Macrofaunal response to phytodetritus in a bathyal Norwegian fjord

The continental margin (rise and shelf) constitutes approximately 1/10th of the surface area of the oceans, but 80–90% of all sedimentary organic matter (OM) is remineralised here. Recent evidence has suggested that macrofauna may play an important role in organic matter remineralisation in deep-sea continental margin sediments, and the deep fjords of western Norway provide a relatively easily accessible opportunity for detailed studies of continental margin macrofaunal communities and their role in C-cycling. We examined the macrofaunal community and assessed its response to a simulated OM pulse in a fjord environment using pulse-chase tracer experiments. In each experiment, 1 g Corgm2 of 13C-labelled Skeletonema costatum was deposited onto intact sediment cores collected from 688m water depth and incubated ex situ for 2, 7 and 14 d. Macrofaunal abundance and biomass were comparable to those of other deep-sea continental margin sediments of similar depths, but in contrast to previous fjord studies, the macrofaunal community was numerically dominated by ostracods. Tracer experiments revealed highest uptake of tracer after 7 and 14 d compared to 2 d. Of the seven deposit feeding polychaete families, only the Paraonidae and Cirratulidae together with the largely carnivorous Lumbrineridae—showed a significant response to our labelled C-source. The lack of response by the majority of deposit feeders and the unexpected feeding mode of the Lumbrineridae may be attributable to species—rather than family specific feeding—ecologies or ontogenetic changes in diet/feeding mode. Total macrofaunal C-turnover was much lower than recorded in the deep Sognefjord in a 3 d feeding experiment, and is possibly a result of (1) distinct differences in macrofaunal community composition between sites, with a predominantly sub-surface-feeding macrofaunal assemblage being found in this study as opposed to a surface-feeding community in the Sognefjord, or (2) variations in OM supply and demand. Overall, this investigation highlights the importance of ecological information at the species level for a detailed understanding of macrofaunal C-cycling and early diagenesis in marine sediments.