Inferring Critical Nitrogen Deposition Loads to Alpine Lakes of Western National Parks and Diatom Fossil Records

Saros, Jasmine

Shifts in algal (diatom) composition in alpine lakes are frequently the first indication of ecological perturbations in these systems. Of the major disturbances affecting alpine lakes, enhanced atmospheric nitrogen (N) deposition is one of the primary concerns. Previous research using sedimentary diatom profiles identified a critical N load of 1.5 kg ha-1 yr-1 wet deposition to lakes in Rocky Mountain National Park (Baron 2006). In this study, we have expanded critical loads determination across parks of the west to more broadly define a threshold for these alpine lake ecosystems. Using a similar approach to Baron (2006), we defined the critical N load as the threshold that elicits key diatom shifts, as determined by changes in lake sediment cores and reconstructed by relating the timing of diatom shifts to the inferred N deposition rate at the time. Sediment cores from the eastern Sierras, Glacier National Park (GNP), and the Greater Yellowstone Ecosystem (GYE) were used in this analysis. We used total wet inorganic N deposition data from the National Atmospheric Deposition Program (NADP), with modifications in the GYE to account for elevation. Our analyses revealed a critical N load of 1.4 kg N ha-1 yr-1 in wet deposition for the east Sierras and for the GYE. Although the GNP region exceeds this value, indicator taxa of N enrichment were only present in very low numbers in lakes of this park. Water chemistry data revealed that, unlike the Sierra and GYE lakes, GNP lakes are phosphorus limited, hence we would not expect to observe N enrichment effects in these lakes. The results of this study are in close agreement with those of Baron (2006) and expand the geographic regions over which resource managers now have a target value needed to improve aquatic ecosystem conditions.