Ships arriving in Chesapeake Bay ports bring more than just cargo — in 2013 they also inadvertently released an estimated 10 billion live zooplankton from other parts of the world, a finding that surprised the researchers who recently reported the results.

Regulations aimed at reducing the risk of aquatic invasions went into effect more than a decade ago, and a team from the Smithsonian Environmental Research Center had expected to see a decrease in live organisms being released from ballast holds of ships.

Instead, they found that concentrations of coastal zooplankton discharged into Bay waters had increased nearly fivefold from releases checked before the new regulations took effect in 2004.

“It wasn’t quite what we expected to see,” said Greg Ruiz, SERC senior marine biologist and a co-author of the study. “We wanted to know how things had changed, and they changed in a way that we didn’t expect.”

So-called “biological pollution” from ships has been a concern since the 1980s, when releases of ballast water led to the devastating invasion of zebra mussels in the Great Lakes, causing billions of dollars in damages, harm to water quality and alterations to the lakes’ food chain.

Ballast water has also been blamed for introducing the Chinese mitten crab, the rapa whelk and other exotic species into the Chesapeake Bay. An Asian strain of Vibrio bacteria, which sickened two people who ate raw Bay oysters in 2010, is suspected to have arrived in a ship’s ballast tank.

Large ships often suck huge amounts of water into large ballast tanks when leaving a port to help stabilize the vessels during their voyage. A single ship can often hold 30,000–40,000 cubic meters of ballast water.

Until about a decade ago, that ballast water — along with any organisms in it — was routinely discharged into the water at destination ports. Starting in 2004, the Coast Guard required ships to exchange ballast water while they are more than 200 nautical miles offshore, replacing water drawn in at ports with ocean water. That was supposed to reduce the density of coastal species in the ballast tanks and replace them with oceanic species less likely to survive in the fresher water of ports.

But Smithsonian scientists were shocked when they examined ballast water collected in recent years from ships arriving in the ports of Norfolk and Baltimore and found that coastal zooplankton concentrations had dramatically increased when compared with samples gathered from 1993 to 2000, before ballast water exchange was required.

The scientists, who reported their findings in the journal PLOS ONE, say that several factors likely contributed to the increases, including the fact that the total amount of ballast water being discharged increased nearly fivefold during that time.

While overall shipping traffic has remained fairly steady in the Bay in recent decades, the amount of traffic by “bulkers,” primarily transporting coal, has increased steadily since 2000, figures in the study show.

Container ships both pick up and drop off cargo, and therefore often do not have to take on as much ballast. But bulkers leave the Bay full, typically taking coal to other countries, and return empty. That means they must take on large amounts of ballast water for their return voyage to stabilize the empty vessel, and most of that gets released when the ship returns to pick up another load.

From 2005 through 2013, the amount of ballast water being discharged into the Bay increased 374 percent, the scientists said. The increase was driven by bulkers, which accounted for more than three-quarters of the total ballast discharge in the final years of the study.

As a result, the effectiveness of the ballast exchange requirement was reduced simply because significantly more water was being released.

But the scientists said that alone did not account for the overall increase in zooplankton concentrations, which are supposed to be greatly diluted by ballast water exchanges.

They suggested several other factors may be playing a role. Zooplankton concentrations have been poorly studied in other areas, the Smithsonian team said, and may have increased in ports where Bay-bound ships drew in water, especially because the origins of those ships changed over time. In the earlier years of the study, almost all of the ships arriving in the Bay came from the Eastern Mediterranean, but none of the vessels calling in more recent years originated there, as shipping patterns have changed.

Also, the length of voyages has decreased as ships have become faster, increasing the chance that zooplankton taken on in the port of origin would survive the trip.

And while all ships reported conducting ballast water exchanges, the paper noted that it is impossible to verify how well those were performed. Highly effective ballast water exchanges can eliminate 90 percent of the organisms from the port of origin, but the scientists said there’s no way of knowing whether the ships achieved that level of efficiency.

