Carbon and nitrogen flows through the benthic food web of a photic subtidal sandy sediment

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 C-13 and N-15 stable isotopes of the consumers and their potential benthic and pelagic resources were measured. delta C-13 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 C-13-bicarbonate and N-15-nitrate. The fate of MPB was followed in the different heterotrophic compartments. Transfer of C-13 and N-15 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 C-13. Within the metazoan community, the incorporation of (13)ZC 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 C-13:N-15 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 C-13:N-15 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. GRAPHICS Sand ripples created by wave oscillations; although they appear to be unvegetated, sandy sediments harbour a very diverse flora and fauna