Trophic relations in a stream food web: importance of fatty acids for macroinvertebrate consumers

Our study is a first attempt to characterize seasonal fatty acid (FA) profiles of foodweb components in a small forested stream. We measured FA content of autochthonous food sources (aquatic primary producers == periphyton, green algae, red algae, bryophytes), allochthonous food resources (terrestrial matter == benthic and transported organic matter [[BOM and TOM, respectively]]), and macroinvertebrate consumers (Hydropsyche spp., Ephemerella spp., isopods, oligochaetes). We examined whether FAs could be used as trophic markers and tried to identify which food sources best provided macroinvertebrates with essential FAs (EFA, ωω3 and ωω6 groups). Primary producers consistently had greater content of several EFAs (18:2ωω6 and 18:3ωω3 in green algae, 20:5ωω3 in diatoms, 20:4ωω6 in bryophytes) than did terrestrial matter. The ratio of ΣΣωω3/ΣΣωω6 FAs, a putative marker of the relative amount of autochthonous vs allochthonous matter, was greatest in macroinvertebrates, followed by algae, and was significantly correlated with chlorophyll a content of food sources (periphyton, ultrafine BOM, and TOM). The seasonal dynamics of EFA content of BOM and TOM varied with particle size. Other FAs were identified as specific markers for diatoms (20:5ωω3 [[eicosapentaenoic acid]], 16:1ωω7, 16:ωω4s, 16C-polyunsaturated FAa [[PUFAa]]), green algae (18:3ωω3 [[αα-linolenic acid]], 18:2ωω6 [[linoleic acid]], 16C-PUFAb), and bryophytes (20:4ωω6, 20:3ωω3). Terrestrial matter had higher levels of bacterial and fungal FAs than did primary producers. Nonmetric multidimensional scaling analyses based on FA spectra of foodweb components in early spring (open canopy) and mid-summer (shaded canopy) confirmed that Ephemerella and Hydropsyche consumed mainly autochthonous food sources, even during the shaded summer period. Isopods and oligochaetes consumed a mixed diet of terrestrial matter and algae. Autochthonous food sources may be a more important part of the diets of benthic macroinvertebrates in forested streams than previously recognized.