Most likely, the Smithsonian team concluded, all of those factors may have contributed to the increase. “There are so many interacting factors, there is no way to pinpoint one,” said Jenny Carney, the lead author on the paper.

The findings don’t mean that ballast water exchange does not help. “The zooplankton have increased surprisingly, but it would be even higher if we didn’t have ballast exchange,” Ruiz said.

But, the scientists said, research shows how shifts in trade and trade patterns can diminish the effectiveness of efforts to reduce the risk of biological invasions through ballast water exchange.

This has implications for the future, the authors said. While the demand for coal has dropped in the past few years, other factors could lead to the importation of more ballast water. A recent expansion of the Panama Canal has led to a new generation of even larger shipping vessels, the authors noted, and the ports of Baltimore and Norfolk are among the few along the Atlantic Coast capable of handling those ships. The expected opening late this year of a new terminal in Maryland to export liquified natural gas could also lead to more shipping and ballast water.

“All of these factors combined can lead to highly dynamic changes in [ballast water] delivery to Chesapeake Bay,” the scientists wrote.

That could increase the risk of further invasions by nonnative species in the Bay, the scientists said.

“People should be concerned about this,” Ruiz said. “We know that invasions are a really major force of change, ecologically, economically and in terms of health, too. We see that with invasions around the world, including in Chesapeake Bay.”

Ballast water is considered to be the primary mechanism through which nonnative species colonize coastal waters around the globe. Zebra mussels, and the closely related quagga mussel, may be the poster children for the impact such invasions can cause. The filter-feeding mussels latch onto solid substrates in such huge numbers that they clog water intakes and have sunk navigational buoys.

They have spread avian botulism, which has killed thousands of birds in the Great Lakes, and they have caused the near extinction of some native mussel species. They filter algae from water, but are selective in the types that they consume, which has resulted in the overconsumption of some algae sought by fish — leading to the collapse of some populations — while allowing algae that contribute to poor water quality to persist.

They also accumulate toxic contaminants, which are passed up the food chain when the mussels are consumed by other species. They are blamed for more than $7 billion in damage to the Great Lakes fisheries alone, and have been spreading across the country. They reached the upper Chesapeake in the last few years, probably by attaching to recreational boats, though it’s unclear whether they will thrive in the Bay’s saltier environment.

But the Bay also has suspected ballast water invaders of its own, including the rapa whelk, a native of the western Pacific Ocean, which consumes native mollusks. It was first discovered in the Virginia portion of the Bay in the 1990s and appears to now be established.

The Chinese mitten crab, which was confirmed to be in Maryland’s portion of the Bay in 2005, has become an economic and ecological problem in other areas where it has become established and abundant.

Although coastal waters, including the Bay, are constantly subjected to new species from ballast water, it’s difficult to predict which will survive and potentially become problems. While those species thought to have entered the Bay through ballast water have not caused ecosystem-altering changes like zebra mussels in the Great Lakes, scientists note that some species may remain at low levels for years, even decades, until the right conditions allow their populations to explode.

“We are not saying that all species delivered in ballast tanks are going to have major impacts, but some subset of them can and will, and we see good evidence for that,” Ruiz said. “So at some point, with business as usual, one of those species will have a pretty big effect” in the Bay.

The risk might be reduced, over time, by new regulations that will be phased in starting this fall. Instead of relying on ballast water exchange, those rules require that ballast water to be treated with approved technologies — typically chlorination or exposure to ultraviolet light — to greatly reduce the number of live organisms that could be discharged.

But it will be years before those technologies are on all new ships and existing ships. Also, they may not perform as well in the real world as they do in tests, said Mario Tamburri, of the University of Maryland Center for Environmental Science, who has been working on the new techniques.

“It is still going to remove more organisms than just doing an exchange or doing nothing,” Tamburri said. “Whether it is actually meeting the intent of the regulations, the discharge standard, that’s what’s questionable.”