Last month, Joe McMullin, coach of the freshman boys rowing team at Bethesda-Chevy Chase High School, sent his boats out into the Potomac only to have one get stuck in a green mat floating near Roosevelt Island. The mass extended off the island’s bank and into the river.Well, yes, plants need nutrients, and the added nutrients from all the shit coming in from up stream do contribute to the growth of the plants, but in so doing, the plants take nutrients out of the water, and at least temprorarily sequester it, moderating its influence farther down stream
“It was also growing up near the steps near the Lincoln Memorial,” McMullin told Answer Man as the team assembled this past week at Thompson Boat Center.
As Answer Man looked across the Potomac, he could see a thin green margin edging Roosevelt Island.
“Two weeks ago, the heavy rains cleared it out,” Joe said. “It was pretty bad the first two or three weeks of September.”
The tide was rising, which was another reason the green carpet was not as visible as it often is, for it grows up from the river bottom. Biologists call it submerged aquatic vegetation, SAV for short.
The SAV in the DMV includes the invasive species hydrilla along with such mellifluously named native species as small pondweed, spiny naiad, water stargrass, wild celery and coontail. It’s all thriving, which has divided experts.
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Dean blames the increased SAV on pollution, namely nutrient runoff: fertilizer, farm waste, pet waste and the overflow from combined storm water and sewage systems in the District and Alexandria. All of it washes into the river and encourages the growth of SAV. In warm, stagnant or slow-moving water, algae will often bloom atop the vegetation
But others feel that the increase in the Potomac’s SAV — including hydrilla, a once much-maligned species whose reputation has improved in recent years — is because the river is actually cleaner. Improvements in wastewater treatment have reduced the overall nutrient load. And a Potomac strewn with vegetation is actually more natural, said R. Christian Jones, a freshwater ecologist and director of George Mason University’s Potomac Environmental Research and Education Center.Mattawoman Creek, a Potomac tributary south of Washington, is just loaded with Largemouth Bass and Northern Snakeheads.
“Our best guess is that all of the shallow areas of the Potomac were covered with submerged aquatic vegetation when the Europeans first came to the area,” Jones said.