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Pesticides applied in agriculture can affect the structure and function of non-target populations at lower doses and for longer timespans than predicted by the current risk assessment frameworks. We identified a mechanism for this observation. The populations of an aquatic invertebrate (Culex pipiens) exposed over several generations to repeated pulses of low concentrations of the neonicotinoid insecticide (thiacloprid) continuously declined and did not recover in the presence of a less sensitive competing species (Daphnia magna). By contrast, in the absence of a competitor, insecticide effects on the more sensitive species were only observed at concentrations one order of magnitude higher, and the species recovered more rapidly after a contamination event. The underlying processes are experimentally identified and reconstructed using a simulation model. We conclude that repeated toxicant pulse of populations that are challenged with interspecific competition may result in a multigenerational culmination of low-dose effects.

Demethylase inhibitors (DMIs) are broad-spectrum fungicides that are ubiquitously used in agriculture and medicine. They comprise chemically heterogeneous substances that share a common biochemical target in fungi, the inhibition of a specific step in sterol biosynthesis. Several DMIs are suspected to disrupt endocrine-mediated processes in a range of organisms and to inhibit ecdysteroid biosynthesis in arthropods. It is unclear, however, whether and, if so, to what extent different DMI fungicides have a similar mode of action in nontarget organisms, which in turn would lead to a common chronic toxicity profile. Therefore, we selected a representative of each of the major DMI classes-the piperazine triforine, the pyrimidine fenarimol, the pyridine pyrifenox, the imidazole prochloraz, and the triazole triadimefon-and comparatively investigated their chronic toxicity to Daphnia magna. No toxicity was detectable up to the limit of solubility of triforine (61 mu mol/L). All other DMIs reduced reproductive success by delaying molting and development and by causing severe developmental abnormalities among offspring. Prochloraz was most toxic (median effective concentration [EC50] for fecundity reduction, 0.76 mu mol/L), followed by fenarimol (EC50, 1.14 mu mol/L), pyrifenox (EC50, 3.15 mu mol/L), and triadimefon (EC50, 5.13 mu mol/L). Mean effect concentrations for fecundity reduction were related to lipophilicity and followed baseline toxicity. However, triadimefon and fenarimol (but none of the other tested DMIs) caused severe eye malformations among exposed offspring. Affected neonates did survive, but a reduced ecological fitness can be assumed. Offspring exposed to fenarimol in mater matured earlier. The investigated different life-history parameters were affected in a substance-specific manner. These qualitatively different toxicity profiles suggest additional, substance-specific mechanisms of action in D. magna that probably are related to an antiecdysteroid action

The aim of this thesis is to evaluate the chronic effects of endocrine disrupting chemicals (EDCs) to the crustacean Daphnia magna under complex exposure situations. It comprises four experimental studies, dealing with the toxicity of single substances as well as mixtures.It is evaluated whether additional life-history parameters, such as molting and embryo abnormalities, may indicate interferences of selected chemicals with ecdysteroid-related processes in the reproduction test with Daphnia magna improving an assessment of endocrine disrupters. A focus was set on structurally diverse demethylase inhibiting fungicides (DMIs), which were comparatively assessed with respect to their single substance toxicity as well as their joint toxicity to Daphnia magna. Here, the two concepts Concentration Addition (CA) and Independent Action (IA) were used to predict mixture toxicity based on single substance data across different endpoints and effect levels. The last experiment focused on the toxicity of binary combinations under conditions of sequential or simultaneous exposure and the predictive abilities of the two mixture toxicity concepts.

The aim of this thesis is to evaluate the chronic effects of endocrine disrupting chemicals (EDCs) to the crustacean Daphnia magna under complex exposure situations. It comprises four experimental studies, dealing with the toxicity of single substances as well as mixtures.It is evaluated whether additional life-history parameters, such as molting and embryo abnormalities, may indicate interferences of selected chemicals with ecdysteroid-related processes in the reproduction test with Daphnia magna improving an assessment of endocrine disrupters. A focus was set on structurally diverse demethylase inhibiting fungicides (DMIs), which were comparatively assessed with respect to their single substance toxicity as well as their joint toxicity to Daphnia magna. Here, the two concepts Concentration Addition (CA) and Independent Action (IA) were used to predict mixture toxicity based on single substance data across different endpoints and effect levels. The last experiment focused on the toxicity of binary combinations under conditions of sequential or simultaneous exposure and the predictive abilities of the two mixture toxicity concepts.

The aim of this thesis is to evaluate the chronic effects of endocrine disrupting chemicals (EDCs) to the crustacean Daphnia magna under complex exposure situations. It comprises four experimental studies, dealing with the toxicity of single substances as well as mixtures.It is evaluated whether additional life-history parameters, such as molting and embryo abnormalities, may indicate interferences of selected chemicals with ecdysteroid-related processes in the reproduction test with Daphnia magna improving an assessment of endocrine disrupters. A focus was set on structurally diverse demethylase inhibiting fungicides (DMIs), which were comparatively assessed with respect to their single substance toxicity as well as their joint toxicity to Daphnia magna. Here, the two concepts Concentration Addition (CA) and Independent Action (IA) were used to predict mixture toxicity based on single substance data across different endpoints and effect levels. The last experiment focused on the toxicity of binary combinations under conditions of sequential or simultaneous exposure and the predictive abilities of the two mixture toxicity concepts.