Abiotic Factors That Affect Toxicity

A number of abiotic factors affect the toxicity of substances. Notable examples in aquatic tests include the pH, temperature, dissolved oxygen content, salinity, alkalinity, and water hardness. The fact that all of these can have a simultaneous effect greatly complicates the application of toxicity data to the field. In most of the following cases, the research has been done on acute toxicity (i.e., those with rapid onset). Much less is known about the effect of these factors on chronic toxicity (i.e., those with delayed onset).

Temperature affects organisms directly as well as by changing the response to toxins. Thermal pollution of surface waters, often associated with cooling-water discharge from electric power plants, can affect fish behavior. By reducing the solubility of oxygen and increasing microbial oxygen uptake rate, it can apply a stress to many aquatic organisms. However, one cannot make generalizations about the effect of temperature on toxicity. The threshold toxicity of zinc to Atlantic salmon was higher at 19°C than at 3°C, but rainbow trout, which are in the same genus, were more tolerant to zinc at 3°C than at 20°C. The undissociated fraction of hydrogen cyanide shows one of the strongest temperature effects. Its LC50 (the concentration that is lethal to 50% of test organisms) increases with temperature in a log-linear relationship. The LC50 increases (i.e., it becomes less toxic) 12-fold, from 44 mg/L at 6.5°C to 530 mg/L at 25°C.

The effect of temperature can depend on other abiotic factors, thus forming a three-way interaction between the toxin dosage, temperature, and the other factor. In soft water, napthalenic acids are twice as toxic to snails at 20°C than at 30°C. However, no differences have been reported in hard water. One rule of thumb that can often be used is that the time it takes for an organism exposed to a lethal concentration to succumb follows a van't Hoff relationship with temperature: survival time decreases by a factor of 2 or 3 with each 10°C increase. For this reason, shorter-term acute toxicity tests could show an increasing toxicity with temperature, while the same test conducted for a longer time could show decreasing toxicity.

Dissolved oxygen (DO) in water is saturated with respect to the atmosphere at concentrations ranging from 7.5 to 14.6mg/L, depending on temperature, salinity, and total pressure (and therefore, altitude). Most fish will tolerate DO levels as low as 2.0mg/L, although their activities may be reduced. DO can affect aquatic toxicity in several ways: (1) by increasing exposure at a given activity level, since at low DO levels fish must circulate more water through their gills; (2) indirectly by increasing organism activity; and (3) by causing stress to organisms at low DO. Very commonly, there is no discernible effect of DO, or the effects observed are due to the oxygen stress and not the toxin. One case of strong interaction has been found in the effect of pulp mill effluent on young salmon. Only 2% of effluent mixed with half-strength seawater caused the fish to gasp for oxygen at the surface when the DO was reduced to 36% of saturation.

The pH range 6.5 to 9.0 is considered harmless to fish. The pH interacts strongly with the toxicity of weak acids and bases, such as cyanide, hydrogen sulfide, or ammonia, by shifting the balance between ionized and nonionized forms (Sections 3.3 and 18.1). The LC50 of total sulfide is 64 mg/L at pH 6.5 and 800 mg/L at pH 8.7. Thus, the undissociated form seems to be about 15 times as toxic as the ions. Metals are usually more toxic in the ionic form. Although phenol is an acid, its toxicity does not change much with pH. Alkalinity may have some effect. High-alkalinity waters are more buffered against changes in pH. The CO2 released by gills can cause a local decrease in pH, affecting the toxicity of weak acids and bases. High-alkalinity waters are more buffered against changes in pH, and therefore have less of an effect from CO2. Thus, at the same pH, the toxicity of ammonia to fish would be expected to be less at lower alkalinities.

Water hardness is due primarily to dissolved calcium and magnesium. A soft water is one with less than about 75 mg/L expressed in terms of CaCO3, and above 300 mg/L as

CaCO3, it is considered very hard. Surface waters tend to be soft; the world's rivers have a median hardness of 50 mg CaCO3/L. Groundwater from limestone regions range from 350 to 380 mg CaCO3/L.

Metal salts are less toxic in hard waters than in soft. For example, a very good correlation was found between the LC50 of copper in mg/L to salmonid fish vs. hardness (H) in mg CaCO3/L:

Based on such simple correlations, it was thought that membrane permeability was being affected by calcium. However, if the tests were done in water with combinations of pH and alkalinity different from those typically found in nature, the simple relationship disappears. It seems that the relationship was observed because in natural waters the hardness and alkalinity vary together. Now it is thought that it is the alkalinity that interacts with pH, and that this is due to the various complex ions formed by the equilibrium among the metal ion, hydroxide ions, and carbonate ions. For example, copper forms at least seven species in solution, each of which may have its own toxicity. Figure 17.2 shows the effect of hardness and pH on the toxicity of copper to rainbow trout. The curve represents a mathematically smoothed response surface. The low toxicity at pH 8 is thought to be due to most of the copper being in the form of carbonates and nonionized hydroxides, which may be less toxic than the other forms.

Salinity is not an important variable in toxicity.

Temperature and humidity strongly affects the sensitivity of plants to toxins in the air. An increase in either stimulates plants to open the leaf stomata, giving gaseous toxins admittance to the interior of the leaves. Light intensity also affects plant response, but the effect may be positive in some situations and negative in others.

Even time of day has been shown to have an effect on toxicity to animals. In rats and mice the cytochrome P450 enzyme system, which metabolizes many toxins, is most active just after dark. Factors relating to the care and handling of test animals, such as whether they are caged individually or in groups, also affect the measured toxicity.


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