Eurasian watermilfoil is pretty to look at -- but otherwise is a nuisance. Common to eastern U.S. lakes, the aquatic weed entangles boat propellers, crowds out native plant species, inhibits water flow and possibly degrades water quality. Now, it has invaded Lake Tahoe, prompting UC Davis researchers this summer to study its impact on the lake. Found mostly at nearshore locations and in the South Lake Tahoe Keys area, the weed is believed to deteriorate water quality by acting as a straw, drawing up phosphorus, which the plant then makes available to other aquatic organisms, enhancing algal growth, says UC Davis graduate researcher Katey Walter, who works with limnology professor Charles Goldman. The weed takes up far more phosphorus than native Sierra Nevada lake plants, Walter says. In the fall, when colder water temperatures prompt the perennial weed to go dormant, bacteria degrading the plant's tissues deplete oxygen from the lake's water -- also a concern to researchers. This UC Davis study -- the first to examine the impact of the weed on water quality in Lake Tahoe -- fits into the Tahoe Research Group's overall quest to determine more about the forces that are dramatically reducing the lake's clarity and quality. The weed, Walter points out, is only one of many impacts on Tahoe clarity: road runoff, fertilizer and auto exhaust lead to far more algal growth than does Eurasian watermilfoil. Still, the weed is spreading rapidly in the lake. Already Tahoe-area residents routinely use the underwater equivalent of lawnmowers to trim the fast-growing weed, Walter says. "The plant is particularly a problem in the marshes, keys and in stagnant near-shore areas because it grows fairly densely." Walter's research will help Tahoe basin agencies determine how to manage watermilfoil. "Is it only an aesthetic nuisance or is it a water-quality issue?" she says. Walter's study includes analysis to quantify the plant's phosphorus uptake, water quality tests in the areas in which the plant grows and aerial flights to map out where the plant is growing this year compared with previous years.