Critical Loads of Atmospheric Nitrogen Deposition for Aquatic Ecosystems in the Sierra Nevada, California

Leora Nanus1, David W. Clow2 and James O Sickman3

High-elevation aquatic ecosystems in Sierra Nevada wilderness areas are impacted by atmospheric nitrogen (N) deposition associated with local and regional air pollution. Documented effects include elevated surface water nitrate concentrations, increased algal productivity, and changes in diatom species assemblages. Critical loads of N deposition for nutrient enrichment of aquatic ecosystems were quantified and mapped using a geostatistical approach, with N deposition, topography, vegetation, geology, and climate as potential explanatory variables. Rather than relying on a single predictive model, multiple predictive models were developed using various combinations of explanatory variables; this approach allowed for better quantification of uncertainty and more accurate identification of the aquatic ecosystems most sensitive to high atmospheric N deposition (> 3 kg N ha-1 yr-1). The lowest critical loads estimates (< 1.0 + 1 kg N ha-1 yr-1) and highest exceedances identified within the Sierra Nevada occurred in high-elevation basins with steep slopes, sparse vegetation, and areas of neoglacial till and talus. Estimated critical load exceedances correspond with areas of high N deposition and vary spatially. Broad areas of the Sierra Nevada, including areas within Yosemite and Sequoia & Kings Canyon National Parks may be impacted by excess N deposition, with greatest impact at high elevations. These results are consistent with previous analyses in the Rocky Mountains, and highlight the sensitivity of high-elevation aquatic ecosystems to atmospheric N deposition in the western United States.

 

1San Francisco State University, lnanus@sfsu.edu
2United States Geological Survey, Denver, Colorado, dwclow@usgs.gov
3University of California, Riverside, james.sickman@ucr.edu