At any point in time, tens of thousands of ships are crossing the oceans of the world, carrying goods to hundreds of distant ports.

Tagging along is cargo of a different sort: algae, worms, plants, fish and other aquatic species, sucked into the vessels along with the millions of gallons of "ballast water" used to help stabilize ships at sea.

That water is routinely dumped at destination ports, along with any organisms that have inadvertently hitched a ride.

Coming with these species are unpredictable impacts: a common comb jelly from the Atlantic Coast has nearly wiped out fisheries in the Azov and Black seas; zebra mussels have brought economic and ecological havoc to the Great Lakes; and toxic "red tide" algae blooms have rapidly spread around the world, threatening fish and humans alike. All are known to have been transported by ocean-going ships.

Some call this free-wheeling movement of species "ecological roulette." And, a new report says the Chesapeake Bay is heavily involved in the game.

The report, prepared by the Chesapeake Bay Commission, warns that the Bay is a potential "hot spot" for invasions by non-native species because of the high volume of shipping traffic at the ports of Norfolk and Baltimore.

"The relevant question is when the next invasion will occur rather than if it will occur," commission Executive Director Ann Pesiri Swanson said at a recent CBC meeting. "That's why this is appropriately labeled 'roulette' - that invasion may be good, it may be bad and you don't know when it will occur. But once you engage in playing it, over time, your odds will diminish and something will happen."

The commission is an advisory body representing the legislatures of the three Bay states. Commission-backed resolutions urging federal and international action on the handling of ballast water have been approved by the general assemblies of all three Bay states this year.

A number of well-known exotic species have already entered the Bay through various means over the years, with mixed results. The grass hydrilla has been able to survive in areas where the Bay's native species were wiped out, accounting for part of the resurgence of grasses - an important part of the Chesapeake ecosystem - in recent years. But the Asian clam has proven troublesome, and some believe the parasite MSX, one of the diseases that has ravaged the Bay's oyster population, originated outside the Bay.

But the widespread movement of ballast water, the commission's report warns, is increasing the risk of new invasions, particularly as world trade increases and faster ships haul the goods.

Ships today have special compartments to hold huge amounts of ballast water. The largest vessels can hold millions of gallons - so much that boats float inside the ships to inspect the ballast tanks. This ballast water helps stabilize the ship as it crosses the ocean and as goods are loaded and unloaded at different ports.

At any one time, scientists calculate that thousands of species are inadvertently being transported by ships around the world.

Nationwide, it is estimated that 57 million metric tons of ballast water - about 2 million gallons an hour - were emptied into U.S. waters during 1991.

That same year, a U.S. Department of Transportation study estimated that about 3.2 billion gallons of ballast water were dumped into the Bay by large international cargo ships arriving at the ports of Norfolk and Baltimore. The figure underestimated the Bay total, according to the commission report, because it did not include smaller vessels, international cargo ships that first visited another U.S. port, Navy ships or vessels going to other destinations in the Bay.

Ships from at least 48 nations visit Norfolk and Baltimore, and they rank second and fifth nationally in the amount of ballast water they receive, the commission's report said. Combined, they receive more ballast water than any Atlantic Coast port.

"What's important about that is that ships are often taking on their ballast in estuaries and bays, waters teeming with life," Greg Ruiz, a researcher with the Smithsonian Environmental Research Center in Edgewater, Md., told commission members who reviewed the report earlier this year. "Virtually everything you can imagine in a Bay system is coming across the ocean from other ports of the world."

In the past two years, Ruiz and his colleagues have examined ballast water from about 70 foreign ships in the Bay, and found that about 90 percent of them contained living organisms.

Most of the organisms are algae, zooplankton, larvae and tiny aquatic worms. "In some ships we've gotten small crabs and even schools of fish," Ruiz said.

No one knows exactly what happens to these organisms when they are dumped in the Bay. Maryland Sea Grant is funding a study at the Smithsonian research center to determine the susceptibility of the Bay to invasions. It will focus on whether such variables as salinity and temperature promote or hinder invasions, and whether viability differs among different taxonomic groups of animals and plants.

So far, Ruiz said, laboratory tests indicate that many of these species may do "quite well" in the Bay. "Many of the species are viable, are capable of invading."

