A high alpine lake in the Colorado Rockies is showing increased algal growth thought to be caused by atmospheric nitrogen deposition from auto emissions and agricultural activity on the heavily populated Front Range, including Denver.
Diane McKnight of the University of Colorado at Boulder said the study showed that since about 1940, changes in Green Lake 4 associated with nitrogen enrichment and climatic changes have increased algal growth, the accumulation of organic sediment and shifts in the dominant algal species. McKnight and her associates have studied algal species distribution in water samples, a sediment trap and a small sediment core in the lake on a weekly to biweekly summer basis since 1998.
"Over the past 20 years, nitrogen deposition has increased in the Green Lakes Valley watershed and the lake ice cover has become progressively thinner," said McKnight, a fellow at CU's Institute for Arctic and Alpine Research. The five lakes in Green Lakes Valley - including Green Lake 4 at 11,500 feet, account for about 40 percent of Boulder's water supply.
"The city of Boulder owns the watershed and makes a substantial effort to protect the water quality of the lakes, including a ban on hikers," she said. "But there are no means for the city to protect the watershed from atmospheric inputs that can cause increased algal growth and potentially have some influence on water quality."
Similar trends have been observed in alpine lakes in Rocky Mountain National Park about 20 miles to the north, McKnight said. The Green Lakes study is part of the National Science Foundation's Long-Term Ecological Research effort, which administers about 20 sites in North American ecosystems where researchers document ecological and climate changes over decades and centuries.
McKnight gave a paper on the subject at the Spring American Geophysical Union Meeting held in Washington, D.C., May 28 to May 31. Co-authors included CU graduate student Summer Waters, CU undergraduates Sean Sundermann and Meghann O'Brien, Alex Wolfe of the University of Alberta in Edmonton and Rolf Vinebrook of the University of Regina in Saskatchewan.
According to McKnight, the ice has become about 12 inches to 14 inches thinner in late March since 1982, possibly due to warming winter temperatures or more accumulation of snow on the frozen lake, insulating the ice.
One species of diatom -- algae that have microscopic shells made of silica -- has grown faster under higher nitrogen conditions and become more abundant in the lake than other diatom species in the past 60 years, she said. The increased algae growth causes more algal pigments like chlorophyll and fucoxanthin, as well as organic carbon from algae, to accumulate in the lake sediments.
"In addition to light, algae need two main nutrients, nitrogen and phosphorus, to grow," she said. "We think the nitrogen deposition in Green Lake 4 stimulated the growth of algae on the lake bottom because there is a supply of phosphorus from the lake sediments."
On a regional scale, there are now proposed levels of maximum nitrogen deposition that may become part of air quality regulations, said McKnight. "These levels have been determined based on avoiding acidification of the water by nitrate in spring snowmelt."
The results of the study indicate that more algal growth in the lake can occur at lower nitrogen levels than those that could cause acidification, she said. They also suggest lower standards than those proposed may be needed to protect water quality in alpine lakes on the Front Range, many of which are sources of water for Front Range communities in Colorado.
Undergraduates O'Brien and Sundermann were supported by CU-Boulder's Undergraduate Research Opportunities Program, which allows undergraduates to receive stipends to work with faculty on research projects. They also were supported by the NSF's Research Experience for Undergraduates program.