CADDIS Volume 2: Sources, Stressors & Responses
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- Low pH
Mine wastes/historic mine sites: Coal mining is a well known source of low pH, particularly where sulfur-bearing rock is present; drainage from other mines and mine wastes can also be very acidic.
Acid generating rocks/soils: Interactions between rocks or soils and water (i.e., weathering) can generate acid. For example, andesite soils of the Great Basin region, derived from weathered volcanic rock, are strongly acidic.
Sources of acidic gases: Acidic deposition, the transfer of acidic compounds via rain, snow, and dry deposition acidifies waters. Sulfur and nitrogen oxides are emitted into the atmosphere from power plants, industrial sources (e.g., pulp mills), and internal combustion engines (where they are transformed into H2SO4 and HNO3). When those acid gasses are deposited into freshwater systems there is an increase in SO42-, NO3-, and H+, which can decrease the pH. Streams and lakes with relatively low buffering capacity or low flow (e.g., intermittent and first order streams) are particularly susceptible to low pH resulting from acidic deposition. Acid deposition has occurred in the Northeastern part of the United States. This is due to the abundance of cities and industrial plants in the Midwest and New England which emit pollutants to air that are then transported and deposited in the northeastern U.S. Acid deposition stored in snow and ice at high elevations or in cold climates can serve as a pulsed source of low pH water to streams when melting in the spring.
Point and nonpoint discharges: Industrial (e.g. metal plating, circuit board manufacturing, and other processes requiring acid) effluents can be discharged at a low pH if not properly treated. Nonpoint sources such as confined animal feeding operations (CAFOs), dairies, and concentrated poultry farms can generate acidic runoff from manure if not controlled adequately. Runoff from urban areas, leaching from improperly functioning septic systems and landfills, and coal pile runoff can be acidic as well.
Instream oxidation or reduction processes: Inputs of ammonium can contribute to acidification because the nitrification of NH4+ produces H+.
Recent draining of naturally inundated wetlands or floodplains: Under natural conditions, sulfur in permanently saturated soils is reduced. When those soils are drained and exposed to air, sulfuric acid can be generated and released to streams.
Alterations in pH are generally not directly observable in the field, and observers usually must measure pH to detect a change. Site observations indicative of low pH are, therefore, indirect and largely restricted to observing the presence of one or more acid producing (above).
Low pH may be indicated by the presence of metal precipitates on stream substrate. Common metal precipitates observed in streams under acidic conditions include iron, manganese, and aluminum which have a yellow (or orange or red), black, or white color, respectively. Metal precipitates, notably iron, can result in a thick floc that physically smothers biota and degrades habitat quality to the point that many fauna (particularly those species requiring interstitial spaces in rocks or accessible grazing surfaces to feed) are extirpated.
Black or brown (tea colored) water, common in southeastern U.S. swamps and bayous (e.g., Okefenokee Swamp, GA) are typically acidic due to humic and other natural organic acids derived from peat and dissolved vegetative material.
Some filamentous algae are tolerant of low pH and may dominate acidified lakes and streams without heavy precipitates (Niyogi et al. 1999). They include the filamentous green algae Chlorophyta, Klebsormidium, Microspora, Mougeotia, Ulotrix,and Stigeoclonium (Stevens et al. 2001).
Low pH has both lethal and sublethal effects on organisms. For many stream species, prolonged periods of pH <5 are likely to be lethal, resulting in significant changes in species composition and diversity. At pHs between about 5 and 6.5, sublethal effects on many stream species result in reduced fecundity, growth, and population size. Acid water also damages the skin and gills of fish, amphibians, and invertebrates. Skin damage increases the susceptibility of fish to fungal infections which may lead to diseases such as epizootic ulcerative syndrome. Gill and skin damage and associated production of mucus on the gill reduce the ability of fish to take in oxygen or regulate their intake of salts and water. Meyer and Barclay's (1990) fish kill manual states that acids (as well as heavy metals and trinitrophenols) can cause “white film on gills, skin, and mouth.” Meyer and Barclay also state that at low pH hyperexcitability and attempts by fish to swim onto shore may be observed. These effects are symptomatic of low pH, but they are not specific enough to be considered diagnostic.