Lessons for the Bay abound in other places. About 200 nonindigenous species are known to inhabit San Francisco Bay, 120 are in the Hudson River estuary, and 137 are in the Great Lakes. Of invasions known to have taken place in the past 20 to 30 years, most are blamed on ballast water. Many of these species, particularly nonnative bivalves such as clams and mussels, are rapidly replacing the native populations, recent studies indicate. The long-term consequences for the food webs are uncertain.

Beyond the species identified in his Bay study, Ruiz said ballast water may contain organisms too small to have been detected, including bacteria or viruses that may spread diseases through native populations.

In 1991, a strain of cholera from Latin America was found in ballast water holding tanks in the port of Mobile, Ala. Ballast water may, as a result, have been responsible for the detection of cholera in oysters found in Mobile Bay, causing public health officials to temporarily close oyster beds and warn consumers against handling or eating raw oysters or seafood.

Transport by ships is thought to be one reason for the increasing number of algae blooms known as "red tides" around the world, the commission report said.

The bloom-forming dinoflagellates - a type of algae - live in "cysts" in the sediments that are sucked into ships along with the water which is then transported from port to port. When the conditions are right, the algae emerge from the cysts and rapidly multiply. The blooms of some species are toxic to fish - and even humans - and may lead to poisoning and fatalities, the report said.

Australian researchers have discovered cysts of bloom-forming dinoflagellates in 40 percent of the ballast tanks of 80 ships studied. One of these algae species has successfully invaded Australian waters, leading that country to become one of the first to raise concerns about ballast water transport.

Concern about ballast water in North America increased dramatically with the introduction of the zebra mussel to the Great Lakes in the mid-1980s. The zebra mussel, a native of the Caspian Sea which has spread across Europe over the centuries, was inadvertently hauled to the Great Lakes in ballast water from an ocean-going ship.

With no natural predators, its population has mushroomed to the point that they clog industrial and municipal water intake pipes and threaten the entire Great Lakes' food web, jeopardizing its fisheries. It is estimated that it will cost cities and industries $3 billion over the next decade to keep the mussels from clogging their water pipes - a figure expected to grow as the mussel spreads through the Mississippi and Hudson river systems.

Congress responded in 1990 with legislation calling for studies of the issue. In 1991, voluntary guidelines were established that urged ships entering the Great Lakes to exchange their ballast water in the high salinity ocean waters before entering the St. Lawrence Seaway. The hope is that the exchange will both remove and kill most fresh water species before they can get into the lakes.

The guidelines became mandatory in 1993. Those regulations, and ones later adopted for the Hudson River, are the only mandatory measures in the world dealing with ballast water.

Resolutions backed by the commission, which were developed in consultation with scientists and the shipping industry, call on the federal government to establish voluntary ballast water management measures for the entire country that are similar to those in the Great Lakes. The measures are to be coupled with a large educational campaign. After two years, the measures could become mandatory.

Swanson said there was consensus that "mandatory measures should not be considered without trying voluntary measures first."

She noted that the Great Lakes requirements were voluntary for two years and "enjoyed a very high level of participation" and therefore caused few problems when the guidelines became mandatory.

The Bay states can do little on their own about ballast water because states cannot pass laws affecting interstate commerce. But, Swanson said, the states could undertake educational campaigns that would distribute multilingual informational material to ship crews visiting Chesapeake ports.

Exchanging water at sea is not the perfect answer, the commission's report notes. There is no guarantee that the exchange will flush out or kill the hardiest of organisms; it only lowers the risk of invasion.

Also, the regulations make many exceptions for practical and safety reasons. Ships often cannot safely exchange ballast water in high seas, and for some large ships it is costly - taking up to three days to complete the flush. The largest of ships could even break in half if they tried exchanging ballast water at sea.

As a result, the commission's report suggests that the U.S. support international efforts to explore design changes for ships or different treatment techniques - such as the use of heat or chemicals - that would minimize the transport of species.

But that solution, the report notes, could take years - both to reach international agreement, then to complete the changes in the world's shipping fleet.

Copies of the report, "The Introduction of Nonindigenous Species to the Chesapeake Bay via Ballast Water," are available from the Chesapeake Bay Commission, (410) 263-3420.