The effects of low pH conditions—or high acidity—must be studied from a variety of perspectives. As an aquatic environment experiences a decrease in the pH, a general trend of deterioration is observed. This results in a host of progressive biological effects from outright death (Figure 10.2) through a variety of reproductive and behavioral impacts. Additionally, changes in pH also can affect the toxicity of certain pollutants and can affect the mobility of others.
The control of pH is regulated by the water’s buffering system. Because geography and geology are important in the buffering system, different environments exhibit varied tolerances to pH changes. The EPA Redbook recommends a pH of 6.5-9.0 for freshwater systems, and a 6.5-8.5 range, with not more than 0.2 standard units above normal range, for marine environments.
Hydrogen sulfide also should be considered deleterious to aquatic organisms. The degree of danger posed by it is dependent upon the temperature, pH, and dissolved oxygen. Because hydrogen sulfide is the predominant form of sulfide at pH 5 and is the most hazardous form, acidic conditions augment toxicity. Fish exhibit a strong avoidance response to H2S, and if escape is possible, the toxic threat is minimized. The EPA Redbook identifies a water quality criterion of 2μg/l for fish and other aquatic life in fresh and marine waters.
Because pH affects chemical reactions, pH control of drinking water supplies is also very important. This is because of the potential effect of low pH on distribution and process piping and equipment. It also should be noted that pH markedly affects water treatment practices such as coagulation and chlorination chemistry.
One of the most important aspects of low pH in potable water distribution systems is the potential for lead to be leached from older plumbing as well as copper and cadmium from pipes and water coolers. This problem is so widespread and significant that methods of control to prevent lead leaching have been implemented. One approach is to raise the pH through chemical addition. Another, employed by New York City, requires the addition of calcium orthophosphate to induce pipe scaling to prevent lead leaching.
Industrial uses of water often result in substantial pH changes, and consequently, this parameter is monitored in most, if not all, NPDES permits. Compliance requires pH adjustment with caustic (e.g., NaOH) or acid (e.g., H2SO4) stored on-site in tanks and delivered by way of automatic feed systems. Due to both wide pH fluctuations and equipment failures, the discharge of wastewater from industrial sources can result in problems to receiving water bodies.
One of the most significant problems related to low pH conditions is associated with acid precipitation/deposition in the northeast and Canada. In watershed areas where the bedrock outcrops or is near the surface, acidic deposition receives little buffering. This is because the bedrock is often igneous rock such as granite, or metamorphic rock such as gneiss. They resist the solvent properties of the water runoff, which is usually soft, and they do little to buffer the acidity.
The spring thaw yields and acid surge or “acid shock” to the surrounding soils and receiving water bodies. The pH often can be as low as 3-3.5, and the surge of acid runoff occurs at the same time as the reproductive cycle of many fish and the growth of food species. The acid conditions are toxic to eggs and fry, and also leach heavy metals such as lead. However, one particular pollutant, i.e., aluminum, deserves special attention.
In 1977, Dr. Carl Schofield of Cornell University observed that aluminum compounds were collecting in the gills of fish fry. In response to the aluminum, the fry secreted mucous on the gill surface, which interfered significantly with respiration and often led to death.
Other studies have demonstrated different effects from the acid-leached aluminum. Aquatic insects are somewhat resistant to low pH conditions and will accumulate excess aluminum. The impact was observed on the songbirds who frequented the area around the lakes. They consumed the aquatic insects as food, and within the birds “factor substitution” took place.
In this case the aluminum was substituted for calcium in the bone marrow. The songbirds along the acidified lakes had fewer eggs and lower hatching success and the eggs were soft or missing shell material. It was found that the aluminum in the bird’s bone marrow counteracted calcium deposition and resulted in the defective eggshells.
Low pH conditions in lakes generally lead to a process of species substitution and lower productivity. In waters with only slightly acid conditions, desmids are common, but as conditions become progressively more acidic, dinoflagellates and blue-green algae become more numerous.
Species diversity decreases with decreasing pH in most lakes. This can be seen particularly during the acid surge associated with the spring thaw. It has often been observed, however, that biomass did not vary appreciably even though the ecosystem’s composition does.
Along with the increase in dinoflagellates and filamentous algae (blue-green), sphagnum mosses often replace shallow water macrophytes. They further acidify the shallow waters by releasing H4 in exchange for cations—such as Ca, Mg, Na, and K—and grow excessively. Mats of filamentous algae are also often characteristic with the Sphagnum mosses, comprised of blue-green algae and some specific types of diatoms.
