Effect of Dissolved Oxygen Concentrations On the Toxicity of Several Poisons to Rainbow Trout (Salmo Gairdnerii Richardson)

The low dissolved oxygen concentrations which are characteristic of many polluted rivers have been shown by several authors to increase the toxicity of poisons to fish. Since most toxicity tests are made in well aerated water, it is important to know what factor to apply to the results when predicting the effect of a reduced dissolved oxygen concentration on toxicity. Evidence presented in this paper suggests that there may be a common relation between the dissolved oxygen concentration and the toxicity of poisons, and this is supported by a theoretical consideration of the problem.

Details have been published of determinations of the effect of various levels of dissolved oxygen concentration on the toxicity of a mixture of monohydric phenols (Department of Scientific and Industrial Research, 1958), zinc sulphate (Lloyd, 1960), lead nitrate and copper sulphate (Department of Scientific and Industrial Research, 1960).

The effect of low dissolved oxygen concentrations on the toxicity of ammonium chloride to rainbow trout was determined in fixed volumes of solution ; pH values and oxygen concentrations were controlled by aeration with known mixtures of air, nitrogen, and carbon dioxide. The trout were fed before they were acclimatized for 18 hr. to the temperature (17·5° C.) and carbon dioxide concentration used in the tests. Other details of procedure were similar to those described by Lloyd & Herbert (1960).

Measurements of the oxygen consumption of rainbow trout at 17·5° C. were made in a simple continuous flow respirometer, similar in essentials to that illustrated by Fry (1957) in his figure 10B. Values for oxygen consumption were first obtained in water saturated with air and the same fish were immediately used again to obtain oxygen-consumption values at a lower oxygen level. The rainbow trout used weighed between 1 and 11 g. and were acclimatized to the temperature and free carbon dioxide concentration (about 8 0 mg./l.) before the test.

When the log. survival times of rainbow trout are plotted against the corresponding log. concentrations of these poisons in well aerated water, a curvilinear relation is obtained, and at those concentrations of the poisons in which periods of survival are long the line is nearly vertical, so that a further slight decrease in concentration is associated with a prolonged period of survival. It is these slightly toxic concentrations of poisons which are important for predicting safe concentrations in a river. If the dissolved oxygen concentration of the water is reduced, the survival time/concentration curve is displaced towards lower concentrations of poison, and a value for this increase in toxicity can be obtained by comparing concentrations of poison which are equitoxic at prolonged periods of survival. This can be expressed as the factor X_S/X, where X_S is the concentration of poison at 100 % of the air-saturation value of oxygen, C_S, and X is the equitoxic concentration at a lower value of dissolved oxygen, C. Values of X_S/X at different levels of dissolved oxygen concentration were derived from the experimental data for monohydric phenols, and zinc, lead, and copper salts for median periods of survival between 1000 and 2000 min. Values of X_S/X for these four poisons are shown in Fig. 1, where it appears that the relation between increase in toxicity and dissolved oxygen concentration is similar for these poisons.

Batches of ten rainbow trout were exposed to various concentrations of ammonium chloride at two levels of free carbon dioxide (3·4 and 19·8 mg./l.) and three levels of dissolved oxygen (37·5, 66·0 and 100% of the air-saturation value); concentrations of ammonia corresponding to 500-minute median periods of survival (at which time the survival time/concentration curve has become practically vertical) were calculated by probit analysis for each series. The experimental values of X_S/X for ammonia (Fig. 2) are higher than those for the previous four poisons, and are affected by the free carbon dioxide concentration of the water; these differences can be explained by the following hypothesis.

Thus, theoretical values of X_S/X for ammonia can be calculated on the assumption that the relation between dissolved oxygen concentration and the toxicity of this poison is essentially similar to the relation for the other four poisons (Fig. 1), but that in water of lowered oxygen content the toxicity is further increased, because of the reduction in the concentration of excreted carbon dioxide at the gill surface. An estimate of this additional increase in toxicity can be derived from the theory given by Lloyd & Herbert (1960). Theoretical curves for the factor X_S/X for ammonia under the conditions of the experiments described here are shown in Fig. 2 where they are in good agreement with the experimental points. From data in a paper by Merkens & Downing (1957) on the effect of a reduction of the dissolved oxygen concentration to 47% of the air-saturation value on the toxicity of ammonia, it can be calculated that the experimental factor for X_S/X for a 500 min. median period of survival was 3·64; the factor expected for the experimental conditions, in which the concentration of free carbon dioxide was between 0·75 and 1·0 mg./l., is between 3·35 and 4·17. The good agreement between predicted and experimental results strengthens the view that the effect of low dissolved oxygen concentrations on the toxicity of this poison is basically similar to that for the other four poisons, but that its toxicity is increased still further by the rise in pH value of the water at the gill surface.

Although the toxic actions of heavy metals, ammonia, and monohydric phenols are probably dissimilar, the common effect on their toxicity resulting from a reduction in the concentration of dissolved oxygen suggests that this is a result of a physiological reaction by the fish to such a change of the environment, and is independent of the nature of the poison. The most obvious reaction of fish to a lowered oxygen content of the water is to increase the volume of water passed over the gills, and this may increase the amount of poison reaching the surface of the gill epithelium, the site at which most poisons are absorbed. Weiss & Botts (1957) have shown that an increase in the oxygen uptake of several species of fish results in a decrease of their survival times in toxic solutions ; they found, however, that a reduction in the dissolved oxygen concentration of the water reduced the oxygen uptake of the fish, yet increased the toxicity of the solution, and thought that this reduction in uptake was insufficient to compensate for the reduced oxygen content of the solution and that it was the increased rate of respiratory flow through the gills which led to an increased toxicity of the poison. However, the design of their experiments does not allow the results to be compared in detail with those from the experiments described here. Therefore, although there is some evidence that an increase in respiratory flow increases the toxicity of poisons, there is no evidence to show that this accounts for the whole of the increase in the toxicity of poisons in water of low dissolved oxygen concentration. The following hypothesis is suggested to explain the relation between respiratory flow and the toxicity of poisons.

