Effects of Acidic Deposition on Freshwater Aquatic Ecosystems

Surface Water Acidification

Acidic deposition degrades surface water quality by lowering pH (i.e., increasing acidity); decreasing ANC; and increasing dissolved inorganic aluminum concentrations. While sulfate concentrations in lakes and streams have decreased in the eastern North America and Europe over the last 20 years, they remain high compared to background conditions (e.g., approximately 20 meql-1). Moreover, improvement in other chemical conditions in many lakes and streams in acid-impacted regions has been limited.

Acidification of surface waters due to elevated inputs of acidic deposition has been reported in many acid-sensitive areas receiving elevated inputs of acidic deposition, including Great Britain, Nordic countries, Northern, Central and Eastern Europe, southwestern China, southeastern Canada, the northeastern US, the Upper Midwest and the Appalachian mountain region of the US. Large portions of the high-elevation western US are also potentially sensitive to acidic deposition; however, atmospheric deposition to this region is relatively low. Concern over effects of acidic deposition in the mountains of western US may be overshadowed by potential effects of elevated nitrogen deposition, including eutrophication of naturally nitrogen-limited lakes.

To illustrate the regional impacts of acidic deposition, a comprehensive survey of lakes greater than 0.2 ha in surface area in the Adirondack region of New York was conducted to obtain detailed information on the acid-base status of waters in this region. Of the 1469 lakes surveyed, 24% had summer pH values below 5.0. Also 27% of the lakes surveyed were chronically acidic (i.e., ANC less than 0 meql-1) and an additional 21% were susceptible to episodic acidification (i.e., ANC between 0 and 50 meq l_ ). An analysis of the anion content of these lakes illustrates that these lakes have predominantly been acidified by atmospheric deposition of sulfate.

Seasonal acidification is the periodic increase in acidity and the corresponding decrease in pH and ANC in streams and lakes, which generally occurs during the higher flow fall, winter, and spring periods. Episodic acidification is caused by the sudden pulse of acids and a dilution of base cations (e.g., calcium, magnesium, sodium, potassium) due to spring snowmelt and large rain events in the spring and fall. Increases in nitrate are often important to the occurrence of acid episodes. These conditions tend to occur when trees are dormant and therefore retain less nitrogen. At some sites, short-term increases in sulfate and organic acids can also contribute to episodic acidification. Episodic acidification often coincides with pulsed increases in concentrations of dissolved inorganic aluminum. Short-term increases in acid inputs to surface waters can reach levels that are lethal to fish and other aquatic organisms. All of the acid-sensitive and acid-impacted regions discussed in this article have documented effects associated with episodic acidification.

Trends in surface water chemistry in Europe and eastern North America indicate that recovery of aquatic ecosystems impacted by acidic deposition is occurring over a large geographic scale since the early 1980s. Some regions show rather marked recovery, while others exhibit low or nonexistent increases in ANC. Based on long-term monitoring, virtually all surface waters impacted by acidic deposition in Europe and eastern North America exhibit decreases in sulfate concentrations. This pattern is consistent with decreases in emissions of sulfur dioxide and atmospheric sulfate deposition. The exception to this pattern is streams in unglaciated Virginia. Watersheds in this region and other portions of the southeastern US exhibit strong adsorption of atmospheric sulfate deposition by highly weathered soils. In Europe, the most marked decreases in surface water sulfate have occurred in the Czech Republic and Slovakia, regions that experienced historically very high rates of atmospheric sulfate deposition. Somewhat more than half of the surface waters monitored in Europe show increases in ANC. The rate of ANC increase in Europe is relatively high. This pattern is, in part, due to the relatively high rates of sulfate decreases, but also to the fact that decreases in base cations only account for about half of the decreases in sulfate plus nitrate, allowing for relatively large rates ofANC increases. In contrast, in the US only three regions show statistically significant increases in ANC: lakes in the Adirondacks, Upper Midwest, and streams in Northern Appalachian Plateau. In the US, decreases in the sum of base cations closely correspond to decreases in sulfate plus nitrate, limiting rates of ANC increase.

Three factors account for the slow chemical recovery of the water quality of acid-impacted surface waters, despite the decreased deposition of sulfate. First, levels of acid-neutralizing base cations in streams have decreased markedly due to a loss of base cations from the soil and, to a lesser extent, due to a reduction in atmospheric inputs of base cations. Second, inputs of nitric acid have acidified surface waters and elevated their concentration of nitrate in many acid-impacted regions. Finally, sulfur has accumulated in the soil and is now being released to surface water as sulfate, even though sulfate deposition has decreased. It appears that the only approach to accelerate the recovery of acid-impacted lakes is to make additional cuts in emissions of sulfur dioxide and nitrogen oxides.

The modest decreases in sulfate concentrations and increases in pH and ANC exhibited in some surface waters is an encouraging sign that impacted ecosystems are responding to emission controls and moving toward chemical recovery. Nevertheless, the magnitude of these changes is small compared to the magnitude of increases in sulfate and decreases in ANC that have occurred in acid-impacted areas following historical increases in acidic deposition. Moreover, as discussed above, in many acid-sensitive regions soils continue to acidify despite decreases in acidic deposition.

