Ships traveling from port to port have become a “long-distance dispersal mechanism” for microorganisms, moving human pathogens such as cholera and other microbes around the globe — and into the Chesapeake Bay, according to a new study.

While it has long been known that fish, clams and other aquatic organisms are transferred from place to place by ships, the analysis by Bay region scientists is the first time anyone examined microbes in ballast water in the Chesapeake.

On average, ballast water samples taken from 15 ships revealed nearly a billion bacteria and roughly 7 billion virus-like particles per liter, the scientists reported in the Nov. 2 issue of the journal Nature.

Those concentrations are not unusually high; marine water is teeming with bacteria under natural conditions. What’s more important, the scientists say, is that the ballast water from other places may contain nonnative types of algae, zooplankton or pathogens that may threaten public health or disrupt the natural system if they became established in the Chesapeake or other coastal waters.

“Of those billions of bacteria, some of them may be nasty,” said Fred Dobbs, an oceanographer at Old Dominion University and a member of the research team.

The greater the amount of bacteria and other microorganisms imported, the greater the odds of an “invasion.” In fact, the scientists said, that may have already happened, but microorganisms are so poorly monitored that past invasions would have been overlooked — even if they caused problems.

The research team, led by Greg Ruiz of the Smithsonian Environmental Research Center, said the Bay received about 12 billion liters (about 3 billion gallons) of ballast water in 1991 alone, suggesting that “ballast water probably delivers large numbers of microbial species and potential pathogens to this estuary.”

Moving water around is a normal activity for freighters. After a ship unloads its cargo, it routinely sucks water into its hold as ballast to steady the vessel for the return voyage, usually releasing the water when it reaches its destination. Ports in the United States receive about 79 million tons of ballast water from overseas annually.

Worries have grown over the years that ballast water is transporting creatures around the world and depositing them in areas where they have no natural predators. The zebra mussel has caused billions of dollars of damage since it arrived in the Great Lakes in the 1980s. It has also sparked concern that it may be altering the lakes’ food web by filtering huge amounts of algae — needed by native fish — out of the water.

In the Bay, researchers at the Virginia Institute of Marine Science recently discovered the rapa whelk, a species native to the Sea of Japan. The whelk, which has been spreading in the Lower Bay, could pose a threat to native shellfish: When they were accidentally introduced into the Black Sea, they caused the near extinction of several shellfish species. Scientists believe the whelk was brought here in ballast water.

The new study suggests those more visible examples of invasions via ballast water may be only the tip of the iceberg. Ruiz said that while there is no reported evidence of outbreaks of human diseases from nonindigenous microbes in ballast water, the findings indicate the need for a much greater emphasis than has been shown.

“Despite growing concern about biological invasions and emergent diseases, the extent and effects of the transfer of microorganisms in ballast water are virtually unexplored,” Ruiz and his colleagues wrote.

Most of the microorganisms counted by the scientists were not identified, although Dobbs said that effort is continuing. Indeed, the scientists wrote that the numbers of organisms and volumes of water involved suggest that coastal waters may be “frequently invaded by microorganisms from ballast water.”

Further, they said that their observations of cholera bacteria serves as a “useful model” of the potential significance of ballast water dispersal. They found one type of cholera in the ballast of all 15 ships examined, and two types in 14 ships. Many of the cholera bacteria were “viable” when they arrive in the Bay, the scientists said, noting that they observed dividing cells during microscopic examinations of ballast water.

That doesn’t mean the Bay is at risk from a cholera outbreak — some strains are already a common component of most freshwater and marine habitats, including the Chesapeake, and local environmental and sanitation conditions make outbreaks unlikely. But ballast water could introduce a “novel genotype” that could become a bigger problem if it were to become established, the scientists said.

When a strain of cholera not native to the United States was introduced to Mobile Bay several years ago, it resulted in the closure of infected oyster reefs.

The research team’s findings could make solutions to the ballast water issue more difficult to find. Efforts under way to stem ballast water invasions include the exchange of ballast water at sea, the installation of filtration systems, and heat or chemical treatments.

But the small size of microorganisms, and the ability of many species to withstand heat and chemical treatments — sometimes by remaining in sediment that builds up in ballast tanks — may mean additional types of treatment are needed.

“If we keep throwing the dice,” Dobbs said, “it’s statistically predictable in the long term that the Chesapeake Bay environment would be a loser if shipping patterns continue as anticipated and the water isn’t treated.” Some follow-up work is aimed at finding ways to treat ballast water for bacteria, he said.

Ships entering the Chesapeake are encouraged, but not required, to exchange ballast water at sea in the hope of minimizing the potential for introducing nonnative species.

In its Chesapeake 2000 Agreement, the Bay Program called for creating a task force to work with ports, the shipping industry, the Coast Guard and others to develop and implement a national program to reduce, or eliminate, the potential for introducing nonnative species through ballast water. That task force has not yet met.

Until a national program is developed, the agreement calls for the development by 2002 of a voluntary ballast water management program for the Bay.

Ann Swanson, executive director of the Chesapeake Bay Commission, which represents state legislatures and has expressed concern about the ballast water in the past, said the new study amplifies the need for the Bay region to take a more aggressive role in dealing with the issue.

“I think if we wait for the federal government, then we are not taking the initiative to protect our waters,” she said. “Ballast water coming into the region is a game of ecological roulette, and the more we accept ballast water, and the more volume that comes in, the greater the odds are that we will be inoculated with something undesirable.”

Swanson said the region should call for stronger federal action to regulate ballast water. In the interim, she said, the states should seek to make the Bay a “no discharge zone” for ballast water, as has happened in other coastal areas. In those cases, all ballast water releases must take place at sea.

“Of course you are putting your ports at risk,” she said. “They may choose to go elsewhere. But I don’t think so.”

Institutions participating in the Sea Grant- funded research included the Center of Marine Biotechnology of the University of Maryland Biotechnology Institute, and Old Dominion University.