Transfer of essential lipids from aquatic to terrestrial ecosystems

To date, aquatic ecosystem research is strongly restricted to investigations in water bodies, such as lakes and rivers. Thus, the importance of water organisms for terrestrial ecosystems remains less clear. Cross-ecosystem fluxes of organic matter can crucially influence the productivity of adjacent habitats. For example, emerging aquatic insects provide an important pathway by which freshwater-derived organic matter can enter terrestrial food webs. Effects of reciprocal resource subsidies on adjacent food web processes depend on the quantity of the subsidy; if and how qualitative differences in resource subsidies can influence adjacent food web processes has not been studied yet. The objective of this project is to evaluate the transfer of essential and potentially limiting biochemical nutrients, i.e. polyunsaturated fatty acids (PUFA), from freshwater to terrestrial systems via emerging insects and to assess the significance of freshwater-derived PUFA for terrestrial invertebrate consumers. Emergence traps will be installed in different lentic and lotic ecosystems (small lakes and streams) to assess habitat-specific PUFA fluxes. Pond experiments with stable isotope-labelled substrates will be used to assess PUFA export via emerging insects and distribution of aquatic PUFA in adjacent terrestrial ecosystems and invertebrate consumers. Laboratory feeding experiments will be conducted with riparian invertebrate predators (i.e. spiders), aquatic and terrestrial insect prey and PUFA-manipulated food to test whether aquatic insects are preferentially consumed and support the growth of riparian predators more efficiently than terrestrial insect prey. This approach will provide novel insight into the potential role of essential nutrients in mediating cross-ecosystem effects.

The AQUATERR research project was able to demonstrate that several hundred kilos of insects from lakes and fish ponds (important habitats for insect larvae) emerge from the water every year and their energy is available for insectivores, such as birds and spiders. In these insects we found high concentrations of omega-3 fatty acids, which are important for the growth and survival of insectivores. AQUATERR was also able to show, for the first time, that spiders get essential dietary energy from aquatic insects and not from terrestrial insects. We were able to make this distinction by identifying stable hydrogen isotopes in fatty acids from spiders and insects.