However, although the agreement between these theoretical points and the experimental curve in Fig. 3 is reasonable, the values of $(V/VS)$ are all slightly lower than would be expected from the experimental curve; this discrepancy may be the result of using the values of P given by Van Dam who obtained them from a rainbow trout which was held in a clamp and which was respiring at a rate close to the standard metabolic rate, whereas the rainbow trout used in the present experiments were free-swimming and presumably respiring at a greater rate. Theoretical values of P required to bring the values for $(V/VS)$ on to the curve for the factor X_S/X are shown in Fig. 4, where they are compared with Van Dam’s data. Since it is reasonable to suppose that the values of P depend upon the rate of respiratory flow, it follows that the curve relating P to C for free-swimming fish should be displaced towards higher values of C when compared with the curve obtained for clamped fish. However, at the asphyxiai level of dissolved oxygen for rainbow trout (about 20% of the air-saturation value) the oxygen uptake of both free-swimming and clamped rainbow trout will be the same (Shepard, 1955), and the curve drawn through the theoretical points in Fig. 4 has been fitted on the assumption that both curves should coincide at that level. If it is accepted that the theoretical values of P shown in Fig. 4 are more likely to apply to free-swimming fish than the curve given by Van Dam, then the slight discrepancy between the calculated factors for $(V/VS)$ and the curve for the factor X_S/X in Fig. 3 is explained. The theoretical values of P were used in equation (1) to calculate the theoretical increase in the toxicity of un-ionized ammonia with a reduced dissolved oxygen concentration of the water ; the reasonable fit of these curves to the experimental points in Fig. 2 lends some support to the assumption that these values of P are valid for free-swimming rainbow trout.

An estimate of the increase in the concentration of poison at the gill surface resulting from a reduction in the dissolved oxygen concentration of the water can be obtained by another approach. It may be assumed that the relation which governs the rate at which toxic molecules diffuse from the bulk of the solution to the surface of the gill epithelium also governs the diffusion of oxygen molecules. Thus, as the rate of respiratory flow is increased, so the rate at which oxygen molecules reach the epithelial surface will also increase if the dissolved oxygen concentration in the bulk of the solution remains constant. If it is assumed that the oxygen uptake of the fish is proportional to the dissolved oxygen concentration at the surface of the gill epithelium, then if the oxygen uptake of the fish was reduced from Q_S to Q, the rate at which oxygen molecules, and also toxic substances, reached that surface would also be decreased by the factor Q/Q_S. However, if the decrease in oxygen uptake was accom-panied by a decrease in the dissolved oxygen concentration of the water from C_S to C, the rate at which other molecules or ions reached the epithelial surface would not be altered by the factor Q/Q_S but by the factor C_SQ/CQ_S. Values for the factor C₈QICQ₈, calculated from data given in Table i, are shown in Fig. 5 where they are compared with the curve fitted to the experimental points for X_S/X. Although these theoretical points follow a curve similar to that given by the experimental factors for X_S/X, the values are somewhat higher, and it may well be that the assumption that the oxygen uptake of the fish is proportional to the oxygen concentration at the gill epithelium is not accurate. It has been suggested that at low dissolved oxygen concentrations in the water the haemoglobin content of the blood is increased (Shepard, 1955) and the rate at which blood is pumped through the gills may also be raised, both of which would increase the rate of removal of oxygen from the gill epithelium. This may account for the higher values obtained for the increase in the rate at which toxic substances reach the gill epithelium when calculated by this method.

Neither of the theoretical methods used here to calculate the relation between the increase in toxicity of poisons and the reduction in the dissolved oxygen concentration entirely agree with the experimental data, and although the discrepancies can be reasonably explained in qualitative terms, there are no data available whereby the extent of these differences can be quantitatively accounted for. Furthermore, there would be some difficulty in obtaining the required data on the percentage removal of oxygen from the respiratory water, and on the rate at which oxygen is removed by the blood from the gill epithelium, since the measurements would have to be obtained from free-swimming rainbow trout. Nevertheless, the close approximation of the points given by the two theoretical methods to the practical values obtained for X_S/X suggests that the majority, if not all, of the increase in toxicity of poisons in water of low dissolved oxygen concentration is caused by the increase in the rate of respiratory flow.

This is of fundamental importance in fish toxicology, since it implies that any environmental or physiological change which affects the rate of respiratory flow of a fish will also affect the concentration of poison at the surface of the gill epithelium, and that a known relation exists between these two factors. It also implies that the relation between the increase in toxicity of poisons to fish and a reduced dissolved oxygen concentration of the water will be the same for all poisons except those whose toxicities are affected by the pH value of the water. Thus, the curve obtained for the factor X_S/X in Fig. 1 should apply to the effect of dissolved oxygen concentration on the toxicity of most poisons to rainbow trout.

The author wishes to acknowledge the assistance given by H. T. Mann and A. C. Wakeford with the experimental work. This paper is published by permission of the Department of Scientific and Industrial Research.