60 Percent Jump in Bay's Blue Crabs
Pennsylvania Penalizes Drilling Company for Water Contamination

"Monster" Invasive Species Helped Return of Native Bay Grasses

Beneath the waves of the Chesapeake Bay, an emerald forest once flourished. Underwater grasses swayed in rippling sunlight. Long ribbons of eelgrass released so much oxygen the water fizzed with bubbles. Feathery jungles were nurseries for crabs and fish.

Grassbed But water pollution smothered these aquatic gardens. By the 1970s, the amount of Bay grasses had fallen from about 200,000 acres to less than a fifth of that. More than just a canary in the coal mine of the Bay’s troubles, the death of the grasses stripped away a natural network of clean water machines.

Recently the U.S. Environmental Protection Agency’s Chesapeake Bay Program released a report with good news about the both the Bay’s grasses and blue crabs.  The rise in crab populations was discussed at length in this blog and elsewhere yesterday.  The increase in grasses has been less noticed, but is also important.  Aerial surveys found about 9,000 more acres of aquatic vegetation in the Bay last year than the year before.  That was a 7 percent increase in one year, which is part of a more than doubling over the last quarter century.

Grasses Richard Batiuk, associate science director at the Bay Program, said that there was less rain last year, and this  meant less muddy runoff that smothers grass. But he said he also sees solid evidence of a long-term improvement.

 “Over the last 25 years there have been some up and down trends, but a pretty strong increasing level of grasses,” Batiuk said.  “Look back 25 years, we only had 35,000 acres of grasses. Now look forward, and we’ve got 86,000 acres.”

That means the Bay’s grasses are still less than half of what they were before the 1970s.  But they are slowly recovering. A major reason is better pollution control systems on sewage treatment facilities.

However, there is also an unlikely hero to this comeback tale: invasive species.  That’s right – the mustache-twirling villains of the plant world.

Back in the 1970s and 1980s, bright-green aquarium plants from Asia called hydrilla and millfoil --  most likely dumped into rivers by people getting rid of their goldfish – began to proliferate in the Potomac River and the northern Bay. 

Newspapers warned of an alien menace that would overwhelm all life.  One 1984 headline blared: “Area Governments to Battle Monster Hydrilla.”  Another warned, “Army to Use Herbicide" on Potomac River.

Thankfully, the government did not bomb the whole Bay with poisons.  Because over the years, researchers found that these invasive plants actually served more as home-rebuilders than home wreckers in Chesapeake tributaries.

“Some of those fears were unfounded,” said Nancy Rybicki, a scientist with U.S. Geological Survey who has been studying hydrilla on the Potomac River since 1978.  “Even though the vegetation in the Potomac is dominated by an exotic plant, the waterfowl members increased tremendously when the vegetation came back.”

Bill Dennison, professor at the University of Maryland Center for Environmental Science, said he does not recommend people introduce invasive species anywhere – because you never know when a kudzu-like disaster will happen.

But at least in this case, the native grasses have adapted and are now thriving alongside the exotics, he said.  In recent years, most of the increase in Bay grasses has been in native grasses.

Dennison said of the exotic grasses: “Underwater, they don’t seem to be a scourge that outcompetes the natives...Not such a bad monster after all.”

The tough, pollution-resistant aquarium plants grew like crazy at first in the Potomac River and elsewhere, colonizing polluted riverbottoms that were barren of all plant life. This stabilized muddy areas, and  allowed native grasses to re-establish themselves. More grasses, in turn, fed more waterfowl, and sheltered more fish and crabs.

“Most fish, shellfish and baby crabs that are born get eaten.  So what you need is a place to hide,” Dennison said.  “You get eaten typically even by your own parents. You know, there is a lot of cannibalism in the marine environment.  So most of what happens out there is, you eat each other’s babies, and sometimes your own. So what the three dimensional grass canopy provides is hiding places, so that more babies can escape predation and grow up to become adults.”

The ironies of the Chesapeake grow as thick as a tangle of eelgrass. Invading plants opened the door for the return of the natives. And the resurgent grassbeds, in turn, protected the Bay’s fish and crabs from their own worst enemies – including themselves.


By Tom Pelton

Photos from Chesapeake Bay Program / Alicia Pimental


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