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Empirical Critical Loads of Atmospheric Nitrogen Deposition for Nutrient Enrichment and Acidification of Sensitive US Lakes
Citation:
Baron, J. S., C. T. Driscoll, J. L. STODDARD, AND E. E. Richer. Empirical Critical Loads of Atmospheric Nitrogen Deposition for Nutrient Enrichment and Acidification of Sensitive US Lakes. BIOSCIENCE. American Institute of Biological Sciences, MCLEAN, VA, 61(8):602-613, (2011).
Impact/Purpose:
A critical load is a “quantitative estimate of the exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge”.
Description:
A critical load is a “quantitative estimate of the exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge”. Critical loads can be either modeled, or calculated empirically; both approaches have been widely used in Europe, but have not been widely embraced in the U.S. We developed empirical critical loads of nitrogen for U.S. freshwaters in the Northeast and western mountains, by conducting a synthesis of the literature and analyzing extant monitoring data. Ecological effects of elevated atmospheric nitrogen deposition include eutrophication and acidification. These effects are most evident in high elevation lakes and streams, which are sensitive to atmospheric deposition and have been minimally impacted by land disturbance. Freshwaters have historically been considered phosphorus-growth limited systems. Recent literature and our analysis suggest instead that nitrogen-limited freshwaters may be more common than previously thought, particularly in high elevation western lakes with very low inputs of nitrogen. Nitrogen-limited freshwaters will exhibit increases in productivity and shifts in the composition of phytoplankton in response to increases in atmospheric nitrogen deposition. We estimate the critical load of nitrogen for eutrophication of western lakes to be 2 kg N ha-1 yr-1. The critical load for eutrophication of eastern freshwaters is more difficult to evaluate because eastern ecosystems have been receiving elevated inputs of nitrogen deposition for many decades. We estimated an empirical nitrogen critical load for eutrophication for eastern surface water to be 7-8 kg N ha-1 yr-1, based on the level of nitrogen deposition which results in elevated leaching losses of nitrate. Additional research is needed to quantify the biological response of nitrogen-limited eastern lakes to changes in nitrogen inputs. Critical loads for acidification effects of nitrogen are difficult to determine in isolation because the additive effects of atmospheric sulfur deposition. Nitrogen acidification effects are largely associated with elevated leaching of nitrate during snowmelt or during storm events in waters with low values of acid neutralizing capacity. The nitrogen critical loads for acidification are higher than the critical loads for eutrophication. Based on our review of the literature and our analysis of monitoring data, we estimate critical loads of nitrogen for acidification to be 4-8 kg N ha-1 yr-1 for high elevation Western lakes and 8 kg N ha-1 yr-1 for high elevation Eastern lakes.