Progress Report - Ecosystem Response

Ecosystem Response

Ecosystem Health

Acidified precipitation can impact lakes and streams by mobilizing toxic forms of aluminum from soils, (particularly in clay rich soils) and/or by lowering the pH of the water, harming fish and other aquatic wildlife. In a healthy well-buffered lake or stream, decreased acid deposition would be reflected by decreasing trends in surface water acidity. Four chemical indicators of aquatic ecosystem response to emission changes are presented here: trends in sulfate and nitrate anions, ANC, and sum of base cations. Improvement in surface water status is generally indicated by decreasing concentration of sulfate and nitrate anions and increasing base cations and ANC. The following is a description of each indicator:

• Sulfate is the primary anion in most acid-sensitive waters and has the potential to acidify surface waters (lower the pH) and leach base cations and toxic forms of aluminum from soils, leaving soils depleted of their ability to neutralize acidic inputs.
• Nitrate has the potential to acidify surface waters. However, nitrogen is an important nutrient for plant and algae growth, and most of the nitrogen inputs from deposition are quickly taken up by plants and algae, leaving less in surface waters.
• ANC is a key indicator of ecosystem recovery and is a measure of overall buffering capacity of surface waters against acidification; it indicates the ability to neutralize strong acids that enter aquatic systems from deposition and other sources.
• Base cations neutralize both sulfate and nitrate anions, thereby preventing surface water acidification. Base cation availability is largely a function of underlying geology, soil type, and the vegetation community. Surface waters with fewer base cations are more susceptible to acidification.

In the central Appalachian region, some watersheds have soils which have also accumulated and stored sulfate over the past decades of high sulfate deposition. As a result, the substantial decrease in acidic deposition has not yet resulted in comparably lower sulfate concentrations in many of the monitored Appalachian streams. A combination of low base cation availability and stored sulfate in the soils means that stream sulfate concentrations in some areas are not changing, or may be increasing, as the stored sulfate slowly bleeds out without adequate base cation concentrations to neutralize sulfate anions.¹

Surface Water Monitoring Networks

In collaboration with other federal and state agencies and universities, EPA administers the LTM program, which provides information on the impacts of acidic deposition on otherwise pristine lakes and streams. This program was designed to track changes in surface water chemistry in the four regions sensitive to acid rain in the eastern U.S.: New England, the Adirondack Mountains, the Northern Appalachian Plateau, and the central Appalachians (the Valley, Ridge, and Blue Ridge geologic provinces).

Acidified Water Trends

Acidic deposition resulting from sulfur dioxide (SO₂) and nitrogen oxides (NO_X) emissions is known to 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 NO_X emission reductions are sufficient to protect aquatic systems.

Regional Trends in Water Quality

• Between 1990 and 2022, EPA found significant decreasing trends in sulfate concentrations in water at all long-term monitoring (LTM) program lake and stream monitoring sites in New England, the Adirondacks, and the Catskill mountains, indicating improved lake and stream health.

Figure 1: Interactive Map of Long-Term Monitoring Program Sites and Trends, 1990–2022

• During that same period, streams in the central Appalachian region have experienced mixed results due in part to their soils and geology. Only 66 percent of monitored streams show statistically significant lower sulfate concentrations, while 4 percent show increased sulfate concentrations.
• Nitrate concentrations and trends are highly variable and many sites do not show consistent improving trends between 1990 and 2022, despite reductions in NO_X emissions and inorganic nitrogen deposition.
• In 2022, levels of acid neutralizing capacity (ANC), a key indicator of aquatic ecosystem recovery from acidification, have increased significantly from 1990 in lake and stream sites in the Adirondack Mountains, New England, and the Catskill mountains. In the central Appalachian region, sites with increasing ANC remain low at 19 percent.

