Sebastian Pietz

• Neighboring ecosystems are connected by fluxes of organic and inorganic matter. At the aquatic-terrestrial interface, researchers have primarily focused on the magnitude of resource fluxes, while largely disregarding differences in their nutritional quality for consumers in the recipient ecosystem. Aquatic resources, such as emergent aquatic insects, contain substantially higher levels of physiologically important long-chain polyunsaturated fatty acids (long-chain PUFA) than terrestrial resources. Since animals cannot typically biosynthesize long-chain PUFA in quantities sufficient to meet their physiological demands, they must obtain these compounds through their diet. Aquatic subsidy by emergent aquatic insects can therefore benefit the physiological condition and breeding success of riparian consumers such as birds and spiders. However, aquatic contaminants may disrupt aquatic insect emergence, altering the quantity and nutritional quality of aquatic subsidy to recipient terrestrial systems. Despite its importance to local consumers, effects of aquatic contaminants on this process are poorly studied and understood. The primary aim of this thesis is therefore to assess the effects of aquatic contaminants on aquatic-to-terrestrial subsidy, focusing on FA fluxes, and to determine whether shifts in subsidy quality affect riparian consumers. Effects were examined in a set of complex laboratory bioassays that also considered environmental factors (e.g., basal food resource) to increase environmental relevance. Metals (i.e., cadmium, Cd and copper, Cu) substantially reduced aquatic insect emergence (up to 95%) and the flux of physiologically important long-chain PUFA (up to 80%) while being efficiently excreted during metamorphosis (reduction by ~90%). Given their relative scarcity in terrestrial ecosystems, local consumers may be particularly vulnerable to changes in the flux of physiologically important long-chain PUFA. Indeed, contaminant-induced alterations of aquatic subsidy quality can affect the physiological condition of riparian spiders, as suggested by a consistent decline in spider growth (40-50%), irrespective of the contaminant group (Cu, Bacillus thuringiensis var. israelensis (Bti) or a mixture of synthetic pesticides) to which their prey was exposed. Since spider neutral lipid FA contents were only reduced in two out of three contaminant treatments compared to the control (up to ~30%, Cu and Bti), this implies contaminant-dependent effect pathways modifying the transfer of energy within food webs at the aquatic-terrestrial interface. Furthermore, results from a multigeneration experiment point to a relatively constant reduction in aquatic insect emergence and thus sustained alteration in the aquatic-to-terrestrial subsidy. Finally, the basal resource quality and sex-specific differences in aquatic insects and terrestrial consumers may further complicate the assessment of contaminant-induced effects. This thesis therefore recommends to consider fluxes of long-chain PUFA to better understand and anticipate implications of aquatic contaminants on the aquatic-terrestrial meta-ecosystem and develop efficient measures to protect riparian systems and their communities.