The stew of non-native species known to be swarming in the Great Lakes just got a little thicker.

The U.S. Environmental Protection Agency announced Monday that a new type of zooplankton, commonly reported in Europe and Asia, has been discovered in the western basin of Lake Erie.

Precisely how the rotifer Brachionus leydigii arrived in the Great Lakes is not known, but contaminated ballast water discharged by oceangoing ships sailing up the St. Lawrence Seaway is a likely answer.

The species was identified in a 2001-2002 survey of ballast tanks of ships sailing into the Great Lakes, though it was not found in the lakes themselves. Nobody can say at this point what type of effect it might have on the Great Lakes, the world's largest freshwater system.

This is the second non-native species recently discovered in the lakes by a team of researchers from the EPA and Cornell University. In late 2016, the EPA announced that another type of zooplankton had also been discovered in Lake Erie.

The two discoveries come after nearly a decade in which no new non-native species had been identified in the Great Lakes, which are now home to at least 187 non-native species.

The rate of species discoveries peaked more than a decade ago when a new organism was being discovered at a rate of nearly two per year.

The shipping industry points to rules requiring overseas ships to flush their ship-steadying ballast water tanks with mid-ocean salt water as a reason for the slowdown in discoveries, but scientists maintain the door to new invasions remains open, and these recent finds bolster that argument.

The new discoveries come at a time when the EPA is under legal pressure to do more to protect the Great Lakes from invasive species. In 2013, the agency established a set of ballast water discharge standards that will eventually require all ships sailing into the lakes, and other U.S. waters, to have onboard water treatment systems to kill ballast hitchhikers.

Conservation groups sued under the Clean Water Act, arguing those standards weren't stringent enough to protect the Great Lakes from the next quagga mussel, zebra mussel, round goby or fish-killing VHS virus - all are invaders believed to have colonized the Great Lakes via oceangoing ships.

The EPA is now developing more stringent ballast water regulations at the same time the shipping industry is pushing for legislation that would pull ballast water enforcement out of the hands of the EPA, a measure conservation groups argue would remove Clean Water Act ballast water protections for the Great Lakes - and keep the door open to more Great Lakes invasions.

While only a single specimen has been found at this point, conservationists are taking it as a sign to strengthen ballast water discharge regulations.

"It's a reminder that we could be one ballast tank away from the next zebra mussel," said Molly Flanagan of the Alliance for the Great Lakes. "I hope this finally puts to rest efforts in Congress to weaken federal ballast water protections."

Explore further Great Lakes invasive species studied

©2017 Milwaukee Journal Sentinel

Distributed by Tribune Content Agency, LLC.