HIGH-RESOLUTION MEASUREMENTS OF SEDIMENT RESUSPENSION ABOVE AN ACCUMULATION BOTTOM IN A STRATIFIED LAKE

A detailed record of suspended particulate matter (SPM) concentrations in the benthic boundary layer (BBL) 1.5 m above an accumulation botto m and 13.5 m below the surface was obtained from frequent (30 min inte rval) beam attenuation measurements made with a Sea Tech transmissomet er in the main basin of Lake Erken, a moderately deep (mean depth 9 m, maximum depth 21 m) dimictic lake in central Sweden. Concentrations o f SPM (g m-3) were not as strongly correlated to the beam attenuation coefficient (c, [m-1]), as were concentrations of the inorganic SPM fr action. Apparently, this was caused by large optically inactive organi c particles which significantly affected the measurements of SPM, but had little effect on the attenuation of light. When the water column w as thermally stratified, SPM concentrations in the BBL showed a season al increase which was related to an increase in the thermocline depth. As the epilimnion deepened, there was also a marked increase in the o ccurrence of rapid and large changes in SPM concentration. After the l oss of stratification, the amount of SPM and the temporal variability in its concentration was reduced. Since surface waves could not influe nce sediment resuspension at the depth of measurement, these data show the importance of internal waves in promoting sediment resuspension i n areas of sediment accumulation. The relatively short period in each summer, when the thermocline reaches a sufficient depth to allow for r esuspension over accumulation bottoms, can have important consequences for both the redistribution of lake sediments and the internal loadin g of phosphorus.