The stalks of hexactinellid sponges in the genus Hyalonema provide hard substrate in soft-bottom, deep-sea environments. The goal of this research was to describe the epifaunal communities of these “stalks” and their physical environment in the abyssal NE Pacific. Several projects were conducted to determine the biological and physical factors influencing the vertical distribution of attached fauna.

In photographic transects, stalks appeared to be covered with suspension feeders. Examination of stalks collected intact from the sea floor revealed that the communities were species-rich, with diverse trophic modes represented. Vertical distributions of most taxa extended over a wide range in height above the sea floor. The distributions of smaller, solitary fauna and certain functional groups of colonial organisms were restricted by sheet and mound-like colonial cnidarians that were dominant space competitors.

Results of an experimental study using artificial Hyalonema stalks indicated that the abundance of fauna in stalk communities is maintained by frequent recruitment of individuals in a few common taxa and infrequent recruitment of many rare taxa. Vertical distributions of foraminiferan recruits were skewed towards the top of the artificial stalks, likely due to behavior.

The physical environment of the stalk communities was examined in terms of the components of advective “food” flux: flow velocity and organic particle concentration. The gradient in the near-bottom flow environment was estimated from long-term records and measured directly with in situ flow visualization. Although the magnitude of flow at the level of the stalk communities was weak (on average < 2 cm/s), the average gradient in flow (calculated from log layer profiles) was such that flow at 50 cm would be twice that at 5 cm above the sea floor. Intact phytoplankton cells, potential food for members of the stalk communities, were found in phytodetritus entering the benthic boundary layer, suspended at ~1 m above bottom, and deposited on the sea floor, suggesting rapid transfer from surface waters to abyssal depths.

Overall, biological interactions appear to be most important in structuring the Hyalonema stalk communities. However, some patterns observed for suspension feeders are potentially a response to a gradient in advective food supply.