Scientists monitor lake algae

Friday, May 25, 2001 | 11:37 a.m.

Algae facts According to the National Park Service:

Water experts are keeping watch on Lake Mead, looking for signs that a harmless green algae blanketing the reservoir could be replaced by a toxic blue-green algae.

The largest green algae growth in the lake's 66-year history began taking hold in February and now extends from the mouth of the Virgin River in the northwest corner of the lake to Hoover Dam, a rare sight as the Memorial Day weekend arrives.

"The bloom appears to be over the whole lake," said Brenda Pohlmann, Las Vegas branch chief of the Nevada Division of Environmental Protection, on Thursday.

Under normal conditions, the algae would die off when the lake's temperature rises to 78 degrees Fahrenheit or more, she said. The water was 76 degrees on Thursday.

After the algae, called Pyramichlamys, disappears, it is possible that the dying cells will allow a blue-green variety to grow, microbiologist Peggy Roefer of the Southern Nevada Water Authority said. Blue-green algae can tolerate hotter temperatures than the green algae, and the lake would still be full of the nutrients that feed the organisms.

Some blue-green species are toxic to fish and mammals, but no human deaths have been reported from the algae.

In 1974 about 10 dogs died from an unknown cause after lapping Lake Mead's water along the shore, National Park Service biologist Bill Burke said. Although blue-green algae was suspected, it was never proven to cause the animals' deaths. Blue-green toxins have killed cows in the Midwest when they drank water from reservoir ponds, he said.

The water authority is monitoring the green algae, because six miles downstream from where the Las Vegas Wash enters the lake, Las Vegas water is supplied to the 1.4 million residents of the valley. There will be plenty of time to warn people if the toxic variety of algae occurs, Roefer said.

The water authority scientists are working with a task force from the Lake Mead Water Quality Forum to monitor the algae.

Tests for bacteria, including disease-causing fecal streptococci, E. coli and other coliforms, show that the water, despite the six-inch-thick green goo, is safe for body contact. The algae also feeds fish and other creatures in the lake, Pohlmann said.

Retired UNLV biologist Larry Paulson said such large green algae blooms may become more common as long as the valley dumps roughly 150 million gallons a day of treated sewage into the lake. The treated wastewater is loaded with phosphorous, a leading nutrient for algae to grow, Paulson said.

A Lake Mead Water Quality Forum committee is studying alternative ways to discharge the valley's sewage without creating an imbalance in the lake.