Benthic community response to temporal and spatial gradients in physical disturbance within a deep-sea western boundary region

Abstract

Western boundary lower slope regions of the deep-sea are often characterized by spatial and temporal gradients in bottom currents and cycles of erosion/deposition. These cycles cause instability in the seabed, provide reactive organic matter and organisms, and physically disturb benthic infaunal populations. Benthic infauna in one such region, the Nova Scotian Rise (western North Atlantic), are modified by and adapted to the periodically strong near-bottom currents and cycles of erosion and deposition. It is likely that the chronic physical disturbance provides the mechanism for structuring and maintaining a distinctive benthic assemblage in this area. Multi-year current records from moorings ≤50 m above bottom, have documented gradients between three study sites: upslope, a “Shallow” study site at 4185 m water depth, where currents average < 3 cm s −1 and rarely exceed 8 cm s −1; downslope, a region close to the base of the rise at ∼ 4950 m (“Deep” study site) where currents are on the average 4 times greater than at the 4185 m water depth, and can exceed 20 cm s −1 for week-long periods; and mid-slope, the HEBBLE site (“Mid” site at 4815–11830 m water depth, where periods of strong near bottom currents (> 15 cm s −1) erode the seafloor, but alternate with periods of currents < 5 cm s −1, when biological activity dominates particle transport. At both the Shallow site and periodically at the Mid site, the presence of low current speeds (≤5 cm s −1), a depositional sedimentation regime, and additions of labile POM, are likely key variables in significantly increasing ( p<0.05) ATP concentrations and standing stocks of microorganisms and meio- and macrofauna compared with the Deep site and the Mid site during times of strong flow conditions ( _ ̆ 15–25 cm s −1). As found previously on the rise, standing stocks are higher than in typical deep-sea regions, mean sizes of individuals are larger, adult organisms are present, and there is a greater diversity of feeding strategies and life habits than encountered in most deep-sea habitats. During times of intermediate flow conditions (5–15 cm s −1) at the Mid site, microbial production and activities are enhanced relative to low flow conditions, total numbers of epifauna decrease, abundances of infaunal burrowers and tube builders increase, and a higher proportion of smaller individuals is found. Strong near-bottom currents (> 25 cm s −1), which occur periodically at both the Mid site and Deep site, erode and remove reactive organic matter and presumably contribute to significant reductions in faunal abundances at both locations. Organism sizes and biomass are reduced and epifauna eliminated. At the Mid slope HEBBLE site, there is a cycling of both physical disturbance and biological responses, with characteristics of both the relatively undisturbed upslope and severely disturbed downslope communities. Temporal comparisons show response and recovery times to disturbance events on scales of days, to weeks, to months.