Know Your Algae

By Wendy Bedwell-Wilson

Pond and water garden owners typically complain about two types of algae: string algae that take on a shag-carpet appearance along the bottom and sides of the water feature, and the tiny phytoplankton that create the pea soup effect, said Carolyn Weise, pond consumer relations manager for Ecological Laboratories

string algae (Courtesy of Ecological Laboratories Inc.)

single-celled algae/phytoplankton (Courtesy of Ecological Laboratories Inc.)

Inc. in Freeport, N.J. “Most people see green in their ponds and panic,” Weise said. “There are about 40 varieties of algae that can be found in a pond at any one given time, but most people only see either string or pea soup.”

String algae, caused by a filamentous species of algae, grows in long strands that eventually tangle together and form thick mats that can double their weight within 24 hours, said Curt Nuenighoff, director of TetraPond in Blacksburg, Va. “Blanket weed or string algae tend to adhere to rocks and waterfalls, which can be unsightly,” he said.

Single-celled algae that remain suspended in the water column cause green water or the pea soup, Nuenighoff said. Given the right conditions -- plenty of nutrients and sunlight -- as 5 million algae cells per milliliter of pond water can be present, he said. “These organisms are so tiny, they can pass through even the finest filter,” he said.

In reasonable amounts, neither type of algae harm pond fish nor aquatic plants, said Gary Jones, corporate and scientific affairs manager for Mars Fishcare in Chalfont, Pa. If left unchecked, however, they can affect the pond or water garden's pH and oxygen levels and threaten pond fish health, he said.

“Algae blooms can cause wide swings in pH and oxygen levels so they can, on rare occasions, cause pH problems,” Jones said. “More importantly, they can cause oxygen levels to fluctuate greatly, and low oxygen is a concern.”

Excess nutrients in the water cause algae blooms, Jones said. Whether they result from pond fish waste, decomposing plant matter or fertilizer runoff, those nutrients feed the single-cell and string algae. Because excess nutrients cause algae, Jones said plants are the best way to control it. “Products, such as algaecides, phosphate removers and selective bacteria, are there for when people don’t have the right balance of plants and fish,” Jones said.

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Know Your Algae

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