Hindcasting nitrogen deposition to determine an ecological critical load

Area of Science:

• Environmental Science
• Ecology
• Atmospheric Chemistry

Background:

• Nitrogen (N) deposition is a significant environmental stressor in alpine ecosystems.
• Historical N deposition levels in alpine lakes are often poorly constrained.
• Eutrophication from N deposition can alter aquatic ecosystems, including diatom assemblages.

Purpose of the Study:

• To reconstruct a historical N deposition record for alpine lakes.
• To identify the critical N deposition load associated with ecological change.
• To validate a hindcasting method for estimating past atmospheric deposition.

Main Methods:

• Utilized a 19-year record of measured wet N deposition from Loch Vale, Colorado.
• Developed exponential equations correlating N deposition with EPA-reported NOx emissions.
• Estimated background N deposition using a 1900 value of 0.5 kg N x ha(-1) x yr(-1).

Main Results:

• Reconstructed N deposition history correlated strongly with regional NOx emissions.
• Mean wet N deposition from 1950-1964 was approximately 1.5 kg N x ha(-1) x yr(-1).
• This value aligns with the timing of observed diatom assemblage alterations in Rocky Mountain National Park.

Conclusions:

• A critical N deposition load of 1.5 kg N x ha(-1) x yr(-1) was identified as a threshold for ecological change in alpine lakes.
• Hindcasting N deposition based on identified thresholds is a viable method for assessing past atmospheric deposition.
• The technique is supported by experimental studies, ecosystem modeling, and paleolimnological data.