Mountain Studies Institute contributed to two new scientific papers published Nov. 6 that explain how extra nitrogen deposited in undisturbed lakes is changing the nutrient balance available to algae, the small aquatic plants at the base of the food chain.
San Juan Mountain lakes studied near the MSI field station in Silverton had relatively low levels of nitrogen, however, compared to many of the lakes studied in the Front Range of northern Colorado and portions of Norway and Sweden.
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 top-notch scientific journals Science and Ecology.
The findings demonstrate that atmospheric nitrogen deposition, 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 higher predators, such as fish.
Algae, like all plants, need nitrogen and phosphorus for growth. Inputs from pollution in the atmosphere appear to shift the supplies of nitrogen relative to other elements, like phosphorus.
“When nitrogen levels get too high, the growth of algae at the base of the food web becomes limited by how much phosphorus they can acquire,” says Koren Nydick, executive director and chief scientist at the Mountain Studies Institute. “Initially, excess nitrogen from air pollution may stimulate growth of algae, but the phosphorus-starved algae are poor quality food for zooplankton, and this may have repercussions for fish.”
James Elser from Arizona State University, who led the collaborative study, likens phosphorus-poor phytoplankton to “junk food” for zooplankton.
Lake sampling and experiments were initially conducted in low and high nitrogen deposition regions of Colorado in 2006. The San Juan Mountains near Silverton served as a “low deposition” region due to the relatively low amounts of nitrogen deposition recorded at a long-term monitoring station at Molas Pass.
Algae in the San Juan lakes tended to be more deficient in nitrogen than phosphorus. This contrasted to lakes on the Front Range of northern Colorado, which receives about three times the nitrogen deposition recorded at Molas Pass. Algae in Front Range lakes tended to be phosphorus-poor (i.e., “junk food” for fish).
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 when atmospheric nitrogen deposition rises even slightly above background natural levels. 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. While the lakes in the San Juans did not show high nitrogen levels or resulting nutrient imbalances in this study, such changes could happen if nitrogen pollution rises.
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.
This study was supported by the National Science Foundation.