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Invasive Grasses Defy Pollution to Bolster Chesapeake's Health

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Last year, record-breaking rains pummeled Maryland and the Chesapeake Bay, flushing farm fertilizer and runoff pollution from parking and roads lots into the estuary.

Many Chesapeake ecologists feared that the unprecedented downpours – partially driven by climate change – would reverse progress that the bay had been enjoying since the year 2010.

Well, the results finally came in last week and they were in some ways predictable.  The University of Maryland Center for Environmental Health’s annual report card on the bay found that the water clarity in the bay last year plummeted to a rating of 7 out of 100 – the second lowest figure on record.

But, here’s the surprising part:  the bay’s overall health dipped only slightly last year, from a 54 rating out of 100 in 2017 to a 46 in 2018. That’s much less of a slip than researchers expected with all the rainfall.

The main reason: Underwater grasses in the bay, which are critical habitat for crabs and fish and perhaps the Bay’s most important single health indicator, hung on despite the downpours, which normally smother grasses with runoff, sediment and mud.

Preliminary data from the Virginia Institute of Marine Science found that underwater grasses covered at least 86,000 acres of the bay last year (and that figure is likely to grow when the final numbers are submitted). That’s less than the almost 105,000 acres in 2017, but still about the fifth highest total on record since monitoring began in the late 1970s.

How could this be?  Normally, murky waters kill underwater grasses, because they block the light.  But not last year, apparently.

Professor Robert Orth runs the aquatic vegetation monitoring project at the Virginia Institute of Marine Science.

“These plants, even though we had an incredible amount of rainfall and the water was very turbid, they were able to deal with it in a way that we had not seen before,” Orth said.

When Orth looked at what types of aquatic plants were thriving last year, he found it was often invasive species called hydrilla and Eurasian watermilfoil – aquarium plants introduced accidentally from Asia. They often grow on the top and are therefore able to absorb light despite the muddy waters below them.

“That may be exactly what’s going on is that these invasive species – they tolerate much better conditions that our native plants can’t,” Orth said.

Despite their reputation as a menace, exotic species have actually helped the Potomac River and other waterways, Orth said.

“If you take the case of the Potomac River, there were no plants in the Potomac River in the Potomac prior to 1980,” Orth said. “Hydrilla was accidentally introduced into the Potomac. And of course – big mistake, hydrilla took over – and it has provided a level of clarity in many parts of the river that now support multiple species, many of which are the natives now.”

Brooke Landry is a biologist at the Maryland Department of Natural Resources who chairs the Chesapeake Bay Program aquatic grasses work group.

“I would say that’s exactly right,” Landry said. “Milfoil and hydrilla both have lower light requirements. And while they are not native species, they have been very instrumental in the recovery of bay grasses. They can go in and colonize areas that aren’t as clear and have turbid water and they help stabilize the sediment and they help clear up the water and that helps facilitate the recovery of our native species.”

So it appears that the new-found resilience of the Chesapeake Bay to record-breaking rainfall may be due in part to hardy immigrants that were not welcomed at first; but that are now providing shelter, oxygen and improved conditions for many bay species.

Tom Pelton, a national award-winning environmental journalist, has hosted "The Environment in Focus" since 2007. He also works as director of communications for the Environmental Integrity Project, a non-profit organization dedicated to holding polluters and governments accountable to protect public health. From 1997 until 2008, he was a journalist for The Baltimore Sun, where he was twice named one of the best environmental reporters in America by the Society of Environmental Journalists.