On the Stability and Significance of Microflocs in Estuarine Sediment Dynamics

Abstract:

Fine-grained suspended sediment in estuarine, coastal and deep-water environments is almost always aggregated and often a distinction is made between smaller and relatively stable microflocs and larger and more fragile macro-flocs. The grain size distribution (GSD) of microflocs tend to be independent on the local hydrodynamic forcing whereas macroflocs typically show strong dependence.

Resuspension of the deposited material is an important process in coastal and estuarine settings and the material may be eroded as individual particles or as aggregates of particles. The relative occurrence of these two types of particles in the erosion of undisturbed, natural sediments has only rarely been studied. In order to examine the contribution of aggregates and single particles to the resuspended material, erosion and settling experiments have been carried out and the GSD of the resuspended particles have been measured. The studies have been carried out in both temperate and tropical estuaries and on inter-tidal and sub-tidal sediments and the results compared to what was found in suspension in those settings. The erosion experiments were carried out with an EROMES erosion chamber and grain size analysis performed with a laboratory set-up of a LISST 100C.

The mean grain size of the eroded particles were up to about an order of magnitude larger than the mean grain size of the primary (individual) particles and the GSD of the resuspended material generally showed no dependence on the applied bed stress in the studied range from 0 to 0.5 N m-2. The GSD of the eroded aggregates were generally also very similar to the GSD of the material found in suspension at the sites. Our interpretation is that the GSD of the microflocs in suspension is to a large degree controlled by the aggregation taking place at the bed. This aggregation appears to be dominated by biological processes with fecal pellets being a very prominent component in some environments.