Temporal variation of suspended particulate matter and turbulence in a high energy, tide-stirred, coastal sea: Relative contributions of resuspension and disaggregation

Measurements of turbulence and suspended particulate matter (SPM) were measured over a 50 h period at a site in high tidal energy, mixed waters of the Irish Sea, NW European shelf. Turbulence parameters included both production (variance method from seabed ADCP) and dissipation (FLY profiler); SPM parameters included mass and volume concentrations and particle size (LISST 100 C). It is shown that the resultant SPM time series was due to a combination of time-varying turbulence at the measurement site and space-varying turbulence advecting through the site. Time asymmetry in turbulence at the site produced an asymmetric M4 signal in SPM volume concentration due to resuspension and disaggregation of flocs at times of peak turbulent energy. In terms of mass, the disaggregation contribution was 43% as much as the resuspension contribution near the bed, and 20% as much integrated throughout the water column. There was aggregation of flocs at high and low slack waters but the largest flocs occurred at low slack waters. Space-varying ambient turbulence was responsible for a horizontal gradient in floc size with small and large flocs at the high and low ends of the gradient, respectively; this generated a M2 signal in SPM properties. SPM concentrations and properties at any time resulted from combination of M2 and M4 variations which are responsible for the well-known twin peaks signature seen in transmissometer time series in tidal waters.