Progress Report

Ecosystem Response

Acidic deposition resulting from sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) emissions may negatively affect the biological health of lakes, streams, forests, grasslands, and other ecosystems in the United States. Trends in measured chemical indicators allow scientists to determine whether water bodies are improving and heading towards recovery or if they are still acidifying. Assessment tools, such as critical loads analysis, provide a quantitative estimate of whether decreases in acidic deposition levels of sulfur and nitrogen resulting from SO₂ and NOₓ emission reductions are sufficient to protect aquatic resources.

Ground-level ozone is an air pollutant that can impact ecological systems like forests, altering a plant’s health and leading to changes in individual tree growth (e.g., biomass loss) and to the biological community. Analyzing the biomass loss of certain trees before and after implementation of NOₓ emission reduction programs provides information about the effect of reduced NOₓ emissions and ozone concentrations on forested areas.

Ecosystem Health Figures

Monitoring Program Site Map
Long-term Monitoring Program Sites and Trends, 1990–2019
Regional Trends in Water Quality
Regional Trends in Sulfate, Nitrate, ANC, and Base Cations at Long-term Monitoring Sites, 1990–2019
Biomass Loss Due to Ozone Exposure
Estimated Black Cherry, Yellow Poplar, Sugar Maple, Eastern White Pine, Virginia Pine, Red Maple, and Quaking Aspen Biomass Loss Due to Ozone Exposure, 2000–2002 versus 2017–2019

Critical Loads Analysis Figures

Critical Load Exceedances Map
Lake and Stream Exceedances of Estimated Critical Loads for Total Nitrogen and Sulfur Deposition, 2000–2002 versus 2017–2019
Critical Load Exceedances by Region
Critical Load Exceedances by Region, 2000–2002 versus 2017–2019