Jakob Wolfram

• Agricultural intensification has increased substantially in the last century to meet the globally growing demand for food, fodder, and bioenergy, thus agricultural cropland became the largest terrestrial biome globally. Pesticides became a central tool to this intensification strategy, thus pesticide application rose drastically over the last sixty years to secure or increase crop yields. However, pesticides are by design biologically active and known to contaminate non-target ecosystems, thereby adversely affecting their function or structure. Even though ecotoxicological knowledge about probable fate and effects has grown, little remains known about the spatiotemporal occurrence, potential effects, and risk drivers of pesticides on larger, i.e. macro, scales. Consequently, the thesis gathered primarily pesticide exposure data via meta-analysis and from public monitoring databases to describe (i) detailed risks in aquatic ecosystems, (ii) the underlying risk drivers, (iii) associated spatiotemporal trends, (iv) the effect of land use and land-protection and (v) the protectiveness of regulatory frameworks. First, a meta-analysis of insecticides occurring in US surface waters (n = 5,817, 259 studies) revealed large-scale risks for aquatic ecosystems based on the exceedance of regulatory threshold levels (RTL) and identified high-risk substances, particularly pyrethroids, with increasing application trends (publication I). Following this, spatiotemporal factors driving insecticide risks were identified via model-building demonstrating that toxicity-weighted pesticide use was the primary driver in surface waters with subsequent model application generating a spatially comprehensive risk assessment for the United States (publication II). The toxicity-weighted pesticide use was subsequently expanded to an ongoing project covering additional species groups and all pesticides used in the US from 1992 – 2016, highlighting a drastic shift of toxic pressures from vertebrates to aquatic invertebrates. Large-scale monitoring data from European surface waters (n > 8.3 million) of 352 organic chemicals identified pesticides as the main class or organic contaminants causing risks in aquatic ecosystems. Additional analyses established links between agricultural intensity and resulting environmental risks for aquatic invertebrates and plants on this macro scale (publication III). Finally, high-resolution monitoring data from Saxony, Germany, provided, for the first time, detailed insights into the occurrence and resulting risks of organic contaminants (primarily pesticides) in protected surface waters of nature conservation areas (publication IV). In summary, the thesis gathered and used large-scale datasets to analyze the impact of agricultural intensification – and later anthropogenic land use – on ecosystems to reduce knowledge deficits in ecotoxicology on macro scales. Insecticides were shown to be important and spatially extensive agents of impairments to surface water quality and being directly linked to their use in respective landscapes. Changes in the pesticide use composition over time shifted environmental risks from vertebrates to other central species groups (e.g. aquatic invertebrates), highlighting a new challenge to the integrity of aquatic environments. The thesis provided novel insights into contaminants' individual risk characteristics, their interaction with various spatiotemporal drivers and their relevance on various macro scales. Overall, a discrepancy remains evident between estimated environmental impacts of pesticides derived during regulatory approval processes contrasted by a posteriori field measurements detailing larger than assumed adverse exposures and effects. This discrepancy led to pesticides being the most impactful chemical stressor for aquatic ecosystems compared to other organic contaminants on a continental scale; a threat that even increased for some species groups. The extensive use of pesticides has reached levels where even strictly protected surface waters in Germany are regularly exposed adversely, hence threatening conservation areas’ function as ecological refugia. Taken together, the thesis provides new macro-scale evidence regarding the contribution of pesticides (and associated drivers) to large-scale changes in biological systems evidenced over the last decades, underlining their likely contribution to the ongoing freshwater biodiversity crisis globally. Particularly agricultural systems will require substantial changes going forward to protect or reestablish the integrity of aquatic ecosystems and their provision of vital ecological services.