The presence of stream species that are either especially sensitive to or especially tolerant of low pH is suggestive of low pH as a candidate cause. The generalizations about acid tolerance and pH effects thresholds presented in this section are imprecise because responses are heavily dependent on co-occurring toxic ions (particularly aluminum) and nutrient ions (particularly sodium and calcium) (Baker et al. 1990). However, they can be helpful. For example, if the impairment is characterized by the loss of brook trout, the cause is not likely to be low pH and is more likely to be stressors to which that species is sensitive—such as high temperature and low DO.
Certain aquatic or semiaquatic floras in the waterbody or its headwater wetlands are indicative of low pH conditions. These include certain spike rushes, water lilies, and many of the insectivorous marsh plants such as venus fly traps, pitcher plants, and sundews.
The diversity of macroinvertebrates and particularly the commonly monitored EPT taxa (Ephemoptera, Plecoptera and Trichoptera) generally declines with declining pH. Of the EPT taxa, the ephemeroptera (mayflies) are generally the most sensitive and the plecoptera (stoneflies) are generally most tolerant. Particularly sensitive species are found in the mayfly genera Heptagenia (Heptageniidae), Ephemerella (Ephemerellidae), and Paraleptophlebia (Leptophlebiidae) and members of the stonefly families Leutridae and Perlidae, which are generally absent from streams with low pH. A few chironomids, including Apsectrotanypus trifascipennis and Rheocricotopus fuscipes (Chironomidae), also are sensitive to acid and were only found in streams with a pH >6 (Orendt, 1999). Invertebrates that require calcium carbonate for shell or cuticle development (e.g., clams, mussels, snails, and amphipods) also are intolerant of low pH (<5) because acidic water dissolves calcium carbonate.
Other stream invertebrates are relatively tolerant of low pH (Figure 7). These include Megaloptera (e.g., Corydalus) and craneflies (e.g., Tipula). Certain genera of Chironomidae (midges) are also tolerant of low pH (e.g., some Orthocladiinae such as Corynoneura) and their dominance may indicate low pH (Orendt 1999). Also, northern pike, Esox lucius, can survive at a pH from 4-4.5 (Mackie 2001).
Low pH can cause reproductive failure and local extinction fish populations due to low egg fertilization rates and failure of normal egg development (Figure 6). In general, minnows (small members of the Cyprinidae) are the first to disappear from acidifying streams. Blacknose dace may disappear at pH below 6.1 and common shiners experience embryo mortality below pH 6.0 (U.S. EPA 2008). Eggs of several fish species (e.g., striped bass, lake trout, fathead minnow), which will not develop properly at a pH less than approximately 5.5 (Howells et al. 1983, Baker et al. 1990). For most North American species, however, juvenile and adult fish are generally more tolerant of moderately low pH (5-6.5) (Baker et al. 1990). Brook trout is a relatively acid-tolerant species—not disappearing until pH is near 5 (U.S. EPA 2008).
Low pH can have secondary effects due to an alteration of food availability for primary and secondary consumers due to pH stress on lower trophic levels. Low pH also can impede many ecosystem processes that depend on proper biological enzyme function—such as photosynthesis and detritus decomposition, which are critical sources of energy and nutrients for higher trophic levels. The sequence of direct and indirect effects of acidification is summarized in Table 1.
|pH Decrease||General Biological Effects|
|6.5 to 6.0||Small decreases in species richness of plankton 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 fish species (e.g., fathead minnow, striped bass).|
|6.0 to 5.5||Loss of sensitive species of minnows and dace, such as fathead minnow and blacknose dace; in some waters, decreased reproductive success of lake trout and walleye, which are important sport fish species in some areas.|
|Visual accumulation of filamentous green algae in near-shore zone of many lakes and in some streams.|
|Distinct decrease in species richness and change in species composition of plankton and benthic invertebrate communities, although little if any change in total community abundance or production.|
|Loss of some common invertebrate species from zooplankton and benthic communities, including many species of snails, clams, mayflies, and amphipods, and some crayfish.|
|5.5 to 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 near-shore areas and streams.|
|Continued shift in species composition and decline in species richness of plankton, periphyton, and benthic invertebrate communities; decreases in total abundance and biomass of benthic invertebrates and zooplankton may occur in some waters.|
|Loss of several additional invertebrate species common in surface waters, including all snails, most species of clams, and many species of mayflies, stoneflies, and other benthic invertebrates.|
|Inhibition of nitrification.|
5.0 to 4.5
|Loss of most fish species, including most important sport fish species such as brook trout and Atlantic salmon. A few fish species are able to survive and reproduce in water below pH 4.5 (e.g., central mudminnow, yellow perch, and in some waters largemouth bass).|
|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 number of species of plankton and benthic invertebrates and further decline in species richness of plankton and periphyton communities; measurable decrease in total community biomass of plankton and benthic invertebrates of most waters.|
|Loss of additional species of plankton 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.|
Low pH is buffered by minerals, such as calcium carbonate. Therefore, if the stream is well buffered, or flows through soils and rocks high in alkaline minerals (e.g., limestone or dolomite), it is less likely that low pH is an appropriate candidate cause of observed effects. The presence of acid-intolerant species such as ephemerellid mayflies, leuctrid or perlid stoneflies, or an abundance of clams or snails would also be evidence supporting exclusion of low pH as a candidate cause.
