Nitrogen Deposition Critical Loads for Phytoplankton Biomass Nutrient Limitation Shifts in Western U.S. Mountain Lakes

Jason Williams1, Jason Lynch2 and Jasmine Saros3

Anthropogenic atmospheric nitrogen deposition has the potential to alter remote mountain lake ecosystems in the Western United States. Relatively low amounts of Nitrogen (N) deposition may increase lake N concentrations, stimulate algal growth, and affect whether N or phosphorus (P) limits algal growth, leading to changes in the lake’s productivity, specie richness and diversity. We calculated critical loads for deposition-induced changes in nutrient limitation of algal biomass growth in Western U.S. mountain lakes using a three-step approach. First, a database with nitrogen and phosphorus water chemistry data for over 3,000 lakes in the Western U.S was assembled from existing data sources. Second, biomass responses to experimental N and P enrichment in bioassays from 50 Western U.S. mountain lakes were synthesized to define threshold dissolved inorganic nitrogen to total phosphorus (DIN:TP) mass ratios at which shifts in phytoplankton biomass nutrient limitation occur. Third, the water chemistry and chemical threshold data were combined with modeled N deposition estimates to calculate critical loads using two different empirical approaches and one model-based approach. Empirical approaches related modeled nitrogen deposition rates directly to phytoplankton biomass responses in nutrient enrichment bioassays with and without considering effects of watershed characteristics on lake DIN:TP. The model-based approach calculates a critical load as the N deposition rate that causes experimentally-defined DIN:TP chemical threshold to be exceeded while accounting for effects of watershed characteristics on lake DIN:TP. Preliminary results indicate an empirically-based critical load of 3.4 kg total N ha-1 yr-1. Contrasting results of these different approaches yields useful information about the data types, methodologies, and uncertainties relevant to critical load calculations for nutrient enrichment effects in mountain lakes.

 

1Washington State University, jason.williams2@wsu.edu
2USEPA, Lynch.Jason@epa.gov
3University of Maine, jasmine.saros@maine.edu