Observation and modelling of the dynamics of benthic fluff resuspended from a sandy bed in the southern North Sea

The dynamics of suspended particulate matter (SPM) have been investigated at a site in the Central Southern Bight of the North Sea characterised by a well-mixed water column and a non-cohesive sand bed. Measurements of SPM concentration, particle size and quasi in situ settling velocity were made over tidal cycles during three periods (January, May, September) and used to test a 1-D tidal resuspension model. SPM comprised two components: a local tidal resuspension component and a background component in long term suspension. The background component comprised organo-mineral microflocs with a seasonally invariant modal settling velocity of less than 10-6 m s-1 and additional populations of biogenic particles produced during phytoplankton blooms. The tidal resuspension component had a modal settling velocity of ∼5×10-3 m s-1 and comprised benthic fluff and not bed sediment. Superposition of quarter diurnal resuspension and semi-diurnal advection along a horizontal background concentration gradient produced a characteristic ‘twin peak’ concentration time series. The model successfully reproduced the observations and demonstrated that local resuspension was source-limited: carbon-rich fluff, which formed a veneer within ripple troughs at slack water, was rapidly exhausted during tidal resuspension so that SPM concentrations peaked before maximum current velocity. Resuspension potential was therefore governed by the supply of fluff, which was greatest in May after a phytoplankton bloom but was still significant in January.