Selective Retention of Long‐Chain Polyunsaturated Fatty Acids in Heptageniidae (Ephemeroptera) of Subalpine Streams

ABSTRACT Benthic macroinvertebrates are crucial for linking dietary energy from the base of aquatic food webs to consumers at higher trophic levels. Ephemeroptera (mayfly) grazers rely on periphyton as their primary source of essential dietary nutrients, such as polyunsaturated fatty acids (PUFA) and other lipids. Along stream continua, lakes may interrupt the flow of dietary nutrients with as yet unknown effects on the lipid and PUFA composition of periphyton and Ephemeroptera located downstream of lakes. In this field study, we investigated four subalpine streams, one feeding and one draining Lake Lunz (Austria), one unaffected by the lake, and one after the confluence of the last two streams, from May–August 2022 to assess temporal and spatial variations in the lipid and fatty acid (FA) composition of different grazing Ephemeroptera taxa of the family Heptageniidae and periphyton as their diet source. Grazing Ephemeroptera were significantly enriched in total lipids and FA compared to periphyton and contained 15 times more eicosapentaenoic acid (20:5n‐3; EPA). Evidence from compound‐specific stable isotopes of PUFA suggested that these Ephemeroptera mainly accumulated PUFA from periphyton and only converted a small proportion of dietary precursors to EPA in lower‐order streams. Even though the lake had a discernible impact on the relative FA composition of downstream periphyton, the amount of dietary lipids available for consumers remained similar across all study streams over time. Seasonal variations in Ephemeroptera FA profiles were evident, with the peak lipid and FA contents observed in June potentially linked to imminent emergence. Rhithrogena spp. contained significantly higher lipid and PUFA contents than the other genera, suggesting a more efficient retention of dietary lipids. These findings highlight the importance of considering site‐specific, temporal, and taxonomic factors when evaluating FA composition patterns within stream ecosystems and the dietary quality of benthic invertebrates for higher‐order consumers.