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Effect of High Acidity on Aquatic Species

The pH of waters is important to aquatic life because pH affects the ability of aquatic organisms to regulate basic life-sustaining processes, particularly the exchanges of respiratory gasses and salts with the water in which they live (Rober-Bryan, Inc., 2004). In other words, pH (or more appropriately H+ activity) has a large influence on many important chemical reactions such as dissociation of organic acids and concentration and speciation of potentially toxic aluminium (Sullivan, 2000). More specifically, the pH of the water is a major physical factor that determines the distributions of organisms in aquatic habitats (Clark et al., 2004). It is noted that in many instances important physiological processes operate normally in most aquatic biota under a relatively wide pH range (e.g., 6-9 pH) (Rober-Bryan, Inc., 2004). In fact, most of the freshwater lakes, streams and ponds have a natural pH in the range of 6 to 8 (Lenntech, 2014). It must be noted, however, that the acceptable range of pH to aquatic life, particularly fish, depends on numerous other factors, including prior pH acclimatisation, water temperature, dissolved oxygen concentration and the concentrations and ratios of various cations and anions (Rober-Bryan, Inc., 2004).

Unfortunately, human activities present aquatic species with numerous environmental challenges including altered pH regimes (freshwater acidification) (Isaza et al., 2020). It must also be noted that the increased acidity caused by AMD has a range of negative effects on aquatic species depending on the severity of the pH change (Coil et al., 2013). Nevertheless, aquatic organisms are affected by acidic water at all trophic levels, resulting in changes in productivity and biomass accumulation, and the extermination of sensitive species (Lacoul et al., 2011). Low pH often stunts the growth of frogs, toads and salamanders (Thoreau, 2002). Table 5.1 is a summary of the effects of a wide range of pH (3-11.5) on aquatic life (Thoreau, 2002; Simate and Ndlovu, 2014). Table 5.1 clearly demonstrates that there is no definitive pH range within which all freshwater aquatic life are unharmed (Rober- Bryan, Inc., 2004).

To sum up, research has shown that the acid streams resulting from mining activities from certain types of mineral deposits are highly toxic to the aquatic environment (Cotter and Brigden, 2006). In extreme cases, many river systems and former mine sites are totally inhospitable to aquatic life, with the exception of "extremophile" bacteria (Coil et al., 2013). The extreme acidity is toxic to most aquatic life and even after neutralisation, the precipitate formed continues to affect aquatic organisms (Cotter and Brigden, 2006). Toxic elements, such as copper, cadmium and zinc that are often associated with AMD also substantially contribute to the devastating ecological effects of AMD (Cotter and Brigden, 2006). Additionally, heightened acidity

TABLE 5.1

Effects of pH on Aquatic Life

pH

Effect

3.5-3.0

Toxic to most fish; some plants and invertebrates can survive such as the waterbug, water boatmen and white mosses

4.0-3.5

Lethal to salmonids

4.5-4.0

Harmful to salmonids, tench, bream, roach, goldfish and the common carp; all stock of fish disappear because embryos fail to mature at this level

5.0-4.5

Harmful to salmonid eggs, fry and the common carp; the lake is usually considered dead and a "wet desert"; it is unable to support a variety of life

6.0-5.0

Critical pH level, when the ecology of the lake changes greatly; the number and variety of species begin to change; salmon, roach and minnow begin to become less diverse; less diversity in algae, zooplankton, aquatic insects, insect larvae; rainbow trout do not occur and molluscs become rare; there is a great decline in salmonid fishing; the fungi and bacteria that are important in organic matter decomposition are not tolerant so the organic matter degrades more slowly and valuable nutrients are trapped at the bed and are not released back into the ecosystem; most of the green algae and diatoms (siliceous phytoplankton) that are normally present disappear. The reduction in green plants allows light to penetrate further so acid lakes seem crystal clear and blue; snails and phytoplankton disappear

9.0-6.5

Harmless to most fish

9.5—9.0

Harmful to salmonids, harmful to perch if persistent

10.0-9.5

Slowly lethal to salmonids

11.0-10.5

Lethal to salmonids, carp, tench, goldfish and pike

11.5-11.0

Lethal to all fish

Sources: Thoreau, 2002; Simate and Ndlovu, 2014.

reduces the ability of streams to buffer against further chemical changes (Coil et al., 2013).

Effect of High Acidity on Human Health

The polluted AMD water has serious impact on both the ecosystem and on any humans who may come into contact with it (Garland, 2011). Whilst there is research documenting the effects of AMD on the ecosystem, less is known about the potential effects of AMD on human health (Garland, 2011). However, the acidity of AMD indirectly affects human health mainly due to the mobilisation, transport and even chemical transformation of toxic metals (Goyer et al., 1985). Through the food chain, the intake of toxic as well as essential elements may be altered in man (Oskarsson et al., 1996). For example, acidification increases bioconversion of mercury to methylmer- cury, which accumulates in fish, thus increasing the risk of toxicity to people who eat fish (Goyer et al., 1985).

Another indirect, but devastating effect of high acidity in AMD concerns the vivid orange colour which forms when AMD is neutralised because of the precipitation of iron oxides and hydroxides (Cotter and Brigden, 2006). This precipitate, often called ochre, is very fine and can deposit and imbed on the river, stream or ocean bed thus cementing substrates for small animals (Cotter and Brigden, 2006). As a result, the small animals that used to feed on the bottom of the river or stream or ocean (benthic organisms) can no longer feed and die due to starvation and thus get depleted (Cotter and Brigden, 2006). In view of the fact that such small aquatic animals are at the bottom of the aquatic food chain, their depletion has an impact higher up the food chain all the way to the fish that feed on them (Cotter and Brigden, 2006). Therefore, even if the acidity and heavy metals are neutralised, AMD still affects humans and wildlife a long way down stream because of the indirect effects (Cotter and Brigden, 2006).

 
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