Resuspension of benthic fluff by tidal currents in deep stratified waters, northern North Sea

Suspended particulate matter (SPM) concentration and properties (particle size and settling velocity), water column and boundary layer dynamics were measured during a 60-d period at a site in 110 in water depth in the northern North Sea. The site was in stratified waters and measurements were made during September-November as the seasonal thermocline was progressively weakening. SPM concentration was low, c. 1 mg dm(-3) in the surface mixed layer and maximum values of 2 mg dm(-3) in the bottom mixed layer. The bottom layer was characterised by larger mean particle size. SPM signals in the two layers were decoupled at the start of the period, when the thermocline was strong, but were increasingly coupled as the thermocline progressively weakened. A spring-neap cycle of resuspension and deposition of SPM was observed in the bottom mixed layer. Bed shear stresses were too small to entrain the bottom sediment (a fine sand) but were competent to resuspend benthic fluff: threshold bed shear stress and threshold current velocity at 10 mab were 0.02-0.03 Pa. and 0.18 in s(-1), respectively. Maximum SPM concentration in the bottom layer preceded peak spring tide currents by 3 d. Simulation of fluff resupension by the PROWQM model confirms that this was due to a finite supply of benthic fluff. the fluff layer was stripped from the seabed so that fluff supply was zero by the time of peak spring flow. SPM was redeposited over neap tides. Fluff resuspension must have been enhanced by intermittent inertial currents in the bottom layer but unequivocal evidence for this was not seen: There was some resuspension due to wave activity. Settling velocity spectra were unimodal or bimodal with modal values of 2 x 10(-4)- 2 x 10(-3) min s(-1) (long-term suspension component) and 0.2-5.7 nm s(-1) (resuspension component). the slowest settling particles remained in suspension at peak spring tides after the fluff layer had been exhausted. There was evidence of particle disaggregation during springs and aggregation during neaps. (C) 2002 Elsevier Science B.V. All rights reserved.