Trophic pathways of omega-3 fatty acids in stream food webs

Research on stream ecosystems is strongly influenced by the well-established River Continuum Concept that suggests that headwater streams in temperate forests are strongly dominated by leaves and other terrestrial material. At the same time, these streams are important habitats for freshwater salmonids, such as trout and charr, which are an important source of omega-3 fatty acids for higher consumers, including humans. However, only very little omega-3 fatty acids occur in such terrestrial material of headwater streams. Thus, the high dietary omega-3 fatty acid demand for salmonids and other headwater consumers may be too low. This research project targets this conundrum and will investigate, a) spatial and seasonal variation in stream consumer dependence (including insects and fish) on elemental and molecular composition of basal resources in pre-alpine streams (ecosystem approach), b) under different light conditions, the effect of terrestrial and stream diet sources on food supply and retention of fatty acids in stream invertebrates (experimental approach), and, c) the ability of freshwater fish to synthesize long- chain omega-3 fatty acids using state-of-the-art radioactive methods in fish liver cells (lipid metabolism in stream fish). This research will develop and use methods, including stream experiments and fish liver cells analyses, linked with field investigations and apply stable isotopes and fatty acids as a new combined effort to understand how diet sources are retained in fish. Results will thus shed considerable light on the long-standing question of how insects and fish in streams manage or fail to obtain essential nutrients and high quality forms of energy. Together with an international team of high-profile researchers this project will clearly further stream biofilm research and stream food webs. This research will provide excellent training opportunities for young scientists at the graduate and post-graduate level considerably contribute to a more comprehensive understanding of dietary energy transfer and lipid dynamics in stream organisms along increasing trophic levels.

This research project examined the fat composition in organisms at the base of stream food webs and trophic transfer to insect larvae and fishes. The overall goal was to understand how insect larvae and fishes of shaded headwater streams obtain their long-chain polyunsaturated fatty acids (PUFA) that are essential for their cell membranes, somatic growth, and survival, and which diet source (terrestrial vs. autochthonous) contains such PUFA. We provided experimental evidence of trophic upgrading, via the use of compound-specific stable isotopes, by invertebrates that consume leaf litter. This research also identified, for the first time ever, stable hydrogen isotope values of fatty acids for the use in aquatic ecology. We could therefore expand our general view of trophic fatty acids/energy transfer to tissue- and organ-specific levels and demonstrate that fatty acids of fish organs differ markedly from dietary or muscle fatty acids. This finding profoundly changes the way we perceive trophic transfer of essential nutrients that were, thus far, 'only' analyzed in whole organisms (e.g., macroinvertebrates or small diet fishes) or in specific tissues (mostly dorsal muscles tissue of fishes). Our findings show that neural tissues of fish, and possibly also other aquatic consumers, depend on the supply of algae-derived essential dietary fatty acids, but their fatty acids composition is different from the typically identified dorsal muscle tissues, suggesting highly selective allocation and retention and/or innate conversion ('upgrading') of dietary fatty acids for various organs. Importantly, terrestrial diets do not provide these essential nutrients that neural or reproductive fish tissues in shady headwater streams require. Based on the generated findings of this project, we could open a promising new research agenda for aquatic food webs: the use of hydrogen stable isotopes in fatty acids will open up completely new ways to advance our understanding of; a) how dietary energy presents itself in aquatic ecosystems, b) how dietary fatty acid sources are retained in aquatic consumers, and; c) how fatty acids are converted in organisms along the aquatic food web and within consumers. The latter point draws attention to ecophysiological processes that will become more and more important in the future as aquatic consumers will be faced with dietary energy (including fatty acids) that get more and more deteriorated in quality, yet is required for the development of neural tissues that will secure survival. This project generated 10 research papers and was the basis for 1 MSc student, 1 PhD student and 1 post-doctoral researcher.