The response of suspended particulate material to upwelling and downwelling events in southern Lake Michigan

Time series measurements of water temperature, beam attenuation, and current velocity were made at three stations located in 28, 58, and 100 m of water off the east coast of Lake Michigan from late August until mid-October 1995. When combined with water intake records and wave measurements our observations show that local resuspension is not responsible for maintaining the benthic nepheloid layer (BNL) during the stratified period. Resuspension of bottom material by surface wind waves occurs in shallow water (13 m), but this material is not transported offshore into the BNL during downwelling events. During up upwellings material mag be transported onshore from the BNL into shallow water, but additional material is required to produce the concentrations observed at the inner stations. Resuspension by internal waves mag be the source of this additional material. Changes in the vertical structure of the benthic nepheloid layer appear to account for most of the changes in suspended particulate material at the two offshore stations. These changes are probably due to a combination of internal wave action and regional changes in current pattern.