature to assess how other aquatic organisms would respond to the low oxygen environments. The reviews show that significant mortality within only hours to days would be expected for animals commonly found in ballast water. However, there are some species—such as those with cyst stages or anaerobic bacteria—that will likely be able to survive the conditions found in a nitrogen treated ballast tank.

“While extremely effective, deoxygenation may not be as comprehensive in removing aquatic organisms as other proposed ballast water treatments but it has the huge advantages of being environmentally benign while also saving ship owners money through rust prevention,” said Tamburri. “Until international law mandates total mortality of all ballast water organisms, deoxygenation at very least deserves further consideration as a high priority treatment.”