Temporal variability of near-bottom particle resuspension and dynamics at the Porcupine Abyssal Plain, Northeast Atlantic

In order to study particle behaviour and its time-variability in the near-b ottom layer on the Porcupine Abyssal Plain (48 degrees 50'N, 16 degrees 30' W, 4850 m), long-term measurements were made of currents, and nephelometry and particle samples were collected using an autonomous lander between mid- 1996 and mid- 1998. Water samples, collected in the Bottom Nepheloid Layer within 1000 ni of the bottom, were filtered for suspended particles whose contents of organic carbon, nitrogen and pigments were determined. This stu dy was co-ordinated with a water column flux study and a detailed programme of benthic studies to understand how the abyssal boundary layer responds t o and modifies inputs of organic matter from the water column (MAST3/BENGAL programme). There were strong seasonal fluctuations in the near-bottom (2 m above the b ottom, mab) particle flux, whose variation were correlated in time with the water column fluxes. During the periods of peak flux, the near-bottom flux was sometimes higher than that recorded higher up in the water column, but not always at other times. These excesses were attributed to the resuspens ion events, since we observed a correlation between current speed and nephe lometry. However, in summer the peak in the particle resuspension flux coul d not be explained by the variations in the tidal amplitude, Instead we att ribute it to the large quantities of fresh large particles (aggregations) t hat had just arrived on the bottom; it was probably linked to the feeding a ctivity and sediment reworking by the rich and varied benthic and benthopel agic megafauna. In both 1997 and 1998, the nephelometry signal (directly related to fine pa rticle concentration) and its variability increased after the peaks in larg e particle flux with a time-lag of 2-3 months. We assume that this time lag corresponds to the time it takes for the large fresh particles, once they have settled on the bottom, to be disaggregated into smaller particles, and hence become subject to resuspension in the quiet current conditions then prevailing in the BENGAL area. The suspended particle analyses confirm the vertical structure of the Bottom Nepheloid Layer, the lower part of which c orresponds to the Bottom Mixed Layer (BML) where resuspension and mixing ar e higher. (C) 2001 Elsevier Science Ltd. All rights reserved.