Table 1: Regional Trends in Sulfate, Nitrate, ANC and Base Cations at Long-term Monitoring Sites, 1990-2022

Region	Water Bodies Covered	Percent of Sites with Improving Sulfate Trend	Percent of Sites with Improving Nitrate Trend	Percent of Sites with Improving ANC Trend	Percent of Sites with Improving Base Cations Trend
Adirondack Mountains	58 lakes in NY*	98%	91%	90%		88%
New England	26 lakes in ME and VT	100%	12%	77%	65%
Catskills / N. Appalachian Plateau	9 Streams in NY and PA**	78%	44%	67%	89%
Central Appalachians	70 streams in VA	66%	76%	19%	51%

* Data for Adirondack lakes from 1992
** Data for PA streams in N. Appalachian Plateau is only through 2015

Notes:
• Trends are determined by multivariate Mann-Kendall tests
• Trends are significant at the 95 percent confidence interval (p < 0.05)
• Sum of Base Cations calculated as (Ca+Mg+K+Na)

Forest Health

Ground-level ozone is one of many air pollutants that can alter a plant’s health and ability to reproduce and can make the plant more susceptible to disease, insects, fungus, harsh weather, and other environmental stressors. These impacts can lead to changes in the biological community, both in the diversity of species and in the health, vigor, and growth of individual species. As an example, many studies have shown that ground-level ozone reduces the health of commercial and ecologically important forest tree species throughout the U.S.^2,3,4 By looking at the distribution and abundance of ten sensitive tree species and the level of ozone at particular locations, it is possible to estimate reduction in growth – or biomass loss – for each species due to ozone pollution levels. Analyzing the biomass loss of certain trees before and after implementation of NO_X emission reduction programs provides information about the effect of decreasing NO_X emissions and ozone concentrations on forested areas. The EPA evaluated biomass loss for eleven common tree species in the eastern U.S. that have a higher sensitivity to ozone (black cherry, yellow poplar, American sycamore, chestnut oak, quaking aspen, sweet gun, sugar maple, eastern white pine, Virginia pine, and red maple) to determine whether decreasing ozone concentrations are reducing biomass loss in the forest.  This analysis reflects new underlying tree abundance data^5,6, updated tree-biomass functions recently published⁴, includes more tree species, and utilizes a new approach that better reflects tree abundance and biomass loss estimates due to ozone exposure.

Ozone Impacts on Forests

• The analysis presented here estimates biomass loss for 10 tree species using updated tree-biomass loss to ozone functions⁴ and tree abundances,^5,6 which better reflects biomass loss estimates due to ozone exposure for the eastern U.S. Of these 10 tree species, four are less sensitive to ozone exposure (sugar maple, Virginia pine, eastern white pine, chestnut oak) and six are more sensitive (black cherry, yellow poplar, American sycamore, quaking aspen, sweet gum, red maple). This analysis focused on modeled relative percent biomass loss of >5 and >10 percent, which are ecologically important indicators for tree and ecosystem health.^4,7
• Tree biomass loss due to ozone exposure declined sharply as air quality improved between 2000–2002 and 2020–2022. The amount of forest area impacted by ozone exposure with >10 percent biomass loss (most impact) decreased by 88 percent while areas with >5 percent biomass loss declined by 40 percent for all tree species across the eastern U.S.
• Individually each of the 10 tree species showed decreases in total forest area with ecologically important biomass loss of >5 percent between 2000–2002 and 2020–2022 in the eastern U.S. The four least sensitive tree species are not expected to experience biomass loss above 5 percent due to ozone exposure for the current period.
• Among the six more sensitive trees, black cherry, yellow poplar, and American sycamore are the only tree species expected to experience ecologically important biomass loss of greater than 5 percent for the current ozone exposure with only black cherry having areas with the more severe level of >10 percent biomass loss (see table).
• While this change in biomass loss cannot be exclusively attributed to the implementation of the NBP, CAIR, CSAPR, CSAPR Update, and Revised CSAPR Update, it is likely that ozone season NO_X emission reductions achieved under these programs, and the corresponding decreases in ozone concentration, contributed to this environmental improvement.

The following map is interactive. Click and slide the bar left and right to see the changes.

