Katharina Ohler

• Streams and their adjacent terrestrial ecosystem are tightly linked via the flux of organisms and matter. Emergent aquatic insects can be an important food source for riparian predators like bats, birds, spiders, and lizards. Information about the quality, quantity and phenology of emergent aquatic insects is necessary to estimate how riparian predators can benefit from them as food source. Though intensive agriculture is a globally dominant land use, little is known about how agricultural land use affects the quantity, quality as well as phenology of emergent aquatic insects. Typically, emergent aquatic insects contain more long-chain polyunsaturated fatty acids (PUFA) than terrestrial insects. Especially long-chain PUFA, were shown to enhance growth and immune response of spiders and birds. In chapter 2, the PUFA transfer to spiders and the effect of food sources differing in their PUFA profiles on spiders was examined in outdoor microcosms under environmentally realistic conditions (i.e., normal weather conditions, possibility to construct orb webs as in their natural habitat). The environmental context determined how PUFA can affect the spiders. For instance, besides PUFA profiles of food sources, environmental variables like the temperature were important for the growth and body condition of spiders. In the third chapter, the effect of agricultural land use on the quantity in terms of biomass as well as abundance, phenology and composition of emergent aquatic insects was assessed. Previous studies were limited to seasons or single time points, which hampered determining annual biomass export and shifts in phenology. Therefore, emergent aquatic insects were sampled continuously over the primary emergence period of one year and environmental variables associated with agricultural land use were monitored. The biomass and abundance in total were higher (61 – 68 and 79 – 86%, respectively) in agricultural than forested sites. In addition to that, a turn-over of emergent aquatic insect assemblages and a shift in phenology of aquatic insects was identified. In agricultural sites, 71% families of aquatic insects emerged earlier than in forested sites. Pesticide toxicity was associated with different aquatic insect order biomass and abundances. During the same experiment spiders were sampled in spring, summer, and autumn. Additionally, the fatty acid (FA) content of the spiders and emergent aquatic insects was determined. These results are presented in chapter 4. The FA export via emergent aquatic insects was higher (26 – 29%) in forested than agricultural sites, which indicated a reduced quality of aquatic insects as food source for riparian predators in agricultural sites. The FA profiles of mayflies, flies and caddisflies differed between land-use types, but not for spiders. Shading and pool habitats were the most important environmental variables for the FA profiles, though environmental variables explained only little variation in FA profiles. Overall, the quantity, quality and phenology of emergent aquatic insects differed between land-use types, which can affect population dynamics in the adjacent terrestrial ecosystem. Our results can be used in modeling food-web dynamics or meta-ecosystems to improve understanding of linked ecosystems.