Acid deposition in the northeastern United States has had a detrimental impact o

Question

Acid deposition in the northeastern United States has had a detrimental impact on both terrestrial

and aquatic ecosystems. For terrestrial ecosystems, the initial effect of acid deposition involves soil.

In aquatic ecosystems, the initial effect involves zooplankton (i.e., microscopic animals). Choose one of these two ecosystems and describe how acid deposition has degraded it. Organize your answer as a process that starts with a more thorough description of the initial effect mentioned above and the consequences for other aspects of your chosen ecosystem.

Explanation / Answer

Acidic deposition is the transfer of strong acids and acid-forming substances from the atmosphere to the surface of the Earth. The composition of acidic deposition includes ions, gases, and particles derived from the following: gaseous emissions of sulfur dioxide (SO2), nitrogen oxides (NOx), ammonia (NH3), and particulate emissions of acidifying and neutralizing compounds.

It is highly variable across space and time, links air pollution to diverse terrestrial and aquatic ecosystems, and alters the interactions of many elements (e.g., sulfur [S], nitrogen [N], hydrogen ion [H+], calcium [Ca2+], magnesium [Mg2+], and aluminum [Al]). It also contributes directly and indirectly to biological stress and to the degradation of ecosystems.

Acidic deposition was first identified by R. A. Smith in England in the 19th century (Smith 1872). Acidic deposition emerged as an ecological issue in the late 1960s and early 1970s with reports of acidic precipitation and surface water acidification both within Sweden and around Scandinavia (Oden 1968).

The first report of acidic precipitation in North America was made at the Hubbard Brook Experimental Forest (HBEF) in the remote White Mountains of New Hampshire, based on collections begun in the early 1960s (Likens et al. 1972).

Controls on SO2 emissions in the United States were first implemented after passage of the 1970 amendments to the Clean Air Act (CAAA). In 1990, Congress passed Title IV of the Acid Deposition Control Program of the CAAA to further decrease emissions of SO2 and to initiate controls on NOx from electric utilities, which contribute to acidic deposition.

The Acid Deposition Control Program had two goals: (1) By 2010, a 50% decrement from 1980 levels of SO2 utility emissions (amounting to 9.1 million metric tons per year, or 10 million short tons); (2) also by 2010, an NOx emission rate limitation (0.65 lbs NOx/m BTU in 1990 to 0.39 lbs NOx/m BTU in 1996), which will achieve a reduction of 1.8 million metric tons per year (2 million short tons) as NO2) in NOx utility emissions from the amount that would have occurred without emission rate controls. Both SO2 and NOx provisions focus on large utilities.

Patterns of precipitation and deposition of S and N

Acidic deposition can occur as wet deposition; as rain, snow, sleet, or hail; as dry deposition; as particles or vapor; or as cloud or fog deposition, which is more common at high elevations and in coastal areas.

Wet deposition is monitored at more than 200 US sites by the interagency-supported National Atmospheric Deposition Program/National Trends Network (NADP/NTN), which was initiated in 1978.

Dry deposition is monitored by the EPA Clean Air Status and Trends Network (CASTNet) at approximately 70 sites, as well as by the National Oceanic and Atmospheric Administration AIRMoN (Atmospheric Integrated Monitoring Network)-dry program at 13 sites.

Terrestrial–aquatic linkages

Many of the effects of acidic deposition depend on the rate at which acidifying compounds are deposited from the atmosphere, compared with the rate at which acid neutralizing capacity (ANC) is generated within the ecosystem.

ANC, a measure of the ability of water or soil to neutralize inputs of strong acid, is largely the result of terrestrial processes such as mineral weathering, cation exchange, and immobilization of SO2?4 and N (Charles 1991).

Acid neutralizing processes occur in the solution phase, and their rates are closely linked with the movement of water through terrestrial and aquatic ecosystems.

The effects of acidic deposition on ecosystem processes must, therefore, be considered within the context of the hydrologic cycle, which is a primary mechanism through which materials are transported from the atmosphere to terrestrial ecosystems and eventually into surface waters.

The effects of acidic deposition on surface waters vary seasonally and with stream flow. Surface waters are often most acidic in spring after snowmelt and rain events. In some waters ANC values decrease below 0 µeq L?1 only for short periods (i.e., hours to weeks), when discharge is highest. This process is called episodic acidification. Other lakes and streams, referred to as chronically acidic, maintain ANC values less than 0 µeq L?1 throughout the year.

