Tests concerned with direct absorption from water

Much of the toxicity testing carried out with aquatic organisms (eg fish, Daphnia, Gammarus pulex) is concerned with direct absorption of chemicals from water. The chemicals may be in solution, in suspension or both. Organisms are exposed to different concentrations of the chemical in water to determine the values for median lethal concentration. Although absorption is primarily direct from water, it is not possible to completely exclude contamination of food and thus some uptake from this source.

One difficulty in aquatic toxicity testing is maintaining a constant concentration of chemical in water during the test. Chemicals are lost in water because of (i) absorption and metabolism by the test organism and (ii) volatilisation, degradation and adsorption from water. (For the purposes of the practical class, these processes are all unlikely to occur for the pollutants under consideration). Where the rate of loss is relatively low, tests may be performed using static or semistatic systems. With static systems, the water is not changed for the duration of the test. With semistatic systems, the water is replaced at regular intervals (usually every 24 hrs). A better but more complex and expensive method for renewing test solutions is provided by continuous flow (also known as a flow through system). With test systems of this type, the test solution is continually renewed, thus ensuring a constant concentration of the test chemical and preventing the build-up of contamination from faeces, algae, mucus etc. If organisms are exposed to a chemical for a sufficiently long time, steady state concentrations will be reached in the tissues.

The toxic effect of a chemical depends on the concentration in the tissues where the site of action is located. This in turn depends on of the chemical in water and the period of time over which exposure has occurred. Thus the median lethal concentration (LC₅₀) is related to the exposure period. With increasing exposure time, the LC₅₀ becomes less until the median lethal threshold concentration (threshold LC₅₀) is reached. At this point further increases in exposure period cause no change in mortality. It may reasonably be supposed that when the threshold LC₅₀ is reached, the system is in a steady state, i.e. the tissue concentration is no longer increasing with time.

In performing aquatic toxicity tests, preliminary ranging tests are necessary to obtain a rough estimate of toxicity. Here, small groups of test organisms (typically two or three individuals per group) are exposed to a wide range of concentrations of the test chemical on a log scale. The results from the ranging test can be used to plan a full toxicity test in which larger numbers of test animals are exposed to a narrower range of concentrations, centering on the LC₅₀ estimated from the ranging test. The percentage mortality in each of the test groups is recorded over various time intervals for the duration of the test. The percentage mortality in each of the test groups is recorded over various time intervals for the duration of the test. The data obtained can be used to determine the LC₅₀ at different exposure times.