In general, a decrease in invertebrate populations also can accompany the decrease in pH of a lake. Many insects are very tolerant of acidic conditions, but a least one notable species, the mayfly, will suffer significant population declines. It should be remembered that the songbirds who eat the insects suffer the effects of the aluminum brought to the water by acidic runoff.
Zooplankton populations decrease in severe acidic conditions, but in Jess extreme circumstances species shift (species substitution) is observed. Other effects noted on bioassay studies of various species included failure to reproduce, failure of eggs to hatch, and larval death at various stages.
Mollusks also are noted to become absent in extreme pH conditions. This is probably due to the loss of bicarbonate in the buffering system and subsequent interference in shell maintenance functions.
Due to the more extensive buffering system that exists in seawater, there is less variation in the pH than in freshwater. Therefore, species have generally less tolerance to pH changes. Benthic invertebrates appear to be more sensitive than fish. Mature forms of oysters as well as larval forms are adversely affected at the extremes of pH 6.5-9.0.
Fish kills attributed to low pH conditions first received attention in Norway and southern Sweden. It was observed that these periodic incidents occurred following the spring thaw or heavy autumn rains. Scientific investigation of this phenomenon pointed to a variety of causes. Principal among these were a multitude of effects related directly to the receiving water’s pH.
Additional problems included the leaching of heavy metals, especially aluminum; increased toxicity of other pollutants; and significantly increased erosion as acidic deposition damaged vegetation surrounding lakes. Investigations also revealed significant problems in the Adirondack Mountains in upstate New York, where many lakes no longer supported fish populations.
As this pollution situation was studied, it became apparent that fisheries suffered from both acute acid shock following the spring thaw as well as chronic gradual deterioration of the lake ecosystem and its biotic elements. The spring thaw consisted of snow melt with its reserve of acidic deposition. The first 30% of the meltwater contained most of the acid, leading to a low pH episode where levels were observed to be as low as 3-3.5. This event coincides with the reproductive cycles of many fish, including those of commercial value.
Mostly young fish are affected directly, but other effects can also be identified. Female fish exhibit avoidance to waters of low pH, which would preclude spawning. If fish do spawn, there may be significant failure of eggs to hatch or death of fry. Further affecting this problem is the destruction or absence of necessary food species for the developing young. Between avoidance, high death rate of fry, and egg failure, acidified lakes saw little recruitment into adult populations.
In fact, acidic lakes that still have fish are characterized as having fewer and larger adults due to recruitment failure. There is a “shift” in population to decreasing densities with increasing populations of larger and older fish. Some lakes even experience the extinction of fish populations as fewer and fewer fish reach maturity and reproductive rates declined.
Lakes with more complex ecosystems could see other effects in fish populations besides the general size increase. In some situations, size decrease was observed. This was attributed to loss of prey with a subsequent shift to less desirable food species. However in most cases, population decrease and size increase were the rule depending on the degree of damage and original complexity of the ecosystem.
The EPA’s Redbook provides data on a variety of pH effects on fish species. Avoidance of spawning areas made many species unable to find suitable nesting substrate. Brook trout and white sucker females indicated avoidance at pH 4.0-4.5 and showed abnormal patterns of calcium metabolism. In some species that did spawn, hatching time was decreased, for example Atlantic salmon at pH 5.0-5.5.
For eggs to hatch and for fry to develop normally, minimum pH values varied widely from species to species. For salmon the minimum pH is 5.0 to 5.5, for brown trout 4.5, and for brook trout 6.0-6.5. For many species reproduction stops entirely: Small- mouth bass at pH below 5.5-6.0, lake trout 5.2-5.5, rock bass and brown bullheads 4.7-5.5, and lake herring, yellow perch, and lake chub at 4.5-1.7.
Many other effects on fish populations also have been identified. The low pH, conditions and excess hydrogen ions resulted in uptake that lowered a fish’s blood pH, leading to a reduced ability of hemoglobin to carry oxygen. The lower oxygen tension in the blood is offset by increased red blood cell production leading to higher levels of hemoglobin and hemocrit.
In some species, the acidic conditions led to deformities and genetic damage. Physiological stress caused abnormal behavior altering predator/prey relationships, initiating avoidance, and resulting in outside recruitment failure. Impaired osmoregulation and ion balance also has been found, as was inhibited hormone production and activity. Lastly, besides the increased toxicity of many contaminants, fish generally were rendered more susceptible to disease.