Wirkung kurzer Belastungsspitzen auf die Kleinlebewesen der Flusssohle
The concentration of pollutants from the sewer system at the beginning of a rain event can be high enough to cause a fish kill in the receiving river. It is known from toxicological tests that for fish, un-ionized ammonia (NH3) perhaps combined with low oxygen concentration, can be the main lethal factor. But only very little is known about the effect of short-term pollutant pulses on the benthic macroinvertebrates.
With short-term experiments in laboratory channels the effects of high NH3- and low O2-concentration (separately and in combination) on larvae of four different insect species were tested.
For each experiment the channels were stocked with natural substrates from a river and 50 to 200 larvae of Baetis fuscatus, Ecdyonurus dispar, Ephemerella ignita (Ephemeroptera) and Leuctra sp. (Plecoptera). Each experiment lasted five hours. Peak concentrations of NH3 and O2 were reached within one hour and kept constant for one hour. Minimum concentrations of O2 were 2.67, 1.63 and 1.19 mg/l. Maximum concentrations of NH3 were 13.12, 18.55 and 28.03 mg/l where the O2-concentration was unaltered and 14.61, 16.40 and 30.37 mg/l where oxygen was reduced to 2.80-2.85 mg/l.
In all experiments the behaviour of the animals was a far more sensitive parameter than drift and mortality, since it was affected long before some effect in drift or mortality became evident. It is proposed that NH3 damages the cerebral system of the insect larvae as it is known for fish. Oxygen concentrations below 2 mg/l increased drift, mainly in those species where drift is a normal behaviour (i. e. B. fuscatus and E. ignita). Mortality was not significantly altered. The effect of un-ionized ammonia was exactly opposite: Mortality increased strongly with increasing NH3-concentration whereas drift hardly changed. Low O2-concentration intensified the effects of NH3, which can be explained by the fact that the cellular defensive reactions against NH3 are very oxygen consuming and therefore less effective under limited oxygen supply.
Some reactions to high NH3 were identical to those to low oxygen (e. g. E. dispar ventilates its gills). This indicates that un-ionized ammonia affects the respiration system of the animals, either by inhibiting the cellular metabolism or by decreasing the O2-permeability of the cell membranes.
Compared to fish it can be concluded that the investigated macroinvertebrates are much less sensitive to high ammonia or low oxygen concentration peaks. Hence, if one succeeds with water pollution control measures so as to prevent the killing of fish it is likely that also the demands of the benthic macroinvertebratcs upon their habitat will be satisfied.
While planning wet weather pollution control measures for the future, we should take into account the requirements of the affected ecosystem. Concentration calculations of different compounds with the aid of models can help to identify problems and to focus on useful measures. An example is included for the assessment of the NH3 problem in a receiving water due to overflow events. Chemical and biological investigations in the receiving river are necessary to calibrate and verify the models. The benthic organisms react as «integrators» for pollution effects and therefore can give us additional information. Before general instructions for the assessmenl of the wet weather situation by means of the affected biocoenosis can be given, a lot more work on this topic has to be done.