Alpine lakes are ideal for studying climate change because of their short ice-free season, typically July to October.
“Any change in the length of that ice-free season has big impacts,” Saros says. “As we see climate changing and the ice-free season becomes longer or shorter, it has a big effect on the species in those ecosystems.”
In 2007, the fourth assessment report of the Intergovernmental Panel on Climate Change noted that there is substantial new evidence that changes in marine and freshwater biological systems are associated with rising water temperatures, as well as related changes in ice cover, salinity, oxygen levels and circulation. These shifts include increases in algae and zooplankton in high-latitude and high-altitude lakes.
Saros is investigating whether changes seen in alpine lakes are purely climate-driven. If they are, shifts in the diatom record should be evident in the sediment cores taken from lakes in the central and northern Rockies that date back 2,000 years.
To interpret the data preserved in such fossil records, researchers need to better understand the relationships between environmental variables, and the growth and distribution of diatoms. In Saros’ research, two of the critical environmental variables are nitrogen and phosphorus — essential nutrients for algae in the right quantities; in excess, the cause of algae blooms and other harmful water quality.
Alpine lakes are removed from the typical sources of human-induced nitrogen and phosphorus pollution — fertilizers, storm water and agricultural runoff, and faulty waste treatment systems. Yet in the highest elevations, humans still have the capacity to affect water quality via atmospheric nitrogen deposition, a form of air pollution resulting largely from the burning of fossil fuels.
According to Saros, human activity has led to a doubling of the amount of atmospheric nitrogen deposited in alpine lakes in the past century. Since 1980, species changes in the diatom communities have indicated nitrogen enrichment.
“We’re seeing rapid changes in the diatoms,” she says. “What’s hard to say with nitrogen is whether, in a low-deposition area like this, we’re seeing a level of saturation due to accumulation over time, or whether there’s a new atmospheric source, such as air pollution from Asia.”