OBSERVATION AND MODELING OF THE DYNAMICS OF BENTHIC FLUFF RESUSPENDEDFROM A SANDY BED IN THE SOUTHERN NORTH-SEA

The dynamics of suspended particulate matter (SPM) have been investiga ted at a site in the Central Southern Eight of the North Sea character ised by a well-mixed water column and a non-cohesive sand bed. Measure ments of SPM concentration, particle size and quasi in situ settling v elocity were made over tidal cycles during three periods (January, May , September) and used to test a 1-D tidal resuspension model. SPM comp rised two components: a local tidal resuspension component and a backg round component in long term suspension. The background component comp rised organo-mineral microflocs with a seasonally invariant modal sett ling velocity of less than 10(-6)ms(-1) and additional populations of biogenic particles produced during phytoplankton blooms. The tidal res uspension component had a modal settling velocity of similar to 5 x 10 (-3) ms(-1) and comprised benthic fluff and not bed sediment. Superpos ition of quarter diurnal resuspension and semi-diurnal advection along a horizontal background concentration gradient produced a characteris tic 'twin peak' concentration time series. The model successfully repr oduced the observations and demonstrated that local resuspension was s ource-limited: carbon-rich fluff, which formed a veneer within ripple troughs at slack water, was rapidly exhausted during tidal resuspensio n so that SPM concentrations peaked before maximum current velocity. R esuspension potential was therefore governed by the supply of fluff, w hich was greatest in May after a phytoplankton bloom but was still sig nificant in January. (C) 1998 Elsevier Science Ltd. All rights reserve d.