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 C-org m(-2) of C-13-labelled Skeletonema costatum was deposited onto intact sediment cores collected from 688 m 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. (C) 2008 Elsevier Ltd. All rights reserved.