Figure 2: Map of Estimated Black Cherry, Yellow Poplar, Sugar Maple, Eastern White Pine, Virginia Pine, Red Maple, Quaking Aspen, American Sycamore, Chestnut Oak, Sweet Gum, and Table Mountain Pine Biomass Loss Due to Ozone Exposure, 2000–2002 versus 2020–2022

Notes:
Trends are determined by multivariate Mann-Kendall tests
Trends are significant at the 95 percent confidence interval (p < 0.05)
DOC is not routinely measured in Central Appalachian streams
Sum of Base Cations calculated as (Ca+Mg+K+Na)
* Some data for Adirondack lakes started in 1992
**Data for PA streams in N. Appalachian Plateau is only through 2015

Table 2: Tree Biomass Loss from Ozone Exposure, By Species

Tree Species	Percent Area with >5% Tree Abundance	Tree Biomass Loss from Ozone Exposure
		2000–2002		2020–2022
		>10%	5-10%	>10%	5-10%
Black Cherry	17.3	17.3	0.0	5.0	12.2
Yellow Poplar	21.1	21.1	0.0	0.1	15.6
American Sycamore	4.1	3.5	0.5	0.0	0.1
Quaking Aspen	13.3	0.8	2.7	0.0	0.0
Sweet Gum	30.6	5.1	4.3	0.0	0.0
Red Maple	51.0	0.0	9.5	0.0	0.0
Sugar Maple	24.9	0.0	0.0	0.0	0.0
Chestnut Oak	1.2	0.0	0.0	0.0	0.0
Virginia Pine	6.6	0.0	0.0	0.0	0.0
Eastern White Pine	12.3	0.0	0.0	0.0	0.0

Critical Loads Analysis

A critical loads analysis is an assessment used to provide a quantitative estimate of whether acid deposition levels are negatively impacting ecosystem health. The analysis here focuses on aquatic biological resources. If acidic deposition is less than the calculated critical load, harmful ecological effects (e.g., reduced reproductive success, stunted growth, loss of biological diversity) are not expected to occur, and ecosystems damaged by past exposure are expected to eventually recover.⁸ Lake and stream waters having an ANC value greater than 50 or 20 μeq/L, depending on the waterbody’s natural level of acidity, are classified as having a moderately healthy aquatic biological community; therefore, this ANC concentration is often used as a goal for ecological protection of surface waters affected by acidic deposition. In this analysis, the critical load represents the amount of combined sulfur and nitrogen that could be deposited annually to a lake or stream and its watershed and still support a moderately healthy aquatic ecosystem (i.e., having an ANC greater than 50 or 20 μeq/L). Surface water samples from 11,073 lakes and streams eastern United States were collected through several water quality monitoring programs. Critical load exceedances were calculated using the Steady-State Water Chemistry model^8-10 using critical load data from the National Critical Load database NCLD).¹¹

Critical Loads and Exceedances

• For the period from 2020 to 2022, 5 percent of the 11,022 studied lakes and streams in the eastern United States still received levels of combined total sulfur and nitrogen deposition exceeding their calculated critical load. This is an 88 percent improvement over the period from 2000 to 2002 when 44 percent of all studied lakes and streams in this area exceeded their calculated critical load. Total sulfur and nitrogen deposition for the same periods of 2000–2002 to 2020–2022 had similar magnitude reductions of 82 percent and 59 percent, respectively, for the eastern U.S.

The following map is interactive. Click and slide the bar left and right to see the changes.

Figure 3: Map of Lake and Stream Exceedances of Estimated Critical Loads for Total Nitrogen and Sulfur Deposition, 2000-2002 versus 2020-2022

Notes:
Surface water samples from the represented lakes and streams complied from surface monitoring programs, such as National Surface Water Survey (NSWS), Environmental Monitoring and Assessment Program (EMAP), Wadeable Stream Assessment (WSA), National Lake Assessment (NLA), Temporally Integrated Monitoring of Ecosystems (TIME), Long Term Monitoring (LTM), and other water quality monitoring programs.
Steady state exceedances calculated in units of meq/m²/year.

• Emission reductions achieved between 2000 and 2022 have led to decreases in total sulfur and nitrogen deposition and have contributed to broad surface water quality improvements and increased aquatic ecosystem protection across the eastern U.S.
• The 5 percent level of critical load exceedances for acidification for the eastern U.S. corresponds to total annual sulfur deposition of 1.4 to 2.5 kg/ha/yr and nitrogen deposition of 6.1 to 8.4 kg/ha/yr.
• Based on this analysis, current sulfur and nitrogen deposition loadings for the period of 2020 to 2022 still exceed levels required for recovery of some lakes and streams, indicating that additional emission reductions are necessary for the most acid-sensitive aquatic ecosystems, to recover and be protected from acid deposition. This is particularly the case for the southeast where total sulfur deposition and percent exceedances are higher than the other regions.

