It looked like a field of black roses on the bottom of the river.
It was a peculiar sight over the side of my canoe as I glided down the Potomac River about 15 miles north of Harpers Ferry, West Virginia, on Saturday. The water was crystal clear, allowing the sun to slice all the way to the bottom, which glittered gold.
Swarms of tiny fish darted in and out of lush stands of ribbony aquatic grass with star-shaped white blossoms (wild celery, or Vallisneria americana). Smallmouth bass cruised through the forests, as did a fat, whiskered catfish.
The scene was so beautiful it was almost hard to believe: sunshine, silver-topped clouds, forested river banks, and a river swimming with life. I tied my canoe to the knuckle of a tree root, and swam through the grasses and white flowers.
Then I saw the alien-looking plants. Goopy blobs of neon green and black algae clung to the tops of wild celery stalks, making the the plants look like black roses. Elsewhere, wads of this algae tumbled down the river.
The common name for this plant is blue green algae, but it is technically not a plant or algae at all. It is a bacteria called cyanobacteria, and it is one of the most primitive forms of life on Earth.Cyanobacteria frequently multiply on the Potomac and other waterways in the hot summer months, fed by excessive amounts of phosphorus and nitrogen from fertilizers, sewage treatment plants, and polluted runoff from developments.