1. δ13C and δ15N stable isotope signatures combined with an in situ microphytobenthic 13C labelling experiment were performed on epilithic biofilms of a large temperate river (the Garonne, France) to infer the trophic positioning of biofilm-dwelling meiofauna and their uptake of microphytobenthic carbon.
2. Chironomidae larvae and Chromadorina spp. nematodes rapidly incorporated freshly produced microphytobenthic carbon in contrast to Rhyacophilidae larvae and Naididae oligochaetes. Quantitatively, macrofaunal Chironomidae incorporated more microphytobenthic carbon per day than did meiofauna. Moreover, Chironomidae seemed more involved in the spatial export of microphytobenthic carbon than nematodes.
3. Rhyacophilidae larvae were predators feeding on large meiofauna (Naididae and Chironomidae) but not on nematodes. Naididae oligochaetes primarily gained their carbon from allochthonous and/or microbial-loop recycled sources.
4. A rapid and significant loss of labelled microphytobenthic carbon was observed. Feeding activity of biofilm-dwelling invertebrates seemed not to be primarily involved in this loss.
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