For awhile, it looked as if water in the Cub Run — and even the Occoquan — might be in jeopardy from a strange, red plant that suddenly covered a pond in Centreville.
Named azolla caroliniana the plant self-propagated, and after it was removed from the pond, it regrew immediately. With a hurricane on the way, the fear was that it would wash into the Occoquan — the source of Fairfax County's drinking water.
"Phil Miley [an engineering technician with the county's Maintenance and Stormwater Management Division] brought it to my attention, Saturday, during Centreville Day — and he freaked," said Ned Foster, who heads Friends of Little Rocky Run, a conservation group dedicated to protecting local streams. "When I saw it, I freaked, too."
"I don't know what to make of it," Foster said Monday, after visiting the pond and seeing the unusual red plant, firsthand. "It kills everything underneath it, including fish. When I saw it for the first time, I was stunned. Sunday afternoon, I sent out an e-mail to everyone in conservation I could think of."
AZOLLA, IT TURNS out, can't live in running water — only stagnant — so there's no danger. But for several days last week and this week, Foster and other water-and-conservation experts were seriously worried.
"It was shocking to see a pond completely covered with it," said Foster. "Even more shocking to learn that the plant growth had occurred in the space of a week — completely covering a two-plus- acre pond."
Last Wednesday, Sept. 10, Heather MacDonald, property director for the Compton Village Homeowners Association, contacted Miley to come out, to investigate and identify the sudden appearance of the weird, red growth covering that community's stormwater management pond. "She told me the pond had turned red in about three days," said Miley.
He, in turn, contacted Willie Woode, senior conservation specialist with the Northern Virginia Soil and Water Conservation District. "We dashed out to the pond as a team. We met Heather on site, and I took photos and Willie took a water sample for analysis by the state," said Miley.
Woode, an aquatic engineer, told MacDonald not to apply any chemicals to the pond until the vegetation was identified. Then he could advise her the best course of action with the least effect on other aquatic life in the pond. The next day, Thursday, Sept. 11, the state identifed it as azolla.
WOODE THEN TOLD MacDonald to immediately start removing the majority of the plant, hand-raking from boats and along the shoreline, bagging it and taking it to a landfill. He also told her to have the pond treated with Diquat, an EPA-approved chemical.
Meanwhile, Foster was still worried. "Azolla, also known as the 'mosquito fern,' is so dense that even mosquito larvae can't live in the water beneath it," he said. "Worse, the pond it has infested is about a half-mile from Bull Run Creek and the Occoquan Reservoir."
However, the wet pond actually held the growth in check, preventing it from migrating elsewhere. "It proves the [value] of having stormwater management ponds," said Miley. "They stop things from getting into our water supply and protect the Chesapeake Bay Watershed."
AZOLLA IS ACTUALLY a blue-green algae, Miley said, that turns red under a certain light intensity. "The problem is it takes over everything," said Miley on Tuesday. "Yesterday, the contractor raked it up and, as fast as he cleaned the pond, it re-formed today. It's like a field of thick ferns and it's smelly."
Before it re-grew, though, MacDonald had Virginia Waters & Wetlands (VWW) of Manassas treat the pond with the recommended chemical. But, she said Tuesday, "Then it filled up with this red stuff." She expected the treatment to take a couple days to take effect. "I've been advised by the company to wait and see what happens and not to scoop anymore because it won't eradicate it," said MacDonald.
"If you put a bottle on it, it floats — that's how thick it is," said Miley. "This is a first for this area. It's a major thing for us." However, he added, "We have no idea [how it got here]."
Native to British Columbia, azolla could hace arrived here via migrating birds or ducks, said Joe Ivers, an aquatic ecologist and president of VWW. "It's like a carpet — it's pretty wild," he said. "The homeowners association is doing the right thing by asking us to treat it. Everybody's on it — the county responded right away."
Still, Ivers said, it'll take about a month to rid the pond of it. He said it's already in the creek leading from the pond to Bull Run and the Occoquan, but not to worry — "It's more of a lagoon or small pond type of plant."
ACTUALLY, IT'S PRIZED in China and Vietnam, where this nitrogen-rich plant is used as fertilizer in rice paddies. It's fed to poultry, cattle and hogs, makes good compost and is used to adorn aquatic gardens.
It controls mosquitoes because its thick mats carpet the water surface, thwarting adults trying to lay eggs and larvae coming up for air. The world's smallest fern, each dime-sized plantlet has 100-200 overlapping leaves less than one millimeter long. It propagates itself by generating spores.
However, since Compton Village's pond is filled with large carp, bass and sunfish, MacDonald's concerned that the azolla will deplete the pond's oxygen. "We do have some dead fish," she said. "They've now gone to the shallower area where they can breathe."
Woode's also worried because azolla "mats the whole surface of the pond, eliminating the chance for oxygen to dissolve in the water." On Monday, he said, the dissolved oxygen level in the water was two milligrams per liter.
"Fish would survive in five milligrams," said Woode. "But, between two and four milligrams, they begin to show signs of stress, popping up above the surface trying to get air. It would eventually cause a fish kill, so the oxygen level [in the pond] is now at a critical level."