New Studies by Colorado State Scientist Show Changes in Alpine Lake Ecological Nutrient Status

Increasing atmospheric nitrogen deposits in lakes and watersheds in Colorado and around the world are directly affecting the life they contain. A Colorado State University research scientist contributed to two new scientific papers published today that explain how extra nitrogen in undisturbed lakes is changing the nutrient balance available to algae, the small aquatic plants at the base of the food chain.

Although increased concentrations of nitrogen in alpine lakes from atmospheric deposition have been widely reported, the subsequent effects on lake biology are not well documented. The results of the studies are published in the scientific journals Science and Ecology this week. The findings demonstrate that atmospheric nitrogen deposits, which have been increasing steadily due to emissions from motor vehicles, energy production and agriculture, could reduce algal diversity and favor algae that are poor quality food for higher consumers such as zooplankton. Zooplankton are small swimming animals in lake food webs that are important food for even higher predators, such as fish.

“We found that, in the presence of too much nitrogen, the algae at the base of the food web are of much poorer quality for zooplankton, the small swimming animals that consume it. This may eventually have repercussions up the food chain for fish that eat the plankton,” said U.S. Geological Survey research ecologist and Colorado State senior research scientist Jill Baron, a co-author on both papers. Koren Nydick, a graduate of the CSU Graduate Degree Program in Ecology, is also a co-author.

Short-term experiments were initially conducted in three regions of Colorado. Nitrogen additions to waters in regions where atmospheric nitrogen deposition is low moved lake nutrient supply away from being relatively balanced toward chemistry that is less favorable for zooplankton. Nitrogen additions to Front Range lakes, where nitrogen deposition and lake nitrogen concentrations are higher, had no effect.

In Front Range lakes, nitrogen no longer limit algal growth, and another important element, phosphorus, is in short supply. This is important because algae that are limited by phosphorus are very poor quality food for the zooplankton that consume them.

The research in Colorado was conducted by a team of scientists led by James Elser from Arizona State University. The Colorado results are strengthened by additional evidence from Norway and Sweden. A comparative analysis definitively shows that the nutrient status of lakes is disrupted in all three regions due to human-caused atmospheric nitrogen deposition. The finding is important for water quality management in remote, low nutrient lakes where sustained increases in atmospheric nitrogen deposition may eventually influence the entire food web, including fish. These lakes are not immune to disturbance from air pollution, and the results imply important ecological impacts.

This work adds to a growing body of information documenting changes in alpine plants, forests and water quality of Colorado from atmospheric nitrogen deposition. In response to the documented changes, the State of Colorado developed a Nitrogen Deposition Reduction Plan in 2007 designed to improve air quality for Rocky Mountain National Park in order to protect sensitive ecosystems from damage.