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Abstract

The relative importance of allochthonous phytodetritus deposition and autochtho-nous microphytobenthos (MPB) production for benthic consumers in an organic carbon (Corg)-poorsandy sediment was assessed using a 13C-stable isotope natural abundance study combined witha dual 13C-tracer addition approach. In a first experiment (Expt 1), a set of sediment cores receiveda pulse of 13C-labelled phytodetritus and the fate of that organic matter was followed in thebenthic food web (bacteria, meiofauna and macrofauna) over a period of 72 h. In a second exper-iment (Expt 2), the MPB present in a set of sediment cores was labelled with 13C-bicarbonate andthe fate of labelled MPB was followed the same way over a period of 96 h. Natural 13C abundancesof sources and consumers revealed that the benthic food web likely relied primarily on MPB. Inparticular, diatoms contributed at least 40% to the diet of 12 out of the 16 taxonomic groupsidentified. The dual approach revealed the complexity of the trophic interactions and gaveevidence for resource partitioning between 2 species of harpacticoid copepods. Both 13C-traceraddition experiments showed a fast transfer of label to most heterotrophs. Bacteria, whichcomprised the largest fraction of the heterotroph biomass, incorporated more 13C than otherconsumers. Meiofauna had similar relative incorporations in both experiments and likely reliedequally on benthic and pelagic inputs. Macrofauna relied significantly more on MPB. In bothexperiments, most of the 13C-label that was incorporated by heterotrophs was respired. Whilephytodetritus-derived Corgconsumed by heterotrophs was 41 mg C m−2, MPB-derived was at leastone order of magnitude higher. The benthic community growth efficiency in Expt 2 (40%) washigher than that of Expt 1 (25%), confirming the pivotal role of MPB