Effects of a treated sewage effluent on behavioural traits in Diamesa cinerella and Daphnia magna

Abstract

Recently, the use of Daphnia magna has been proposed in on-line and real-time biomonitoring programmes as an early warning system for evaluating the effluent quality of sewage treatment plants (STPs). These systems are based on recording behavioural changes in the test organism resulting from the stress caused by the effluents. Indeed, altered behavioural signals could be induced at sublethal concentrations that are significantly lower than the corresponding EC50. However, at present, it is unknown whether the sensitivity of D. magna can be representative of that of other aquatic organisms, particularly benthic macroinvertebrates. An experiment was designed to verify whether D. magna can be employed in biomonitoring programmes for STPs located in alpine areas as a surrogate for cold freshwater best-adapted species. The responses of survival and behaviour alteration to exposure to the effluent of the Tonale Pass plant (Trentino, Italian Alps, 46°N, 10°E; 1799 m a.s.l.) were compared in a laboratory population of D. magna and a wild population of the chironomid Diamesa cinerella. These larvae were collected from the Vermigliana stream 50 metres upstream of the effluent input. Both organisms were exposed for 24 and 48 hrs to the effluent as it is and to three dilutions (/10, /100, /1000). The mortality rate and behavioural responses (using video tracking systems) were recorded. No significant mortality or change in behaviour was observed in the two species when exposed to the undiluted effluent. Exposure to serial dilutions of the treated effluent did not affect the survival of either species but notably altered their behaviour at both exposure times (e.g., the time spent in activity in D. magna and the average speed of movement and the cumulative distance travelled in both), especially when exposed to the ten-times-diluted effluent. Overall, the findings of this study emphasize that even though D. magna and D. cinerella use different behavioural strategies to cope with adverse environmental conditions, their overall sensitivity to treated effluents is similar. Accordingly, the use of D. magna in biological early warning systems protocols seems to also be sufficiently protective for local, cold-adapted species of alpine freshwater ecosystems.