However, criteria or other benchmarks alone should not be used to exclude low pH as a cause, because of variance in sensitivity among species, effects of natural water chemistry, and effects of other agents such as metals that enhance the effects of pH.
High pH can be caused by discharges from industries that use lime, lye, or NaOH, from agricultural runoff of fertilizers high in lime, or industrial landfill leachates that contain solvents or lye. In particular, cement, asphalt, and soap manufacturing may be sources of high pH due to the use of lime or lye. Run-off from limestone gravel roads may increase pH. High pH can be caused in rare cases by natural conditions and mineralogy (e.g., weathering of chalk rock high in carbonates or olivine basalts); however, even in these cases, it is rare for stream pH to exceed 9.5. Leaching of naturally alkaline rocks and soils is exacerbated by physical disturbances such as tilling, mining, and construction. An additional cause of elevated pH is high photosynthetic activity, which removes carbon dioxide from water favoring equilibrium toward carbonate and a higher pH.
Alterations in pH are not directly observable in typical field settings and observers usually must measure pH to detect a change. However, high photosynthetic activity as evidenced by algal blooms or mats, or high macrophyte density, can cause pH to increase above tolerable levels (pH > 9), at least during daylight hours. Certain species of Microcystis and Coccochloris algae grow optimally at a pH of 10 and do not grow below 8. Also, Cladophora glomerata is only found in streams with a pH greater than 8, while other species such as Oedogonium kurtzi (green algae) and Phaeospaera perforate (yellow-green algae) are found in streams with a pH less than 7 (Mackie 2001). High pH also may be evidenced by certain mineral deposits in the stream notably precipitates of metal hydroxides (e.g., nickel and cadmium), which are insoluble above pH = 9.
As with low pH, effects of high pH usually are not specific enough to be considered symptomatic. Short-term exposures of fish to high pH (pH ~ 9.5) are rarely lethal to most fish species, but prolonged exposure to pH between 9.5 and 10 can damage outer surfaces such as gills, eyes, and skin. Cypriniformes (e.g., minnows) often are less sensitive to high pH than Perciformes (e.g., perch). High pH also can affect the sensory epithelium of the fish olfactory system, making it difficult for fish to detect food, sex hormones or pheromones, alarm substances from conspecifics, or toxic chemicals.
In addition to direct effects of high pH, an important indirect effect to consider is the interaction between high pH and ammonia. As pH increases, unionized ammonia (NH3), which is more toxic to aquatic life than the ionized form (NH4+), becomes the predominant form. At pH > 9, the fraction of unionized ammonia is > 100 times the fraction at pH = 7 (> 90% of the total ammonia), which could result in ammonia toxicity if sufficient ammonia is present (e.g., >5 mg/L total ammonia at 25° C). Because ammonia toxicity can occur over relatively short exposure durations (hours to 4 days depending on the concentration), even short-term high pH events (due for example to high photosynthetic activity during daylight hours) can contribute to biological effects. Therefore, when considering high pH as a candidate cause, bear in mind that it might be contributing to ammonia toxicity.Step 2 of the Step-by-Step Guide and in Tips for Listing Candidate Causes. We strongly caution against using benchmarks of effects (e.g., water quality criteria) as evidence for excluding high pH from your initial list of candidate causes because different species have different pH requirements, different sites have different naturally occurring levels of pH, and other agents may enhance the effects of pH.