The ballast water that keeps merchant vessels entering Chesapeake Bay stable -long known to carry exotic plankton, shellfish and fin fish - also carries potentially dangerous microorganisms from around the world, a new study says.
The study by a group of bay scientists, which appears today in the British journal Nature, found high concentrations of microbes in the ballast water of ships arriving from foreign ports.
Greg Ruiz, the lead researcher, said there have been no reports of disease tied to the foreign organisms but the findings show a need for more research.
While previous studies have documented the spread of such exotic species as green crabs, the rapa whelk and water milfoil, this is the first time scientists have studied the spread of microorganisms through ballast water.
Oceangoing vessels commonly discharge ballast water to adjust for weather changes or changes in cargo loads. Federal regulations require that ships heading for the Great Lakes and California and Washington state dump ballast at least 200 miles out to sea.
But that is only voluntary for other ports, including Norfolk and Baltimore in the Chesapeake Bay.
The study found that 87 million tons of foreign ballast water is discharged into U.S. ports annually.
Ruiz, of the Smithsonian Environmental Research Center in Edgewater, and scientists from the Center of Marine Biotechnology at the University of Maryland and Old Dominion University in Virginia, began searching ballast water of vessels in the ports of Norfolk and Baltimore for zooplankton and other invertebrate species eight years ago.
Three years ago, they began to "wonder what was happening with microorganisms," Ruiz said.
The scientists, who collected samples from 200 vessels for the earlier study, obtained samples from 15 ships from Europe and the Mediterranean Sea for the microorganism study. They found an average of 31 billion suspected viruses and 3.5 billion bacteria, including some that cause cholera in humans.
But while those numbers sound astronomical, they are not out of the ordinary, cautioned Fred Dobbs, a member of the research team from Old Dominion.
"There's 5 million bacteria in a teaspoon of normal seawater, and sometimes the concentrations are higher in Chesapeake Bay," he said.
"But we found cholera on all those ships, and that leads you to ask what else is out there? What else is being shipped around?"