Periphyton as a key diet source of essential fatty acids for macroinvertebrates across a nutrient and dissolved organic carbon gradient in boreal lakes

We studied how physiologically important long-chain polyunsaturated fatty acids (PUFA) in benthic macroinvertebrates (Asellus aquaticus, Chironomidae, and Oligochaeta) were related to those in periphyton and terrestrial organic matter (tree leaves), collected from littoral areas of 17 boreal lakes that differed in their dissolved organic carbon (DOC) and nutrient (phosphorus and nitrogen) concentrations. We also analyzed fatty acid (FA)-specific stable carbon isotopes (δ13CFA) to investigate the dietary origin (periphyton vs. terrestrial organic matter) of PUFA in the consumers. In contrast to periphyton, terrestrial organic matter was deprived of long-chain PUFA, such as eicosapentaenoic acid (EPA), but rich in short-chain PUFA. The FA composition of macroinvertebrates was primarily taxon-specific despite the large differences in DOC and nutrient concentrations of the lakes. An increase in DOC concentration had a negative impact on the EPA content of Asellus, chironomids, and oligochaetes as well as the total FA content of chironomids and oligochaetes. However, the FA content of macroinvertebrates was not related to lake total phosphorus concentrations, although the total FA and EPA content of periphyton increased with the trophic status of the study lakes. The δ13CPUFA values of macroinvertebrates were positively related with the δ13CPUFA of periphyton and weakly with δ13CPUFA of terrestrial leaf material. The results indicate that EPA in the studied macroinvertebrate taxa was mainly derived from an algal-based diet and not via biosynthesis from allochthonous precursor FA. Thus, macroinvertebrate production in lakes may be limited by the available algae-based food sources.