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Science: Researcher sees the best kind of scum
By Jason Lesley
It was the best of retention ponds and the worst of retention ponds.
Dr. Erik Smith, director of research and resident coordinator for the North Island-Winyah Bay National Estuarine Research Reserve, described adjoining ponds in a high density development. One was covered with a green scum that looked like pea soup. The other was clear and healthy looking.
The only difference in the development’s ponds, Smith said, was that a nice woman feeds the ducks in one pond and a mean, old man chases them out of the other.
“Everything else is the same,” Smith said. “I don’t know that it’s the ducks conclusively, but biology will follow the ecology of a pond. While the duck feeder is well-intentioned, he said, her pond contains processed duck food in one form or another — and too much algae.”
South Carolina has 14,446 man made ponds covering more than 21,000 acres. There was a 91 percent increase in the number of ponds from 1994 to 2006 between North Inlet and Myrtle Beach, Smith told the local Sierra Club chapter at a talk this week.
Ponds are necessary for both flood control and as a source of fill dirt along the coast. “Increasingly, they enhance adjacent property development,” he said. “People are fairly proud of their ponds oftentimes, and they have become a major feature of coastal lands.”
They are designed to capture the “first flush” of stormwater runoff, about a half-inch, he said.
“Creation of ponds has changed the plumbing on the coastal zone,” Smith said. “We have gone from forested wetlands under a permanent canopy to retention ponds with no canopy.”
Smith, who is based at the USC Baruch Marine Lab at Hobcaw Barony, said he has studied the water in retention ponds and how it affects seawater.
“These are not swimming pools,” he said. “They have ecology. The question is: Do they behave as natural lakes?”
A healthy pond contains living algae, Smith said. Algae need nitrogen and phosphorus in a 16-to-1 ratio.
Nitrogen reaches a pond naturally through rainfall, decaying plant matter and weathering of soil and rock, Smith said. Phosphorus, along with some nitrogen, reaches a pond through soaps, fertilizer and fossil fuel combustion. Animal waste is high in phosphorus relative to nitrogen, he said.
“Without nitrogen there is no algae,” Smith said. “Without algae there is no life. Ponds can serve as important ecosystems.”
He said ponds get caught in “the Goldilocks dilemma” regarding nutrients. Without them there is no life. With the right amount, there is a healthy, balanced ecosystem. Too many nutrients lead to an unbalanced, dysfunctional ecosystem — and that layer of green scum on the surface.
Smith and his students studied 26 residential ponds last year. The ponds averaged 2.2 acres in size and were typically 5 feet deep.
The nitrogen was not that variable, he said. Phosphorus increased with residential density, but most of the ponds were deficient in phosphorus relative to nitrogen concentrations. Phosphorus became a predictor of algae in the pond, Smith said.
When ponds get unbalanced, communities have turned to what Smith called “lake management” companies. Typically, they use copper sulfate to kill algae and temporarily clear the water, he said.
“One week later,” he said, “the algae have reestablished themselves. The ponds get a visit every two weeks. Once you go down that road, it’s like beach renourishment. You can’t go back.”
Smith is studying the effects of aeration on ponds. Aerators add oxygen to the water. There is plenty of anecdotal information but no data, he said. His study includes four ponds: two in Murrells Inlet and two in Surfside Beach. After all treatment was stopped, water was sampled for a year. Smith will install aerators on two of the four ponds this week so he can sample and compare the water for another year.
“I would love to encourage aeration versus copper sulfate,” he said.
The larger question for researchers, Smith said, is what happens if pond water reaches the ocean.
He conducted an experiment using saltwater from North Inlet and water from a residential retention pond.
He found that the introduction of the pond water’s nitrogen caused bacteria in ocean water to grow rapidly. “Bacteria get the first cut of oxygen and nitrogen,” Smith said. “They live fast, consume quickly and die young.”
With the oxygen depleted, fish kills would follow, he said, starting with the larger species.