Duckweed a problem for canal boaters


An accumulation of duckweed is seen on the surface of the Dismal Swamp Canal Tuesday afternoon. The photo was taken looking north from the dock of the Dismal Swamp Canal Welcome Center.


By Chris Day
Multimedia Editor

Thursday, November 23, 2017

SOUTH MILLS — A culprit known as duckweed is lending an extra emphasis to the word “dismal” on the Dismal Swamp Canal.

Donna Stewart, director at the Dismal Swamp Welcome Center, says she’s certain the unusual amount of duckweed in the canal is the reason many boaters have opted not to use it.

"I know it's a reason," Stewart said, adding some boaters who’ve stopped at the welcome center since the canal’s reopening late last month have told her they wished they hadn't taken the waterway.

"I wish I'd turned around and gone back the other way," some boaters have commented to her, she said. 

"I feel like this is the big reason people are choosing the Virginia Cut," Stewart added.

While boaters using the Dismal Swamp Canal must pass through the lock at South Mills, the Virginia Cut takes them around the canal and through the lock at Great Bridge, in Virginia. The Virginia Cut winds roughly 15 miles into the Currituck Sound and eventually through Coinjock along the North River and into the Albemarle Sound. 

Traffic on the canal came to a halt last October when the U.S. Corps of Engineers closed the waterway in the aftermath of Hurricane Matthew to perform dredging work and clear trees and other debris.

While the canal has been back open since Oct. 31, traffic appears to be sluggish.

In October 2015, a total of 394 boaters used the canal, according to numbers counted by the lock master at the South Mills Lock. Just three boaters used the canal on Oct. 31, the only day in October this year the canal was open, Stewart said.

The number of boaters using the canal this month isn’t yet available, but there’s been decreasing use in recent years. According to the South Mills lock master, 294 boaters passed through the canal in November 2013. That number fell to 259 in November 2014 and just 199 in November 2015.  

Duckweed is a marine vegetation that floats on the surface of the water. Ted Manzer, an agriculture teacher at Northeastern High School, writing about duckweed in his weekly column in The Daily Advance once described it as “a floating flowering plant without stems” whose “growth rate can be phenomenal.”

“Under good conditions duckweed can double its biomass in less than 24 hours,” Manzer wrote. ”This means it can cover an acre of water in less than a month and a half.”

Also helping duckweed spread, Manzer said, are wild fowl like ducks who eat the plant.

“The problem is that the birds don’t really control it. They spread it to other waters,” he wrote.

The closing of the canal for roughly a year likely contributed to the growth of the duckweed. Because the canal was closed, its waters were still and undisturbed. Add to that the summer's heat, the result was an environment in which duckweed thrives.

Duckweed is a nuisance for boaters. That’s because it can accumulate in the vessel’s engine’s sea strainer, and if the strainer is not routinely removed and cleaned the result can be engine overheating and other problems. Raw water used for inboard engine cooling is pumped aboard and passes through the strainer, which filters out unwanted debris. That water is then pumped back overboard.  

But Duckweed isn’t just the bane of boaters. It can be deadly for other plants that live in affected waterways. 

“These floating plants cover the surface (of the water) and eliminate light to submerged aquatic plants, causing their demise,” Manzer wrote in his column.

While the outbreak of duckweed in the canal has been bad, not all boaters have complained about it. 

"We've had mixed reviews" from boaters, said Stewart.

In the last month boaters moored at Mariners’ Wharf also reported mixed reviews of the duckweed. While some said they had to routinely clean their strainers while using the canal, others said it wasn’t a problem for them. Changing daily wind conditions may play a role in how duckweed accumulates in certain patches of the canal. 

Stewart encourages boaters to visit the welcome center's facebook page (facebook.com/DismalSwampWelcomeCenter/) for daily photos of the duckweed accumulation around the center's docks.