Precipitation (which includes snowmelt) can raise the water table from the subsoil into the upper soil horizons, where acid neutralizing processes (e.g., mineral weathering, cation exchange) are generally less effective than in the subsoil. Water draining into surface waters during high-flow episodes is therefore more likely to be acidic (i.e., ANC value less than 0 µeq L?1) than water that has discharged from the subsoil, which predominates during drier periods.

Both chronic and episodic acidification can occur either through strong inorganic acids derived from atmospheric deposition, by natural processes, or both. Natural acidification processes include the production and transport of organic acids derived from decomposing plant material or of inorganic acids originating from the oxidation of naturally occurring S or N pools (i.e., pyrite, N2 fixation followed by nitrification) from the soil to the surface waters. Here, we focus on atmospheric deposition of strong inorganic acids, which dominate the recent acidification of soil and surface waters in the northeastern United States.

Effects of acid deposition on soils

The observation of elevated concentrations of inorganic monomeric Al in surface waters provided strong evidence of soil interactions with acidic deposition (Driscoll et al. 1980, Cronan and Schofield 1990). Recent studies have shown that acidic deposition has changed the chemical composition of soils by depleting the content of available plant nutrient cations (i.e., Ca2+, Mg2+, K+), by increasing the mobility of Al, and by increasing the S and N content.

Acidic deposition has increased the concentrations of protons (H+) and strong acid anions (SO2?4 and NO?3) in soils of the northeastern United States, which has led to increased rates of leaching of base cations and to the associated acidification of soils. If the supply of base cations is sufficient, the acidity of the soil water will be effectively neutralized. However, if base saturation (exchangeable base cation concentration expressed as a percentage of total cation exchange capacity) is below 20%, then atmospheric deposition of strong acids results in the mobilization and leaching of Al, and H+ neutralization will be incomplete (Cronan and Schofield 1990).

Mineral weathering is the primary source of base cations in most watersheds, although atmospheric deposition may provide important inputs to sites with very low rates of supply from mineral sources. In acid-sensitive areas, rates of base cation supply through chemical weathering are not adequate to keep pace with leaching rates accelerated by acidic deposition. Recent studies based on the analysis of soil (Lawrence et al. 1999), the long-term trends in stream water chemistry (Likens et al. 1996, 1998, Lawrence et al. 1999), and the use of strontium stable isotope ratios (Bailey et al. 1996) indicate that acidic deposition has enhanced the depletion of exchangeable nutrient cations in acid-sensitive areas of the Northeast. At HBEF, Likens et al. (1996) reported a long-term net decline in soil pools of available Ca2+ during the last half of the 20th century, as acidic deposition reached its highest levels. Loss of ecosystem Ca2+ peaked in the mid-1970s and abated over the next 15–20 years, as atmospheric deposition of SO2?4 declined.

Without strong acid anions, cation leaching in forest soils of the Northeast is driven largely by naturally occurring organic acids derived from the decomposition of organic matter, which takes place primarily in the forest floor. Once base saturation is reduced in the upper mineral soil, organic acids tend to mobilize Al through formation of organic–Al complexes, most of which are deposited lower in the soil profile through adsorption to mineral surfaces. This process, termed podzolization, results in surface waters with low concentrations of Al. Such concentrations are primarily in a nontoxic, organic form (Driscoll et al. 1988). Acidic deposition has altered podzolization, however, by solubilizing Al with inputs of mobile inorganic anions, which facilitates transport of inorganic Al into surface waters. Acidic deposition to forest soils with base saturation values less than 20% increases Al mobilization and shifts chemical speciation of Al from organic to inorganic forms that are toxic to terrestrial and aquatic biota (Cronan and Schofield 1990)

Water shed input–output budgets developed in the 1980s for northeastern forest ecosystems indicated that the quantity of S exported by surface waters (primarily as SO2?4) was essentially equivalent to inputs from atmospheric deposition (Rochelle and Church 1987). Those findings suggested that decreases in atmospheric S deposition, from controls on emissions, should result in equivalent decreases in the amount of SO2?4 that enters surface waters. Indeed, there have been long-term decreases in concentrations of SO2?4 in surface waters throughout the Northeast following declines in atmospheric S deposition after the 1970 CAAA (Likens et al. 1990, Stoddard et al. 1999). However, recent studies of watershed mass balances in the Northeast have shown that watershed loss of SO2?4 exceeds atmospheric S deposition (Driscoll et al. 1998). This pattern suggests that decades of atmospheric S deposition have resulted in the accumulation of S in forest soils. With recent declines in atmospheric S deposition and a possible warming-induced enhancement of S mineralization from soil organic matter, previously retained S is gradually being released to surface waters (Driscoll et al. 1998).

