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CADDIS Volume 2: Sources, Stressors & Responses

pH

Low pH | High pH

Low pH: Detailed Conceptual Diagram

detailed conceptual diagram for low pH

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Diagram narrative

pH is a measure of hydrogen ion concentration in an aqueous solution:  pH decreases as hydrogen ion concentration increases, and these acidic conditions can adversely affect aquatic biota. This conceptual diagram illustrates linkages between low pH and associated pH fluctuations (middle of diagram), the human activities and sources that can contribute to low pH (top of diagram), and the biological responses that can result (bottom of diagram). In some cases, additional steps leading from sources to stressors, modes of action leading from stressors to responses, and other modifying factors also are shown. This narrative generally follows the diagram top to bottom, left to right. 

Linking sources to stressors

Certain human activities and land uses can result in increased input of hydrogen ions into aquatic systems.  Land drainage can reduce soil saturation and lead to sulfur oxidation and the release of sulfuric acid.  Other sources of hydrogen ions include acid mine drainage and other mining wastes from active and historical mines; natural, acid-generating geologies and lithologies subjected to weathering; natural organic acids (e.g., humic acids); animal wastes from CAFOs, dairies, and aquaculture facilities; coal piles and landfills associated with residential, municipal, commercial, and industrial facilities; and emissions and effluents from coal-fired power plants, metal plating plants, and other industrial facilities.  Hydrogen ions from these sources can be introduced into aquatic systems via four main transport pathways (or transport-defined sources):  stormwater runoff, leakage or leachate into groundwater sources and subsequent transport, atmospheric emissions and deposition, or direct effluent discharges.  Each of these transport-defined sources can contribute to increased hydrogen ion inputs into surface waters.  When atmospheric deposition of hydrogen ions occurs in the form of snowfall, snowmelt can result in pulsed delivery of hydrogen ions to surface waters.    

Whether inputs of hydrogen ions to aquatic systems lead to decreases in pH depends upon buffering capacity, or the ability of the system to neutralize those inputs.  Streams with high bicarbonate concentrations are highly buffered, and may not become acidic even with significant hydrogen ion inputs; once buffering capacity is exceeded, however, pH will decrease.  Instream oxidation-reduction processes also may influence pH.  For example, nitrification and respiration both produce hydrogen ions, so nutrient (especially nitrogen) concentrations may play a significant role in pH dynamics.  Decreases in pH also can affect other stressors, such as by increasing free metal ions, increasing the bioavailability and toxicity of toxic substances, and increasing ionic strength (see the metals, ionic strength, and toxic substances modules for more information on these pathways).        

Linking stressors to biological responses

Decreases in pH and associated increases in pH fluctuation can adversely affect aquatic organisms via many potential modes of action.  Low pH can lead to dissolution of calcium carbonate shells, ultimately leading to decreases in taxa with calcium carbonate shells (e.g., mussels and snails).  When low pH solutions (e.g., acid mine drainage) are neutralized upon entering higher pH streams, metals in those solutions can precipitate and smother or armor stream bottoms.  Low pH also can compromise ionoregulatory and osmoregulatory function, or lead to changes in food availability.     

These different modes of action all may contribute to decreased condition, decreased growth, altered behavior, and increased susceptibility to other stressors in affected biota.  Possible decreases in condition include gill hyperplasia, gill and fin erosion, lesions and skin damage (increasing susceptibility to fungal infections), and increased mucous secretion; possible changes in behavior include hyperexcitability.  Ultimately, these effects may result in increased mortality and decreased reproductive success, particularly in terms of impaired egg fertilization and development.  This can lead to changes in population and community structure and ecosystem function.  Taxa sensitive to low pH (e.g., certain mayfly and stonefly taxa) may decrease, while more tolerant taxa (e.g., tipulids, megalopterans, spike rushes) may increase; sensitive life stages (e.g., eggs in fish) also may decline.  These changes in community structure may in turn affect ecosystem functions such as leaf decomposition. 

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High pH: Detailed Conceptual Diagram

detailed conceptual diagram for high pH

Click diagram to view larger version


Diagram narrative

High pH levels occur when hydroxide ion concentrations are high and hydrogen ions are scarce.  Although acidic conditions more commonly result from human activities, alkaline conditions also can occur and adversely affect aquatic biota. This conceptual diagram illustrates linkages between high pH and associated pH fluctuations (middle of diagram), the human activities and sources that can contribute to high pH (top of diagram), and the biological responses that can result (bottom of diagram). In some cases, additional steps leading from sources to stressors, modes of action leading from stressors to responses, and other modifying factors also are shown. This narrative generally follows the diagram top to bottom, left to right. 

Linking sources to stressors

Certain human activities and land uses can result in increased input of hydroxide ions into aquatic systems, leading to increases in pH. These sources include:  runoff of lime-rich fertilizers applied to agricultural cropfields, golf courses, and lawns; runoff from limestone gravel roads, asphalt roads, and other asphalt waste; and effluents and leachate from oil and gas brine mining wastes or from industries that use lime, lye or sodium hydroxide (e.g., asphalt and cement manufacturing plants, soap manufacturing plants). Atmospheric emissions and deposition are not significant transport sources for hydroxide ions and thus generally do not contribute to high pH conditions, a marked difference from low pH conditions. There also are natural sources which can result in high pH conditions, such as naturally alkaline geologies and lithologies and high levels of photosynthesis. Because photosynthesis produces hydroxide ions, elevated nutrient concentrations may contribute to pH increases. High pH levels also can affect other stressors, most notably by increasing the proportion of ammonia in its unionized, toxic form (see the ammonia and ionic strength modules for more information on these pathways).        

Linking stressors to biological response

Increases in pH and associated increases in pH fluctuation can adversely affect aquatic organisms via many potential modes of action. For example, metal hydroxides can form and precipitate, smothering or armoring stream bottoms. Ammonia excretion may be impaired, adversely affecting ionoregulatory function and protein metabolism. Indirect effects such as changes in food availability also may occur.     

These different modes of action all may contribute to decreased condition, decreased growth, altered behavior, and increased susceptibility to other stressors in affected biota. Possible decreases in condition include gill hyperplasia, gill and fin erosion, lesions and skin damage (increasing susceptibility to fungal infections), and increased olfactory damage; possible changes in behavior include lethargy. Ultimately, these effects may result in increased mortality, decreased reproductive success, and changes in population and community structure and ecosystem function.  For example, taxa sensitive to high pH (e.g., perciform fishes) may decrease, while more tolerant taxa (e.g., cypriniform fishes, Cladophora) increase; these changes may result in reduced taxa richness or diversity. 

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