Smithsonian Scientists Help Block Ship-borne Bioinvaders Before They Dock

The global economy depends on marine transportation. But in addition to cargo, the world's 50,000-plus commercial ships carry tiny stowaways that can cause huge problems for the environment and economy. A new model created by Smithsonian scientists will facilitate accurate screening of vessels for dangerous species before they unload. The team's findings are published today in the journal Environmental Science & Technology. Ballast water taken up by ships in coastal waters teems with plankton and microbes. When discharged at the next port of call, these hitchhikers can wreak havoc on receiving ecosystems. Under current federal regulations, ships exchange their ballast water in the open ocean to flush out unwanted species. However, some survive the process, and not all ships travel across oceans. Environmental regulators have known about this problem for decades. But while regulators check ship records and can sometimes test salinity to verify compliance, unlike many pollutants, there are no federal requirements limiting the number of viable, potentially dangerous organisms.

That is about to change. The U.S. Coast Guard has proposed a new set of rules limiting the number of organisms allowed, in line with current International Maritime Organization standards. For larger zooplankton (length, width or height at least 50 microns, or one-half the thickness of a piece of paper), the number must be fewer than 10 viable organisms per cubic meter (264 gallons). On-board ballast water treatment technologies offer a promising solution, enabling ships to substantially cut the risk of delivering dangerous species. But while a few systems have entered the market, the challenge of testing the ballast water—and the technology—remains. A major stumbling block is simply understanding how such testing should occur and how much ballast water must be tested in order to count very sparse numbers of organisms.

To help regulators and engineers develop and test such treatment systems, and ultimately enforce these standards, a team of researchers developed a statistical model to see how to count small, scarce organisms in large volumes of water accurately. Led by Whitman Miller, research ecologist at the Smithsonian Environmental Research Center, the scientists took samples that exceeded the limit and ran them through various tests to see which violations would be spotted. Larger samples gave the best results: sampling only 0.1 cubic meter of water (26.4 gallons) made it difficult to detect concentrations even twice as high as the standard. By contrast, when they raised the volume to 7 cubic meters (1848 gallons), the test regularly picked up violations as low as 13 zooplankton per cubic meter.