Observations of particle density and scattering in the Tamar Estuary

To investigate the relationships between the optical scattering and the physical properties of suspended sediments such as size and density, the absorption and scattering coefficients ( a and b), mass concentration and particle size spectra have been measured at 30 stations along a transect of the Tamar Estuary and Plymouth Sound. The apparent density of the suspended sediment ( ρ a ), which dropped significantly in the upper reaches of the estuary, was found to vary negatively as a function of both the chlorophyll concentration and median particle size by volume ( D v ). A relationship of the form, ρ i = (ρ p /D p −α ) D i −α relating the density of individual particle size fractions ( ρ i ) of diameter D i has been assumed and used to predict the total apparent density of the size spectrum. By comparing this to the measured apparent density, the size and density of the primary particles ( D p and ρ p ) and value of α (a measure of the fractal dimension) which in combination produce the best fit of predicted to actual ρ have been found. They were found to be 3.9 μm, 1787 kg m – 3 and 0.65, respectively. These values were closely reproduced by repeating this method for the specific scattering coefficient ( b*) which was determined by applying Mie theory to the measured particle size spectra for a refractive index consistent with that used by the LISST instrument. Using the calculated values of D p and ρ p the best fit value for the fractal dimension was determined for each station. The fractal dimension is shown to be strongly correlated with the apparent density and varies significantly from 2.05 in turbid, chlorophyll-rich Tamar water to 2.56 in the clearer water out in Plymouth Sound. Results suggest that the relationship between fractal dimension and particle density is robust and can be used to predict primary particle size and density from bulk properties. Until now these values, which are important for aggregation models have not been easily determined.