Response of Aquatic Biota to Acidification of Surface Waters by Acidic Deposition

Decreases in pH and elevated concentrations of dissolved inorganic aluminum have resulted in physiological changes to organisms, direct mortality of sensitive life-history stages, and reduced the species diversity and abundance of aquatic life in many streams and lakes in acid-impacted areas. Fish have received the most attention to date, but entire food webs are often adversely affected.

Decreases in pH and increases in aluminum concentrations have diminished the species diversity and abundance of plankton, invertebrates, and fish in acid-impacted surface waters. A detailed summary of the response of aquatic biota to the acidification of surface waters is provided in Table 2.

In the Adirondacks, a significant positive relationship exists between the pH and ANC levels in lakes and the number of fish species present in those lakes (Figure 4). Surveys of 1469 Adirondack lakes conducted in 1984 and 1987 show that 24% of lakes (i.e., 346) in this region do not support fish. These lakes had consistently lower pH

Table 2 Biological effects of surface water acidification in North America pH decrease General biological effects

6.5-6.0 Small decrease in species richness of phytoplankton, zooplankton, and benthic invertebrate communities resulting from the loss of a few highly acid-sensitive species, but no measurable change in total community abundance or production Some adverse effects (decreased reproductive success) may occur for highly acid-sensitive species (e.g., fathead minnow, striped bass)

6.0-5.5 Loss of sensitive species of minnow and dace, such as blacknose dace and fathead minnow; in some waters decreased reproductive success of lake trout and walleye, which are important sport fish species in some areas Visual accumulations of filamentous green algae in the littoral zone of many lakes, in some streams Distinct decrease in the species richness and change in species composition of the phytoplankton, zooplankton, and benthic invertebrate communities, although little if any change in total community biomass or production

5.5-5.0 Loss of several important sport fish species, including lake trout, walleye, rainbow trout, and smallmouth bass; as well as additional nongame species such as creek chub Further increase in the extent and abundance of filamentous green algae in lake littoral areas and streams

Continued shift in the species composition and decline in species richness of the phytoplankton, periphyton, zooplankton, and benthic invertebrate communities; decrease in the total abundance and biomass of benthic invertebrates and zooplankton may occur in some waters Loss of several additional invertebrate species common in oligotrophic waters, including Daphnia galeata mendotae, Diaphanosoma leuchtenbergianum, Asplanchna priodonta; all snails, most species of clams, and many species of mayflies, stoneflies, and other benthic invertebrates Inhibition of nitrification

5.0-4.5 Loss of most fish species, including most important sport fish species such as brook trout and

Atlantic salmon; few fish species able to survive and reproduce below pH 4.5 (e.g., central mudminnow, yellow perch, and in some waters, largemouth bass)

(Continued )

Table 2 (Continued)

pH decrease General biological effects

Measurable decline in the whole-system rates of decomposition of some forms of organic matter, potentially resulting in decreased rates of nutrient cycling Substantial decrease in the number of species of zooplankton and benthic invertebrates and further decline in the species richness of the phytoplankton and periphyton communities; measurable decrease in the total community biomass of zooplankton and benthic invertebrates in most waters Loss of zooplankton species such as Tropocyclops prasinus mexicanus, Leptodora kindtii, and Conochilis unicornis; and benthic invertebrate species, including all clams and many insects and crustaceans

Reproductive failure of some acid-sensitive species of amphibians such as spotted salamanders, Jefferson salamanders, and the leopard frog

This table was previously published in Baker JP, Bernard DP, Christensen SW, and Sale MJ (1990) Biological effects of changes in surface water acid-base chemistry, Report SOS/T 13 for the National Acid Precipitation Assessment Program, Washington, DC.

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Air equilibrated pH class

Figure 4 Distribution of the mean number of fish species for ranges of pH 4.0-8.0 in lakes of the Adirondack region of New York. N represents the number of lakes in each pH category.

• Population-level effects (increased mortality). Bioassay experiments show greater mortality in chronically acidic streams than in high ANC streams. Eggs and fry are sensitive life-history stages for fish.

• Community-level effects (reduced species richness). The species richness of fish and other aquatic organisms decreases with decreasing ANC and pH.

Although chronically high acid levels stress aquatic life, acid episodes are particularly harmful because abrupt, large changes in water chemistry allow fish few areas of refuge. High concentrations of dissolved inorganic aluminum are directly toxic to fish and pulses of aluminum during acid episodes are a primary cause of fish mortality. High acidity and aluminum levels disrupt the salt and water balance of blood in a fish, causing red blood cells to rupture and blood viscosity to increase. Studies show that the viscous blood strains the heart of a fish, resulting in a lethal heart attack.

and ANC, and higher concentrations of aluminum than lakes that contained one or more species of fish. Experimental studies and field observations demonstrate that even acid-tolerant fish species such as brook trout have been eliminated from some waters in New York.

Similar relationships are evident in surface waters in acid-impacted regions throughout the world. Studies demonstrate effects of acidic deposition on fish at three ecosystem levels:

• Effects on single organisms (condition factor — the relationship between the weight and the length of a fish). Fish condition factor is related to several chemical indicators of acid-base status, including minimum pH. This analysis suggests that fish in acidic streams use more energy to maintain internal chemistry that would otherwise be used for growth.

See also: Acidification; Air Quality Modeling; Atmospheric Deposition; Carbon Cycle; Nitrogen Cycle; Sulfur Cycle.

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