Critical Load Example: Rocky Mountain NP
A critical load is the quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment are not expected to occur according to present knowledge. Critical loads are calculated for specific ecosystem components and ecological indicators. For example, in Rocky Mountain National Park, nitrogen deposition has increased significantly over natural background levels and caused subtle changes to high elevation lakes and alpine vegetation. For high elevation lakes, the selected indicator for the critical load is eutrophication, that is, unnatural enrichment. The critical load for this indicator in the park is 1.5 kilograms per hectare per year (kg/ha/yr) measured as wet nitrogen deposition, because at this level, eutrophication begins to occur. For alpine vegetation, the selected indicator is a significant change in species abundance. The critical load for alpine vegetation in the park has not been established. However, research in nearby areas suggests that nitrogen deposition limited to 2.7 kg/ha/yr would protect alpine vegetation from significant change. This level has been set as the target load for alpine vegetation.
A target load represents a deposition goal based on economic constraints and a timeframe for resource protection. Generally, target loads may be higher or lower than the critical load, depending on how each factor is weighed in the park.
If nitrogen deposition continues to increase, effects, including episodic and chronic acidification of lakes, streams, and soils with associated effects to aquatic and terrestrial organisms, are expected to increase in extent and severity.