The SES experimental campaign took place over a 16 month period between March 1995 and August 1996 along the Hebridean continental slope (56° - 58° N), to the west of Scotland. This study was associated with, in particular, examining the distribution and fluxes of particulate matter based on observations with moored instrumentation and regular water column surveys. Despite the poor performance of some of the instruments during the study, a valuable dataset was collected comprising PMC, current velocity, particle size and settling velocity data, substantially enhancing the existing database.
A seasonal cycle in PM distribution was observed at the Hebridean shelf edge, which is dominated by phytoplankton production in Spring and storm-driven resuspension in Winter. Despite strong tidal currents and internal wave activity in the summer months no significant resuspension due to these agents was observed. PM distribution at the upper slope is influenced by the poleward flowing slope current. This is confined to the upper 500 m and is characterised by a core of warmer, more saline water with higher PMC than the surrounding water. Further down the slope, benthic nepheloid layers form a typical feature throughout the year, but especially in Winter and late Spring. Intermediate nepheloid layers, following density surfaces were also observed in the aftermath of a storm.
Near-bed PM flux calculations indicate that PM is persistently transported
northwards along-slope by the slope current, and off-shelf by Ekman transport in the benthic boundary layer, even at depths of 1500 m. Along-slope fluxes are 2-3 times greater than across-slope fluxes and are greatest on the shelf and upper slope.
Intermittent resuspension by storms of PM on the shelf and slope enhances near-bed PMC and off-shelf flux, albeit for relatively short periods (days). Cross-slope fluxes are also enhanced by phytodetritus settling into the benthic boundary layer in the aftermath of phytoplankton blooms, this time over longer periods (weeks).
Estimates of off-shelf PM flux in the benthic boundary layer are three orders of
magnitude greater than the estimated vertical flux. This lateral source of carbon may account for discrepancies observed by other SES workers between the sediment community demand for carbon and the vertical supply, and represents an important transfer of organic matter to the deep ocean.