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

Abstract

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 M 4 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 M 2 signal in SPM properties. SPM concentrations and properties at any time resulted from combination of M 2 and M 4 variations which are responsible for the well-known twin peaks signature seen in transmissometer time series in tidal waters.