Past accumulation of atmospherically deposited S is demonstrated by a strong positive relationship between wet deposition of SO2?4 and concentrations of total S in the forest floor of red spruce stands in the Northeast (Figure 5a). It is now expected that the release of SO2?4 that previously accumulated in watersheds from inputs of atmospheric S deposition will delay the recovery of surface waters in response to SO2 emissions controls (Driscoll et al. 1998). Imbalances in ecosystem S budgets may also be influenced by the weathering of S-bearing minerals or by the underestimation of dry deposition inputs of S. Further effort is needed to quantify such processes more accurately.

Accumulation of nitrogen in soils

Nitrogen is generally considered the growth-limiting nutrient for temperate forest vegetation, and retention by forest ecosystems is generally high. As a result, concentrations of NO?3 are often very low in surface waters that drain forest landscapes. However, recent research indicates that atmospheric N deposition has accumulated in soils, and some forest ecosystems have exhibited diminished retention of N inputs. Total N concentration in the forest floor of red spruce forests is correlated with wet N deposition at both low (Figure 5b) and high elevations in the Northeast (McNulty et al. 1990). A record of stream chemistry in forest watersheds of the Catskill Mountains (New York) has shown increasing NO?3 concentrations since 1920, apparently in response to increases in atmospheric N deposition (Charles 1991). Increased stream NO?3 concentrations have also been observed after experimental additions of N to a small watershed in Maine (Norton et al. 1994). Nitrate behaves much like SO2?4, thereby facilitating the displacement of cations from the soil and acidifying surface waters.

Increased losses of NO?3 to surface waters may indicate changes in the strength of plant and soil microbial N sinks in forest watersheds. Because microbial processes are highly sensitive to temperature, fluctuations in microbial immobilization and mineralization in response to climate variability affect NO?3 losses in drainage waters. Murdoch et al. (1998) found that annual mean NO?3 concentrations in stream water were not related to annual wet N deposition but rather to mean annual air temperature. Increases in temperature corresponded to increases in stream-water concentrations. Mitchell et al. (1996) found that unusually low winter temperatures that led to soil freezing corresponded to the increased loss of NO?3 to surface waters. The sensitivity of NO?3 release to climatic fluctuations tends to increase the magnitude and the frequency of episodic acidification of surface waters.

Despite the linkage between the atmospheric deposition of NH+4 and NO?3 and the loss of NO?3 from forest ecosystems (Dise and Wright 1995), future effects of atmospheric N deposition on forest N cycling and surface water acidification are likely to be controlled by climate, forest history, and forest type (Aber et al. 1997, Lovett et al. 2000). For example, forests regrowing after agricultural clearing or fire tend to have a higher capacity than undisturbed forests for accumulating N without release to surface waters (Hornbeck et al. 1997, Aber et al. 1998). The complexity of linkages of NO?3 loss to climatic variation, to land-use history, and to vegetation type has slowed efforts to predict how ANC in surface waters will respond to anticipated changes in atmospheric N deposition associated with NOx or NH3 emission controls. Improved predictions will depend on continued progress in understanding how forest ecosystems retain N and in determining regional-scale information about land-use history. Despite this uncertainty, it is apparent that additional NH+4 and NO?3 inputs to northeastern forests will increase the potential for greater leaching losses of NO?3, whereas reductions in NOx and NH3 emissions and subsequent N deposition will contribute to long-term decreases in watershed acidification.

Effects on aquatic biota

Acidification has marked effects on the trophic structure of surface waters. Decreases in pH and increased Al concentrations contribute to declines in species richness and in the abundance of zooplankton, macroinvertebrates, and fish (Schindler et al. 1985; Keller and Gunn 1995).

