Bell, T.A. (2009)

A shift in diatom composition due to increased weathering and nitrogen deposition

Thesis (M.S.)--University of Colorado, 69 p.

Green Lake 4 is an oligotrophic alpine lake that is part of the Niwot Ridge Long Term Ecolocial Research (LTER) site in Colorado. Nitrogen has been shown to be increasing in alpine lake systems of the Rocky Mountain Front Range due to atmospheric deposition, doubling in some cases over a twenty-year period. Additionally, warming events associated with climate change have also been shown to introduce other nutrients into the system through chemical weathering. Many Rocky Mountain alpine lakes similar in productivity and elevation to Green Lake 4 have shown a recent shift in diatom species as a result of these nutrient additions. As a result, diatom species associated with oligotrophic systems are being replaced with more common species that are tolerant of higher nutrient concentrations. In order to explore whether a similar trend was occurring in Green Lake 4, we examined biogeochemical and diatom proxies in a sediment core from Green Lake 4 and long-term monitoring data from the same watershed. Above the 1.25 cm level (corresponding to ̃1940s), a change in diatom community was observed. This initial change is associated with nitrogen deposition in the alpine a result of increased population and the use of the Haber-Bosch process to create nitrogen fertilizers in the Colorado Front Range. Post 1970s the change becomes more pronounced, specifically, small benthic diatoms became more abundant while planktonic species decreased in number. This was caused by warmer longer summers that thermally stratified the lake and an increase in nutrient concentrations from chemical weathering of the bedrock, specifically phosphorus. Once introduced into the lake, nutrients became trapped in the hypolimnion, due to thermal stratification, promoting the growth of small benthic algal communities. In conclusion, the data supports that Green Lake 4, like other high alpine Rocky Mountain lakes, has undergone a diatom species shift due nutrient increases associated with atmospheric deposition and chemical weathering processes