Congress will soon give careful consideration to renewal of the Clean Air Act and its amendments. In anticipation of this event, a group of scientific experts met last year to discuss their research of acid deposition, often called acid rain. The results of this meeting have been summarized and are now available in a new report from the Ecological Society of America entitled, "Acid Deposition: The Ecological Response."
The group of experts was composed of 40 of the nation's top ecological researchers. They met in March 1999 with three specific goals in mind: to evaluate the long term trends emerging from research data on ecosystem response to acid deposition; to examine whether changes have occurred in the ecological damage caused by acid deposition, and to compare those changes to observations and projections of the 1990 National Acid Precipitation Assessment Program (NAPAP) Report. During this same year, the Clean Air Act Amendments (CAAA) were passed to reduce the adverse effects of acid deposition through phased reductions of annual emissions of its precursors, sulfur dioxide (SO2) and nitrogen oxides (NOx). The scientists also wanted to ascertain whether ecosystems were experiencing any kind of recovery from previous acid deposition.
The most important conclusions drawn from the workshop regard regional differences in both deposition levels and recovery rates. In the Midwest and Northeast, the scientists agreed, emission reductions of sulfur dioxide correspond to decreases in the fluxes of sulfate in both precipitation and surface waters. But in some regions and ecosystems, these reductions may not have resulted in ecosystem recovery.
Several factors may be limiting this recovery. First, acid deposition can deplete base cations from soils, such as calcium and magnesium, that are essential for plant growth. Besides being essential for plants these elements also neutralize further acid inputs.
Second, nitrogen deposition may also be limiting ecosystem recovery. Nitrogen plays an important role in controlling plant growth, species diversity, and nutrient cycling in terrestrial and aquatic ecosystems. When nitrogen deposition occurs for long periods of time there is the potential in some ecosystems for nitrogen demand to be exceeded or "saturated." Saturated ecosystems typically lose the excess nitrogen via leaching and denitrofication. Nitrogen saturation is being observed in some regions of the US. Estuarine ecosystems along the Atlantic and Gulf of Mexico coasts, for instance, are normally characterized by low levels of nitrogen availability. Recent increases in nitrogen have led to changes in these aquatic communities and losses in biological diversity.
Nitrogen saturation in forested ecosystems can lead to diminished forest health and degraded water quality. In the western US, scientists have observed the early stages of nitrogen saturation in high elevation ecosystems of the Colorado Rockies Front Range. The effects of saturation are also seen in the Los Angeles Basin of California, and the more exposed regions of the San Gabriel and San Bernardino Mountains. In these areas, some of the wildland systems report the highest concentrations of nitrogen on record for North America.
For all of these reasons, say the researchers, the role of nitrogen deposition should be taken into consideration when policy is being formulated.
Fish are especially sensitive to the release of aluminum that is leached from forest soils as a result of these kinds of increases in nitrogen and sulfur deposition. High concentrations and extensive exposure of lakes and streams to aluminum is a main cause for chronic acidified systems. These systems are toxic to host populations of brook trout and many other fish species.
In addition, some of the sulfur in the systems has not been accounted for. The researchers hypothesize that this sulfur could be the result of an underestimation of deposition. It may also be the result of weathering minerals or mineralization of sulfur which naturally occurs in the soil.
The report also highlights important new information regarding episodic acidification. Episodes of intense acid deposition, such as those which may occur during times of high precipitation or snowmelt, are now known to have significant biological effects. Acidic episodes can cause, for instance, losses in biodiversity and changes in community structure, which can cause more widespread damage than waters which are chronically acidified.
Based upon this information, the continued monitoring of deposition into surface waters from the atmosphere is extremely important. The scientists conclude that monitoring should be enhanced, and should include both wet and dry deposition. Also, the researchers assert, biological indicators of ecosystem response to acid deposition should be included in monitoring efforts.
"In order to evaluate the effectiveness of environmental policies and programs," states the report, "a firm commitment is needed to long-term monitoring programsŠ Many questions must be answered to fully assess whether the environmental goals of the 1990 Clean Air Act Amendments are being met as a result of sulfur and nitrous oxide emission reductions."
In addition to this report, ESA will soon release two other publications focused upon atmospheric deposition. Another report examines the role of atmospheric deposition in the Gulf of Mexico Hypoxic Zone and third focuses upon atmospheric deposition to the Pacific Coast. For more information or to download a copy of the reports visit the publications section of the ESA website: http://esa.sdsc.edu/whitepapers.htm
The Ecological Society of America (ESA) is a scientific, non-profit, over 7000-member organization founded in 1915. Through ESA reports, journals, membership research, and expert testimony to Congress, ESA seeks to promote the responsible application of ecological data and principles to the solution of environmental problems. ESA publishes three scientific, peer-reviewed journals: Ecology, Ecological Applications and Ecological Monographs. Information about the Society and its activities are published in the Society's newsletter, NewSource, and in the quarterly Bulletin. More information can be found on the ESA website: http://esa.sdsc.edu.