Importance of phytodetritus and microphytobenthos for heterotrophs in a shallow subtidal sandy sediment

Abstract

The relative importance of allochthonous phytodetritus deposition and autochthonous microphytobenthos (MPB) production for benthic consumers in an organic carbon (C-org)-poor sandy sediment was assessed using a C-13-stable isotope natural abundance study combined with a dual C-13-tracer addition approach. In a first experiment (Expt 1), a set of sediment cores received a pulse of C-13-labelled phytodetritus and the fate of that organic matter was followed in the benthic food web (bacteria, meiofauna and macrofauna) over a period of 72 h. In a second experiment (Expt 2), the MPB present in a set of sediment cores was labelled with C-13-bicarbonate and the fate of labelled MPB was followed the same way over a period of 96 h. Natural C-13 abundances of sources and consumers revealed that the benthic food web likely relied primarily on MPB. In particular, diatoms contributed at least 40% to the diet of 12 out of the 16 taxonomic groups identified. The dual approach revealed the complexity of the trophic interactions and gave evidence for resource partitioning between 2 species of harpacticoid copepods. Both C-13-tracer addition experiments showed a fast transfer of label to most heterotrophs. Bacteria, which comprised the largest fraction of the heterotroph biomass, incorporated more C-13 than other consumers. Meiofauna had similar relative incorporations in both experiments and likely relied equally on benthic and pelagic inputs. Macrofauna relied significantly more on MPB. In both experiments, most of the C-13-label that was incorporated by heterotrophs was respired. While phytodetritus-derived C-org consumed by heterotrophs was 41 mg C m(-2), MPB-derived was at least one order of magnitude higher. The benthic community growth efficiency in Expt 2 (40%) was higher than that of Expt 1 (25%), confirming the pivotal role of MPB.