Table 3: Water Bodies in Exceedance of Critical Load

Region	Number of Water Bodies Modeled	Years				Percent Reduction
		2000–2002		2020–2022
		Number of Sites	Percent of Sites	Number of Sites	Percent of Sites
Northeast (CT, MA, ME, NH, NY, RT, VT)	2,766	779	28%	74	3%	91%
Adirondack (NY)	1,892	1,074	57%	90	5%	92%
Mid-Atlantic (DE, MD, NJ, PA, VA, WV)	3,082	1,498	49%	111	4%	93%
Southeast (AL, GA, MS, NC, SC, TN)	2,501	1,264	51%	258	10%	80%
Midwest (KY, IL, IN, OH, MI, WI)	781	279	36%	31	4%	89%
Total Units	11,022	2,951	38%	458	5.8%	84%

More Information

• Surface water monitoring at EPA
• Acid Rain
• Ozone W126 Index
• National Acid Precipitation Assessment Program (NAPAP) Report to Congress

---

¹ Burns, D.A., Lynch, J.A., Cosby, B.J., Fenn, M.E., & Baron, J.S. (2011). National Acid Precipitation Assessment Program Report to Congress 2011: An Integrated Assessment. U.S. EPA, National Science and Technology Council, Washington, D.C.: 114 p
² Chappelka, A.H. & Samuelson, L.J. (1998). Ambient ozone effects on forest trees of the eastern United States: A review. New Phytologist 139: 91–108.
³ Ollinger, S.V., Aber, J.D., & Reich, P.B. (1997). Simulating ozone effects on forest productivity: interactions among leaf-canopy and stand-level processes. Ecological Applications 7(4), 1237–1251.
⁴ Lee, H.E., Andersen, C.P., Beedlow, P.A., Tingey, D.A., Koike, S., Dubois, J., Kaylor, S.D., Novak, K., Rice, R,B., Neufeld, H.S., Herrick, J.D. 2022. Ozone exposure-response relationships parametrized for sixteen tree species with varying sensitivity in the United States. Atmospheric Environment, 119191. https://doi.org/10.1016/j.atmosenv.2022.119191
⁵ Peters, M.P., Iverson, L.R., Prasad, A.M., Matthews, S.N., 2019. Utilizing the density of inventory samples to define a hybrid lattice for species distribution models: DISTRIB-II for 135 eastern US trees. Ecology and Evolution 9, 8876-8899. https://doi.org/10.1002/ece3.5445
⁶ Tree species ranges from USFS Climate change Atlas (version 4) where Forest Inventory and Analysis records from 2000-2016 were used to calculate a relative abundance for its potential suitable habitat for a particular tree species (https://www.fs.usda.gov/nrs/atlas/tree/).
⁷ U.S. EPA, 2014 (b). Welfare Risk and Exposure Assessment for Ozone. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. EPA-452/R-14-005a.
⁸  Nilsson, J. & Grennfelt, P. (Eds) (1988). Critical loads for sulphur and nitrogen. UNECE/Nordic Council workshop report, Skokloster, Sweden. Nordic Council of Ministers: Copenhagen.
⁹ Dupont, J., Clair, T.A., Gagnon, C., Jeffries, D.S., Kahl, J.S., Nelson, S.J., & Peckenham, J.M. (2005). Estimation of critical loads of acidity for lakes in the northeastern United States and eastern Canada. Environmental Monitoring and Assessment, 109:275–291.9
¹⁰ Sullivan, T.J., Cosby, B.J., Webb, J.R., Dennis, R.L., Bulger, A.J., & Deviney, Jr. F.A. (2007). Streamwater acid-base chemistry and critical loads of atmospheric sulfur deposition in Shenandoah National Park, Virginia. Environmental Monitoring and Assessment, 137: 85–99.
¹¹ Lynch, J.A., Phelan, J., Pardo, L.H., McDonnell, T.C., Clark, C.M., Bell, M.D., Geiser, L.H., Smith, R.J.  2022. Detailed Documentation of the National Critical Load Database (NCLD) for U.S. Critical Loads of Sulfur and Nitrogen, version 3.2.1, National Atmospheric Deposition Program, Wisconsin State Laboratory of Hygiene, Madison, WI