Background/Question/Methods
Green Lake 4 is an oligotrophic alpine lake in Rocky Mountain National Park, 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.  
Results/Conclusions
Above the 9-10cm level (corresponding to 1940), organic carbon, chlorophyll and fucoxanthin concentrations, and the fluorescence index progressively increased, indicating increases in algal-derived organic material.  Small benthic diatoms became more abundant after 1940, indicating that benthic diatom growth is associated with the warmer summers and increase in nutrient concentrations that have been documented in the long-term monitoring data.  Specifically, warmer summer temperatures strengthen thermal stratification trapping phosphate and silica (released by chemical weathering) in the hypolimnion, and promote the growth of 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.