CHICAGO (Reuters) - Environmentalists warn that invasive species disgorged from ships’ ballast tanks have put the Great Lakes near an ecological tipping point.

Light from the rising sun is reflected on ice sheets in Lake Michigan as temperatures dropped to 6 degrees Fahrenheit in Chicago, February 2, 2007. REUTERS/John Gress

But rules enacted this year require all inbound ships to flush their ballast tanks with seawater, allaying most fears about more invasions of the five lakes, which collectively hold one-fifth of the world’s fresh surface water.

“I just wonder sometimes if (environmental groups) appreciate that they’ve won the battle. And whether we ought to move on to other things like sewage runoff, and infrastructure, and other things that are problems for the Great Lakes,” Collister Johnson, administrator of the Saint Lawrence Seaway Development Corporation said.

Johnson, in Chicago to discuss increasing container trade on the lakes, said in an interview that forcing every ship to flush ballast tanks in the Atlantic Ocean before entering the Seaway is close to 100 percent effective at controlling the invasive species problem.

He would like even stricter regulations, which are hung up in the U.S. Congress, that may require shipowners to install equipment to scour ballast tanks with chemicals, gas, or some other method to guarantee any invaders are killed.

When ships offload their cargoes, they fill their ballast tanks, then empty them again when taking on another load. Even empty ballast tanks can harbor harmful species.

Previous colonizers of the Great Lakes such as zebra and quagga mussels, the round goby, and dozens of other species have proliferated and altered the lakes’ ecology.

“It only takes one critter to make a difference. You have to question the ability to enforce the (flushing) rule,” said Jeff Skelding of the environmental group Healing Our Waters.

Invasive mussels have blanketed vast areas of the lakes, clogging water intake valves of municipalities and utilities and triggering algae blooms that foul beaches. A University of Notre Dame study put the cost of invasives at $300 million a year.

But a proposal floated last year to ban ocean-going vessels from the Great Lakes altogether to keep out invasive species appears to have faded.

Proponents had pointed to a disputed study that concluded ocean-going traffic was not that large and using alternate modes including trains, trucks, and lake-only ships and barges would cost only an additional $55 million annually.

Johnson disputed the figure, and called the idea of closing the waterway to ocean-going vessels “absurd.”

The Seaway, a system of locks and canals that opened in 1959 and jointly owned with Canada, is not large enough to handle the large container ships plying the seas. Most of the traffic on the lakes are bulk cargoes -- imports of steel and exports of grain and iron ore.

But Johnson has high hopes for a proposed container port to be located in Nova Scotia, for which the Seaway could provide a feeder system to the U.S. interior.