Carbon and nitrogen flows within the food web of a subtidal sandy sediment were studied using stable isotope natural abundances and tracer addition. Natural abundances of 13C and 15N stable isotopes of the consumers and their potential benthic and pelagic resources were measured. δ13C data revealed that consumers did not feed on the bulk microphytobenthos (MPB) but rather were selective in their food uptake, preferring either benthic diatoms (–16‰), or benthic cyanobacteria (–20‰). MPB was labelled through a pulse-chase experiment with 13C-bicarbonate and 15N-nitrate. The fate of MPB was followed in the different heterotrophic compartments. Transfer of 13C and 15N to consumers was fast, although only a small fraction of total label was transferred to the heterotrophic compartments within the 4 d of the experiment. Heterotrophic bacteria were responsible for most of the total heterotrophic incorporation of 13C. Within the metazoan community, the incorporation of 13C by the meiofauna was more than 2-fold that of the macrofauna, despite a significantly lower biomass. The dual labelling also revealed differential feeding or assimilation strategies in meio- and macrofauna. The low 13C:15N ratios of the meiofauna (the smaller organisms) seemed to indicate that they preferentially assimilated N or specifically grazed on N-rich resources. However, the macrofauna (larger organisms) seemed to feed on bulk sediment, consistent with high 13C:15N ratios. This dual approach, which combined natural abundance and a pulse-chase addition of stable isotopes, revealed crucial information on the key role of MPB in structuring benthic communities in sandy sediments.