Processing of C-13-labelled phytoplankton in a fine-grained sandy-shelf sediment (North Sea): relative importance of different macrofauna species

On-board and in situ experiments with C-13-labelled diatoms were carried out to investigate the processing of algal carbon by the macrofauna community of a fine sandy-shelf site in the southern German Bight (North Sea). The time series (12, 30, 32 and 132 h incubations) was supplemented by additional laboratory experiments on the role of the dominant macrofauna organism, the bivalve Fabulina fabula (Bivalvia: Tellinidae), for particulate organic matter subduction to deeper sediment layers. The specific uptake of algal C-13 by macrofauna organisms was visible after 12 h and constantly increased during the incubation periods. F. fabula, a facultative (surface) deposit- and suspension-feeder, Lanice conchilega (Polychaeta: Terebellidae), a suspension-feeder and the (surface) deposit-feeder Echinocardium cordatum (Echinodermata: Spatangidae) were responsible for the majority of macrofaunal carbon processing. Predatory macrofauna organisms like Nephtys spp. (Polychaeta: Nephtyidae) also quickly became labelled. The rapid subduction of fresh organic matter by F fabula down to ca. 4 to 7 cm sediment depth could be demonstrated, and it is suggested that entrainment by macrofauna in this fine-grained sand is much more efficient than advective transport.