Effects of a Pesticide Mixture on Plankton in Freshwater Mesocosms from single substance studies to combination impacts

In the presented thesis direct and indirect reactions of plankton organisms to an insecticide and a herbicide applied jointly were investigated. The insecticide used was a-Cypermethrin (CYP), a pyrethroid, and the herbicide Isoproturon (IPU), a phenylurea. Both were chosen because they act independently from each other on a molecular basis. Addressing the toxic effects to either pesticides was enabled by this approach. The investigation consisted of three parts: Both pesticides were applied separately in a first mesocosm experiment, and combinations of them were used in another one. Monitoring of effects in all outdoor experiments was continued for two vegetation periods. Additionally, single species tests and biomonitoring experiments were performed with zooplankton organisms. The key objectives were: examining the differences in the impact of the three pesticide treatment regimes, ascertaining how direct and secondary treatment effects of the three treatment scenarios affected the test systems ecosystematically, and finding possibilities to deduce (qualitatively) or predict (quantitatively) combination effects of the toxicant mixture. Results of the insecticide study indicated that the sensitivity towards the active ingredient rose with the position in the food chain. The most sensitive organism was Chaoborus crystallinus with a NOEC of <0.015 µg/L CYP (i.e. determining the overall NOEC for this part of the thesis). This was the top predator in the system. - Recovery could be demonstrated for all endpoints. Functional parameters were not influenced. Secondary effects could therefore easily be deduced by tracking effects from top-down. In the herbicide study, functional parameters were affected. A distinct DO-pH-alkalinity-conductivity syndrome was observed. The system reaction had to be divided into two groups depending on whether macrophytes were influenced or not. Secondary effects were triggered from bottom-up. Oscillating patterns that were related to the grazing pressure and the amount of structural change by the effect on the macrophytes in the system were observed. Secondary effects totally outweighed the direct toxic ones in the plankton. The overall NOEC was 4 µg/L IPU. Recovery was often linked to the re-growth of the macrophytes. In the combination, three principles for the system reaction to the pesticides used could be derived: 1. Sensitive taxa to either active ingredient showed no difference in their reaction to the independently acting pesticides unless they were integrated in very tight food web interactions. 2. The reaction of moderately susceptible taxa was altered via food web interactions. Top-down control was more important but effects only came into being if they were facilitated by a bottom-up process. Insignificant effects with a single active ingredient were sufficient for boosts in the effects of the combination treatment. Species richness in the zooplankton was affected more severely in the joint treatment. 3. Taxa that were not influenced by either pesticide showed reactions in the combination study. These were of secondary nature. They were more intensely controlled from top-down. Depending on the DT50 of the pesticides, direct toxic effects were more important shortly after the treatment, later one secondary interaction took over. Recovery was linked to the development in macrophytes, again, for several effects. BLISS independence was able to predict direct and simple secondary effects in taxa data and water quality parameters. This is a promising approach for risk assessment procedures.