Consumers feeding at the aquatic–terrestrial ecosystem interface may obtain a mixture of aquatic and terrestrial diet resources that vary in nutritional composition. However, in lake riparian spiders, the relative significance of aquatic versus terrestrial diet sources remains to be explored. We investigated the trophic transfer of lipids and polyunsaturated fatty acids (PUFA) from emergent aquatic and terrestrial insects to spiders at varying distances from the shoreline of a subalpine lake in Austria, using differences in fatty acid profiles and compound-specific stable carbon (δ¹³C) and hydrogen (δ²H) isotopes. The omega-3 PUFA content of emergent aquatic insects was higher than that of terrestrial insects. Emergent aquatic insects contained on average 6.6 times more eicosapentaenoic acid (EPA) and 1.2 times more α-linolenic acid (ALA) than terrestrial insects, whereas terrestrial insects contained on average 2.6 times more linoleic acid (LIN) than emergent aquatic insects. Spiders sampled directly on the lake and in upland habitats had similar EPA contents, but this EPA was derived from different diet sources, depending on the habitat. The δ¹³CEPA and δ²HEPA values of ‘lake spiders' revealed an aquatic diet pathway (i.e. EPA of aquatic origin). In contrast, EPA of spiders collected in terrestrial habitats was depleted in both ¹³C and ²H compared to any potential food sources, and their ALA isotopic values, suggesting that EPA was partly bioconverted from its dietary precursor ALA (i.e. internal pathway). The δ²H values of fatty acids clearly indicated that diet sources differed depending on the spider's habitat, which was less evident from the δ¹³C values of the fatty acids. Our data highlight that spiders can use two distinct pathways (trophic versus metabolic) to satisfy their physiological EPA demand, depending on habitat use and dietary availability.
• The ecological role of emergent aquatic insects from lakes in exporting dietary polyunsaturated fatty acids (PUFA) across the freshwater-land interface is still poorly understood. • In this field study, we explored the seasonal biomass export of emergent insects from three subalpine lakes and investigated how lipids of emergent insects were related to lake bathymetry, lipids of organic matter in lake sediments (i.e., basal resources), and the taxonomic composition of insects. • The total lipid and PUFA fluxes of emergent insects were strongly related to taxonomy and lake bathymetry, but weakly associated with the PUFA content of the uppermost lake sediment layers. PUFA flux estimates of the dominant taxon, Chironomidae, from the shallowest lake (3 m depth; 125 g PUFA m⁻² season⁻¹) were considerably higher than those from the deepest lake (33 m depth; 56 g PUFA m⁻² season⁻¹), due to the higher per area biomass of emergent insects from this shallow lake. Insect taxonomy also affected the composition of PUFA transfer to land: Chironomidae were richer in ω-6 PUFA, such as linoleic acid (18:2n-6) and arachidonic acid (20:4n-6), whereas Ephemeroptera and Trichoptera contained more ω-3 PUFA, especially α-linolenic acid (18:3n-3) and eicosapentaenoic acid (20:5n-3). • Our findings suggest that taxon-specific differences in PUFA content and lake bathymetry jointly shape PUFA fluxes and thus the provisioning of emergent insects as dietary sources of physiologically important PUFA for riparian consumers.
Emerging aquatic insects (EAI) are important vectors through which freshwater-derived organic matter can enter terrestrial food webs. Aquatic-derived dietary energy can support terrestrial consumer fitness, especially via its polyunsaturated fatty acids (PUFA) that are otherwise short in supply from terrestrial diet. Considering qualitative aspects of resource subsidies is thus crucial for understanding energy and nutrient fluxes between ecosystems and for assessing effects on food web processes in recipient habitats. In this context, the objective of this study is to, (1) quantify the export of PUFA from four peri-alpine lakes, (2) evaluate the PUFA transfer from aquatic to terrestrial invertebrate consumers via EAI along an altitudinal lake gradient, and, (3) identify trophic pathways for riparian consumers. We hypothesize that, a) PUFA-flux via EAI will decrease with increasing lake depth, and, b) the effect of aquatic subsidies on terrestrial invertebrate consumers decreases with distance to lakes. To test this assumption, insect biomass and total export of PUFA will be quantified from four lakes at different altitudes. In addition to bulk stable isotopes and fatty acids, compound-specific stable isotopes (d13C and d2H of fatty acids) will be used to assess PUFA export via EAI and the distribution of aquatic PUFA in adjacent terrestrial ecosystems and invertebrate consumers. Laboratory feeding experiments will be conducted to test which aquatic or terrestrial insect prey are preferentially consumed and support the growth of riparian predators (i.e. spiders) more efficiently. This approach will provide novel insight into the potential role of essential nutrients in mediating cross-ecosystem effects.