USING GLACIALLY-DERIVED NITROGEN PULSES INTO ALPINE LAKE ECOSYSTEMS AS INDICATORS OF CLIMATE CHANGE

Because nitrogen content of high alpine lakes may be at least partially driven by fluctuations in glacial meltwater, lacustrine indicators of nitrogen inputs may provide climate inferences from glacially-fed alpine lakes. In the central Rocky Mountains, the Beartooth region contains over 600 permanent lakes. Most of the lakes in this region are sustained by snowmelt, but some receive glacial meltwater as well. Rainbow Lake is one of the few lakes in the area receiving inputs from two glaciers, making it ideal for examination of the effects of glacial meltwater on nitrogen loading. In an effort to test this hypothesis, we obtained a 57 cm surface core from Rainbow Lake in August, 2008. The length of this core should allow us to examine whether nitrogen loading to the lake varied during the mid-19th century shift from cooler temperatures of the Little Ice Age to present-day warming. The core was extruded lakeside at 0.5 cm intervals and transported on ice to the Paleoecology Research Laboratory at the Climate Change Institute, University of Maine, Orono, Maine. Analyses include 210Pb dating, total inorganic content by loss-on-ignition, and diatom and pollen assemblages. Diatom assemblages typically show strong and rapid responses to nitrogen enrichment in alpine lakes; they will be used as the primary indicator of changes in nitrogen loading to this lake. Pollen analyses are necessary to determine whether changes in lake chemistries relate to changes in vegetation on the catchment landscape or to changes within the lake basin. We present the dated changes in total inorganic carbon and the results of diatom analysis for Rainbow Lake, along with our preliminary interpretations of climate-related changes in nitrogen loading.