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Abstract

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 13 C and 15 N stable isotopes of the consumers and their potential ben- thic and pelagic resources were measured. δ 13 C 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 experi- ment with 13 C-bicarbonate and 15 N-nitrate. The fate of MPB was followed in the different heterotrophic compartments. Transfer of 13 C and 15 N 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 bac- teria were responsible for most of the total hetero- trophic incorporation of 13 C. Within the metazoan community, the incorporation of 13 C by the meio- fauna was more than 2-fold that of the macrofauna, despite a significantly lower biomass. The dual labelling also revealed differential feeding or assim- ilation strategies in meio- and macrofauna. The low 13 C: 15 N ratios of the meiofauna (the smaller organ- isms) seemed to indicate that they preferentially assimilated N or specifically grazed on N-rich resources. However, the macrofauna (larger organ- isms) seemed to feed on bulk sediment, consistent with high 13 C: 15 N ratios. This dual approach, which combined natural abundance and a pulse-chase addition of stable isotopes, revealed crucial informa- tion on the key role of MPB in structuring benthic communities in sandy sediments.