Treatment for Ballast Tanks Also Kills Aquatic Invasives
The shipping industry moves four-fifths of the commodities worldwide — and also spreads invasive aquatic species in ballast water. New research suggests that a cost-effective ballast water treatment will please both ship owners and conservationists: purging the water of oxygen extends the life of ballast tanks while killing many invasive aquatic species.
“This novel deoxygenation technique may therefore have direct benefits to both marine conservation and the shipping industry,” say Mario Tamburri, formerly at the Monterey Bay Aquarium Research Institute and now at the University of Maryland’s Chesapeake Biological Laboratory in Solomons, and two co-authors in the March issue of Biological Conservation.
Because ballast water is pumped in at one port and discharged at another, it transports aquatic species — from bacteria and dinoflagellates to invertebrates and fish — around the world. Introduced species can displace native species and cost millions of dollars per year to control. Many estuaries and other coastal habitats have been invaded by non-native species; the San Francisco Bay and Delta, for instance, have at least 234 invasives. However, “until mandated to do so, the shipping industry is unlikely to voluntarily install expensive ballast water treatment technologies,” say the researchers.
A new anti-corrosion treatment for ballast tanks — purging oxygen with nitrogen gas — has the economic benefit of being far cheaper than the current treatment of painting the interior of tanks. Tamburri and his colleagues studied whether the new treatment would also have the ecological benefit of killing invasive aquatic species.
The researchers determined the effects of decreased oxygen on larvae of three of the worst aquatic invaders: an Australian reef-building tubeworm (Ficopomatus enigmaticus) that has invaded many estuaries in Europe and the Americas, damaging pilings and interfering with lock operations; a European green shore crab (Carcinus maenas) that has invaded the coasts of North America, Australia, and South Africa, contributing to the decline of the soft shell clam industry on the Atlantic coast of the U.S.; and the European zebra mussel (Dreissna polymorpha), which has invaded the Great Lakes region, displacing other bottom-dwelling organisms and incurring enormous control costs. In addition, the researchers reviewed existing studies of the low-oxygen tolerance limits of a wide variety of aquatic species.
Tamburri and his colleagues found that few of the larvae studied survived more than a few days at oxygen levels as low as those in nitrogen-treated ballast tanks. Specifically, 79 percent of the tubeworm and 97 percent of the crab larvae died within two days, and 82 percent of the zebra mussel larvae died within three days. “Given the high mortality after 2-3 days, we predict that all the larvae tested would have been killed during the 2-3 week duration of most ocean crossings,” say the authors.
The review of existing studies confirmed that very few aquatic animals would be able to survive typical ocean crossings in nitrogen-treated ballast tanks. For instance, studies of 20 invertebrate larval forms showed that they can only tolerate low oxygen levels for a few hours or days. Similarly, studies of 14 adult invertebrates and fish showed that the same is true of most them.
Although deoxygenation kills most organisms in ballast water, Tamburri and his colleagues caution that nitrogen treatment is not perfect. For instance, deoxygenation would not kill bacteria that are adapted to low oxygen environments or species with cysts and other stages that can resist low oxygen levels.
But the researchers stress that nitrogen treatment is far better than nothing and is likely to be adopted voluntarily by the shipping industry. “It’s a rare win-win solution for conservation and industry,” says Tamburri.
For more Information
Tamburri, M.N. et al. 2002. Ballast water deoxygenation can prevent aquatic introductions while reducing ship corrosion. Biological Conservation 103:331-341.