Effects of acidity on stream ecosystems

Fresh waters may be naturally acidic due to the decay of organic matter, and anthropogenically acidified by atmospheric deposition of strong

FIGURE 4.7 The mean annual concentration of chloride increases with impervious surfaces area (an indicator of urbanization) for streams along a rural to urban gradient near Baltimore, Maryland. Dashed lines represent thresholds for damage to some land plants and chronic toxicity to sensitive aquatic organisms. (Reproduced from Kaushal et al. 2005.)

FIGURE 4.7 The mean annual concentration of chloride increases with impervious surfaces area (an indicator of urbanization) for streams along a rural to urban gradient near Baltimore, Maryland. Dashed lines represent thresholds for damage to some land plants and chronic toxicity to sensitive aquatic organisms. (Reproduced from Kaushal et al. 2005.)

inorganic acids formed from sulfate and nitrous oxides released in the burning of fossil fuels, or from acids leached from mining deposits. Naturally acidic waters, tea-colored from the breakdown of organic matter, occur in diverse settings including northern peatlands, tropical regions such as the aptly named Rio Negro, and blackwater rivers draining swamp forests such as the Ogeechee River in the southeastern United States. Acid precipitation is a relatively recent phenomenon due to industrialization, and has its greatest influence in regions of poor buffering capacity, especially in granitic catchments. Substantial areas of northern Europe, the northeastern United States and eastern Canada, and the Rocky Mountains have been significantly impacted.

The deleterious effects of acidic streamwater are well established, primarily in terms of reduced numbers of species and individuals (Figure 4.8), but there also is evidence of altered ecosystem processes. The degree of acidification is of course very important, and depends upon both inputs and buffering capacity. Organisms evidently are harmed via diverse pathways, including metal toxicity, and taxa differ in their susceptibility. Consequently, while one can assert that anthropogenic acidification is generally harmful once pH falls much below 5.0, the details depend upon many factors.

The River Duddon in the English Lake District has received acid precipitation from the industrialization of Europe for probably the last 100 years, and prior to this a long and gradual period of postglacial acidification is thought to have occurred (Sutcliffe and Carrick 1973, 1988). The range of pH in this region is approximately 4-7 and varies with geology, upstream-downstream location, and season. Permanently acid (pH < 5.7 at all times) streams have a characteristic and restricted macroinvertebrate fauna, consisting of six plecopterans, four trichopterans, and three

FIGURE 4.8 Influence of pH on the number of species occurring in streams of the Ashdown Forest, southern England. (a) Microarthropods: mainly mites (Hydrachnellae), copepods (Harpacticoida and Cyclopoida), and Clado-cera. (b) Macroarthropods: mainly aquatic insects. Note that the number of species of macroarthropods found at a site was a fairly constant 40-50% of the species known to occur under the conditions of water chemistry found at that site. (Reproduced from Rundle and Ormerod 1991 and Townsend et al. 1983.)

FIGURE 4.8 Influence of pH on the number of species occurring in streams of the Ashdown Forest, southern England. (a) Microarthropods: mainly mites (Hydrachnellae), copepods (Harpacticoida and Cyclopoida), and Clado-cera. (b) Macroarthropods: mainly aquatic insects. Note that the number of species of macroarthropods found at a site was a fairly constant 40-50% of the species known to occur under the conditions of water chemistry found at that site. (Reproduced from Rundle and Ormerod 1991 and Townsend et al. 1983.)

dipterans. Such streams occur in areas of relatively hard, slow-weathering rocks, often in the headwaters. Locations with pH permanently >5.7 have additional taxa, including a number of mayflies, two other species of trichopterans, the limpet Ancylus, and the amphipod Gammarus. They are found in regions of sedimentary slates and exposed veins of calcite. These streams have not changed in acidity since at least the 1950s, evidently because bicarbonate ion availability counters the continued inputs from acid rain.

Naturally acid streams seem less affected than those acidified by atmospheric pollutants. A survey of 34 streams in New Zealand that exhibited a pH range from 4.1 to 8.1 due to variation in natural concentrations of humic substances found no correlation between pH and taxonom-ic diversity (Winterbourn and Collier 1987). Similar numbers of taxa were obtained from streams with pH <5.5, between 5.6 and 6.9, and >7.0. Only at a pH below 4.5 was there any evidence of faunal exclusion, in marked contrast to the Duddon example. These contrasting cases suggest that the mechanism must involve more than acidity alone. Indeed, there is growing evidence that lowered pH is accompanied by a number of chemical changes, and also that organism response is due to various physiological, behavioral, and indirect effects.

Leaching of metals from soils is an important consequence of hydrogen ion deposition. Aluminum occurs in elevated concentrations in acidic waters (Figure 4.9), and is known to impair osmoregulatory processes by damaging the ion-regulating organs. Separate and combined additions of aluminum compounds and inorganic acids to stream channels have been used to distinguish the direct influence of hydrogen ion concentration from the effects of elevated aluminum. In a short-term (24 h) manipulation of a soft-water stream in upland Wales, two salmonid species exhibited far greater susceptibility to the combined effects of acid and aluminum versus sulfuric acid alone, apparently because of respiratory inhibition (Ormerod et al. 1987).

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