Apparent Flexibility of Fatty Acid Metabolism in Shredders Depends on Terrigenous Organic Matter Type

ABSTRACT Headwater streams receive large amounts of terrestrial organic matter (tOM) such as leaves from a dense riparian matrix, forming a critical link between terrestrial and aquatic ecosystems. Terrestrial OM rarely provides essential macronutrients such as polyunsaturated fatty acids (PUFAs), and microbial colonisation of tOM is an important process that makes macronutrients more readily available to invertebrate tOM consumers. We investigated how the fatty acid (FA) composition of a tOM consumer varies with tOM type and its microbial colonisation. In a laboratory study, Sericostoma caddisfly shredders were fed alder, maple and beech leaves microbially preconditioned under oxic or anoxic conditions. We expected that shredder FA profiles would improve when fed oxic preconditioned tOM and that different tOM types would result in distinct shredder FA profiles as shredders may metabolise stored FAs when consuming tOM low in essential resources such as PUFAs. Our results show that tOM type significantly influenced the FA profiles of shredders. Shredders feeding on alder leaves under oxic conditions had higher total lipid and PUFA concentrations than before the experiment. In contrast, shredders feeding on beech or maple leaves had lower lipid and PUFA levels, suggesting alder provides a more favourable resource for FA accumulation in shredders. Shredder biomass increased in all treatments, indicating growth. However, biomass increase corresponded to higher PUFA concentrations only in alder‐fed shredders. No such increase was observed in beech or maple‐fed shredders, indicating these tOM types did not provide sufficient PUFAs for further accumulation. Microbial preconditioning did not significantly affect FA profiles, highlighting the dominant role of tOM type in shaping shredder FA composition. Our results highlight the importance of tOM quality and microbial preconditioning in shaping resource quality and PUFA availability in headwaters. More importantly, they emphasise the role of tOM–aquatic consumer interactions in land‐water connectivity, regulating the transfer of organic matter and nutrients across ecosystem boundaries and influencing reciprocal subsidy flows.