A model ecosystem experiment to study the recovery capability of a macrozoobenthos community after a pesticide stress

The recovery capability is a fundamental component of the vulnerability of populations and communities, particularly relevant in case of non-continuous stress factors, such as pesticide pollution. The present work was performed using an experimental system composed of 5 artificial streams (flumes), fed by unpolluted river water, which reproduced a mountain river ecosystem under controlled conditions. The environmental conditions are comparable to those of rivers in the same geographic area, subject to pesticide pollution from intensive agricultural basins, mainly apple orchards and vineyards. Three flumes were exposed for 24 hours to chlorpyrifos (nominal concentration: 1.5 mg/L), other two flumes were the untreated controls. After exposure, at the inlet of two of the treated streams and of one control stream, a metal-net filter system was posed in order to avoid recolonisation by drift. In the other two flumes drift was allowed. So, the specific role of drift and direct colonisation in the recovery capability may be assessed. To avoid obstruction, the filter systems were cleaned daily. The experiment started early July 2013 and the community was sampled for five weeks after stopping exposure. The community structure was checked by periodical sampling using a Hess sampler. Drift at the outlet of the flumes was also sampled regularly. Samples were taken in all flumes before exposure, immediately after exposure stop, 24 hours after exposure and then every 4 days up to the end of the experiment. Selected taxa were also sampled, for the measurement of a set of biochemical parameters (e.g. acethilcholinesterase, glutathione-S-transferase, alkaline phosphatase, and catalase) before exposure, immediately after and 24 hours after exposure. During exposure, water samples were taken for the analytical control of concentrations. The exposure to the pesticide caused a significant increase in the drift in most of the taxa present in the flumes. This produced a dramatic decrease in abundance and diversity of the benthic community at the end of the exposure period. Three weeks after exposure, the new colonisation was almost complete, and the benthic community was restored in all the flumes. Preliminary results suggest a low resistance but a good resilience of the macro-zoobenthos community