Franziska Anna Fiolka

• Pesticides are widely used in modern agriculture and have been linked to insect decline and biodiversity loss. They enter freshwater systems via various pathways, such as spray drift and surface runoff. Small streams are a widely distributed type of freshwater ecosystem, which are typically characterized by an adjacent riparian zone that is subject to frequent flooding. As part of this aquatic-terrestrial transition zone, riparian zones are considered biodiversity hotspots. Yet, the possibility of flood mediated pesticide transfer into these ecosystems remains largely unexplored, particularly with respect to varying flooding regimes and the potential for trophic propagation within riparian food webs. Furthermore, climate change is projected to increase the frequency and intensity of flooding events, potentially amplifying the flood-mediated pesticide transfer. This thesis investigated the flood-mediated pesticide transfer at three experimental scales. Organic pesticides were analysed by high-performance liquid chromatography tandem to triple quadrupole mass spectrometry. In a mesocosm experiment, the detection of six pesticides in previously unflooded soil following repeated flooding events demonstrated the occurrence of the flood-mediated pesticide transfer. Further, pesticide concentrations in riparian soils increased both with repeated flooding and longer flooding durations. A field study, conducted in small to medium sized streams, confirmed the occurrence of the flood-mediated pesticide transfer into riparian soils, with subsequent uptake and accumulation of flood-borne pesticides observed in riparian plants. Finally, a climate chamber experiment investigated the uptake of 31 pesticides from floodwaters into the root-zone soil of stinging nettle (Urtica dioica) and their subsequent accumulation in stinging nettles and aphids. Across all studies, riparian soils acted as sinks for flood-borne pesticides, with accumulation increasing with both flooding frequency and duration. Riparian plants bioaccumulated these pesticides and transferred them further into higher trophic levels. In aphids feeding on contaminated stinging nettles approximately half the pesticides present in floodwater were detected, with evidence of biomagnification for selected compounds. These findings demonstrate that pesticides can infiltrate the riparian food web via flooding. With climate change projected to intensify flooding events, especially in small streams, this exposure pathway may become increasingly relevant. Moreover, the flood-mediated pesticide transfer described in this thesis is only one of many exposure pathways that reach riparian zones, with potential consequences to the riparian food-web and functionality of the riparian ecosystem.