Tahoe weed could threaten lake’s clarity

Wednesday, Aug. 25, 1999 | 3:15 a.m.

SOUTH LAKE TAHOE, Calif. - An aggressive aquatic weed already choking waterways at Lake Tahoe could also threaten water clarity and scientists are trying to gauge the threat.

A study is underway by researchers from the University of California's Tahoe Research Group in an effort to determine the level of danger posed by Eurasian watermilfoil.

The exotic weed, which first appeared in the lake in the 1960s, has proliferated in shallow areas - virtually taking over the Tahoe Keys on Tahoe's south shore. Colonies of the weed have been found all around the lake, including in Emerald Bay, Crystal Bay and Logan Shoals.

Eurasian watermilfoil can cause plenty of problems, ranging from clogged boat engines to increased water temperatures. A fragment of the plant only a half-inch in size can grow a new plant. A small clump can start a new colony.

Researchers want to find out how the weed might contribute to the algae growth that is robbing Lake Tahoe's clarity at the rate of more than a foot each year. Tahoe's famous clarity allowed a person to see more than 100 feet deep in 1968. Now clarity has dropped to roughly 70 feet.

Air pollution, urban runoff and sediment suspended in streams pump the key algae nutrients of phosphorus and nitrogen into the lake and are considered to be the primary sources of algae growth. But watermilfoil could contribute to the problem in at least two ways.

The weed essentially acts as a straw, pumping up phosphorus from bottom sediment and discharging it into the water, where it becomes available for algae.

Thick colonies of the weed can grow so thickly they block sunlight, also allowing for more algae to grow.