Tempe Town Lake growth sown by winter rains
By Dennis Welch, Tribune
August 17, 2005

An aquatic plant threatening to turn Tempe Town Lake green is here to stay despite a series of weekend storms that swept most of the duckweed downstream. "It came here suddenly, but there’s no reason to think it’s going to leave here any time soon," said Eric Swanson, urban fishing program manager for Arizona Game and Fish Department.

He said heavy winter rains created the right conditions for the duckweed to spread into Indian Bend Wash and the Salt River, which will continue to supply the lake with the plants.

Once the rains overfilled the state’s northern lakes and ponds — where duckweed is normally found — the excess water spilled into the rivers, carrying the plant downstream and into the lake.

While the weekend storms blew most of the duckweed out of the lake, Swanson said the city will have to work to keep it in check for the foreseeable future.

On Friday, workers with Tempe-based Aquatic Consulting and Testing treated about 2 acres of the affected area with a herbicide, but the storms blew most of the duckweed down stream before they could fully gauge its effectiveness.

It costs the city about $100 per acre to treat, said Rick Amalfi, vice president of Aquatic Consulting. All the chemicals used by the company are registered with the Environmental Protection Agency and not hazardous for fish, he said.

Last week, city officials estimated that duckweed covered about 15 percent to 20 percent of of the lake. The remaining duckweed has gathered in a few places along the shore, with the highest concentration at the marina on the north side.

The plant became visible last month. While the green film on top of the lake may not be very attractive, the plant does offer some benefits for the lake’s long-term health.

The plant keeps the algae population down because the two plants compete for the same food sources. But, in some parts of the country, duckweed has been reported to kill fish populations by absorbing all of the oxygen.

Duckweed, which earned its name because it’s a duck delicacy, is also harvested and fed to pigs, poultry and cattle.

The small plant floats on the water’s surface, feeding off nitrogen and phosphorous and can double its biomass within 10 days, according to some reports.

Because it takes in nitrogen and phosphorous, the plant has also been used to purify water.

On a related note, organizers of the Fat Tire Triathlon say the Aug. 20 event was not canceled because of duckweed. Officials said warm water temperatures and the possibility of monsoon storms threatened to hurt participation.

Contact Dennis Welch by email, or phone (480) 898-6573