Size distributions of the suspended particles of the Chesapeake Bay turbidity maximum

Abstract

It has been suggested by POSTMA (1967) that the suspended particles in a turbidity maximum produced by the net non-tidal estuarine circulation should have a narrow size distribution. Particles larger than a certain size would presumably be deposited while smaller particles would be carried out of the area by the net seaward flow of the upper layer. There have been very few pertinent data to support or deny this suggestion. An intensive investigation of the Chesapeake Bay's turbidity maximum has shown that its suspended particle population is determined by the physical processes, local resuspension and the net non-tidal estuarine circulation, which combine to produce the turbid zone; and that the suspended particles have a narrow size distribution both in terms of equivalent projected diameters and Stokes' diameters or settling velocities. Determinations were made both of the number-size distributions by a photomicrographic technique and of the volume-settling velocity (“size”) distributions by a sedimentation technique. The suspended particle population of the Chesapeake Bay's turbid zone is composed of two sub-populations. One sub-population is comprised of particles with settling velocities of the same order as the vertical mixing velocities. These particles form a “natural background” of suspended sediment found throughout the water column and whose concentration and particle size distribution are very uniform over periods of weeks or months. The second sub-population consists of particles which are alternately suspended and deposited. The presence of this sub-population is made manifest by semi-tidal fluctuations of the concentration of suspended sediment in the lower layer at stations deeper than about 4 m and throughout the water column at shallower stations. The fluctuations produced by tidal “scour and fill” are accompanied by marked changes in the volume-size distribution and increase in magnitude near the bottom—the sediment source.