The foraminiferan macrofauna from three North Carolina (USA) slope sites with contrasting carbon flux: a comparison with the metazoan macrofauna

Food supply exerts a strong influence on benthic faunal abundance and community structure. Here, we compare community-level responses of macrofaunal foraminiferans and metazoans (>300 μm fraction) in relation to a gradient of organic carbon flux [Site III>II>I] along the 850 m contour on the North Carolina slope. Foraminiferan density, species richness E(S100), and dominance were positively correlated with organic carbon flux. Foraminiferans were more abundant at Site III, displayed lower diversity and higher dominance, and tended to live deeper in the sediment column than at either Sites I or II. The Site I fauna was dominated by agglutinated taxa (mainly simple monothalamous forms and hormosinaceans) and included large epifaunal species, some of which projected from the sediment surface and probably fed on fresh phytodetritus. Hormosinaceans and monothalamous taxa also were abundant at Site II, although large epifaunal taxa were not present. The Site III fauna was dominated by calcareous taxa. The most abundant species was Globobulimina auriculata, an infaunal, low-oxygen tolerant, deposit feeder with a calcareous test sometimes obscured by an agglutinated cyst. Plate-like or flattened fragments of small xenophyophore species occurred at Site I, an unusually shallow record for this taxon and the first from the North Carolina margin. Most of these fragments were dead. Xenophyophores were not present at Sites II and III. The metazoan macrofauna exhibited trends in density, diversity, dominance and vertical distribution within the sediment that parallel those of the foraminiferans and were correlated with between-site differences in food availability. However, metazoans were 4.5–6.5 times less abundant than the foraminiferans, were more diverse, exhibited lower dominance and (at least at Sites I and III) tended to penetrate the sediment less deeply. These differences suggest that foraminiferans, considered as a group, are more opportunistic than metazoans, tolerate oxygen depletion better, and have population dynamics that are more closely coupled to organic matter inputs than those of metazoans. Foraminiferan diversity trends are even more similar to those of the polychaetes at these sites, suggesting that there are ecological parallels between the two taxa despite their fundamental phylogenetic and structural differences. Foraminiferans are a ubiquitous yet frequently overlooked component of the macrofauna on continental margins that experience a broad range of organic input regimes. They deserve to be considered more often in macrofaunal studies addressing interactions between organisms and their environments.