High concentrations of both H+ (measured as low pH) and inorganic monomeric Al are directly toxic to fish (Baker and Schofield 1982). Although Al is abundant in nature, it is relatively insoluble in the neutral pH range and thus unavailable biologically. Acid-neutralizing capacity largely controls pH and the bioavailability of Al (Driscoll and Schecher 1990). Thus, surface waters with low ANC and pH as well as high concentrations of inorganic monomeric Al are less hospitable to fish. Calcium, however, directly ameliorates the toxic stress caused by H+ and Al (Brown 1983). Watershed supply of Ca2+ also contributes to ANC. Therefore, lakes with higher Ca2+ are more hospitable to fish, as indicated by the synoptic survey conducted by the Adirondack Lakes Survey Corporation (ALSC; Gallagher and Baker 1990). Of the 1469 lakes surveyed by the ALSC, at least one fish species was caught in 1123 lakes (76%), whereas no fish were caught in 346 lakes (24%). The 346 fishless lakes in the Adirondack region had significantly (p < .05) lower pH, Ca2+ concentration, and ANC values, as well as higher concentrations of inorganic monomeric Al, when compared to lakes with fish (Gallagher and Baker 1990).

Small, high-elevation lakes in the Adirondack region are more likely to be fishless than larger lakes at low elevation (Gallagher and Baker 1990), because they have poor access for fish immigration, poor fish spawning substrate, or low pH, or they may be susceptible to periodic winterkills. Nevertheless, small, high-elevation Adirondack lakes with fish also had significantly higher pH compared with fishless lakes; acidity is likely to play an important role in the absence of fish from such lakes.

Numerous studies have shown that fish species richness (the number of fish species in a water body) is positively correlated with pH and ANC values (Figure 8; Rago and Wiener 1986, Kretser et al. 1989). Decreases in pH result in decreases in species richness by eliminating acid-sensitive species (Schindler et al. 1985). Of the 53 species of fish recorded by the ALSC (Kretser et al. 1989), about half (26 species) are absent from lakes with pH less than 6.0. Those 26 species include important recreational fishes, such as Atlantic salmon, tiger trout, redbreast sunfish, bluegill, tiger musky, walleye, alewife, and kokanee (Kretser et al. 1989), plus ecologically important minnows that serve as forage for sport fishes. Significantly, the most common fish species caught by the ALSC (brown bullhead, yellow perch, golden shiner, brook trout, and white sucker) also show the greatest tolerance of acidic conditions, as evidenced by their occurrence in lakes with relatively low pH and high Al concentrations (Gallagher and Baker 1990).

A clear link exists between acidic water, which results from atmospheric deposition of strong acids, and fish mortality. In situ bioassays conducted during acidic events (pulses of low pH, Al-rich water after precipitation events or snowmelt) provide an opportunity to measure the direct, acute effects of stream chemistry on fish mortality. Those experiments show that even acid-tolerant species, such as brook trout, are killed by acidic water in the Adirondack region (Figure 9; Baker et al. 1996, Van Sickle et al. 1996). Episodic acidification is particularly important in streams and rivers (compared with lakes) because these ecosystems experience large abrupt changes in water chemistry and provide limited refuge areas for fish. Baker et al. (1996) concluded that episodic acidification can have long-term negative effects on fish communities in small streams because of mortality, emigration, and reproductive failure.

The ERP study showed that streams with moderate to severe acid episodes had significantly higher fish mortality during bioassays than nonacidic streams (Van Sickle et al. 1996). The concentration of inorganic monomeric Al was the chemical variable most strongly related to mortality in the four test species (brook trout, mottled sculpin, slimy sculpin, and blacknose dace). Because of their correlations with Al, variations in pH and Ca2+ concentrations were of secondary importance in accounting for mortality patterns. The ERP streams with high fish mortality during acid episodes had lower brook trout density and biomass and lacked the more acid-sensitive species (blacknose dace and sculpins). In streams that exhibited episodic acidification, radio-tagged brook trout emigrated downstream during episodes, whereas radio-tagged fish in nonacidic streams did not. In general, trout abundance was lower in ERP streams with median episode pH less than 5.0 and with a concentration of inorganic monomeric Al greater than 3.7–7.4 µmol L?1. Acid-sensitive species were absent from streams with median episode pH less than 5.2 and with a concentration of inorganic monomeric Al greater than 3.7 µmol L?1.

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