Two Pathways of Essential PUFA Supply from Terrestrial Invertebrates to Fish in Small Shaded Rivers

In small shaded forest rivers, autochthonous primary production is limited, creating a deficiency of essential polyunsaturated fatty acids (PUFAs) for consumers. Previously, we demonstrated that allochthonous subsidy provides a significant flux of physiologically essential PUFAs into the shaded Krutaya Kacha River. Here, we aimed to reveal the specific terrestrial taxa driving this nutritional subsidy and compare them to aquatic taxa. Principal component analysis separated aquatic from terrestrial taxa. Aquatic taxa, particularly Ephemeroptera, remained the primary source of physiologically valuable eicosapentaenoic (EPA, 20:5n‑3) acid for fish. Terrestrial phytophagous insects (Hemiptera, Homoptera, Lepidoptera, Hymenoptera, and Coleoptera) were rich in linoleic (LIN, 18:2n‑6) and/or alpha‑linolenic (ALA, 18:3n‑3) acids but contained very low levels of EPA and arachidonic acid (ARA, 20:4n‑6). By contrast, terrestrial predators and scavengers, especially Tetragnathidae (spiders) and Carabidae (carabid beetles), showed high EPA and ARA contents, comparable to those of aquatic insects. Spiders contained up to 10.6% EPA and 3.2% ARA, while carabid beetles contained up to 9.4% EPA and 4.0% ARA. Therefore, terrestrial invertebrates subsidized essential PUFAs via two discrete pathways: a direct pathway of LIN and/or ALA from terrestrial plants via phytophages and a “boomerang” pathway of ARA and EPA — from aquatic prey to terrestrial predators and scavengers and back into the river. These two pathways may be critical for essential lipid nutrition in shaded rivers occupying vast areas of boreal forests.