Relationships between particle properties and scattering in coastal waters of the northern Gulf of Mexico influenced by the Mississippi River

Bruce Alan Spiering

Abstract

This study was conducted to determine how the characteristics of an assemblage of suspended particles, including their composition and size affect the relationship between the optical scattering coefficient b (m-1 ), and the dry mass of the particles in suspension. Knowledge of the scattering to total suspended matter (TSM) ratio, i.e. the mass specific scattering coefficient b * (m2 /g), is important because the light detected by optical imaging sensors used for remote sensing is the light that has been scattered by particles back through the water surface. If this ratio is not known or varies within the region of interest, accurate estimates of TSM using remotely sensed data will not be accurate. The hypothesis of this study is that b * is not constant in the study area, and that change in b * within the study area can be attributed to change in particle composition and change in the number concentration of particles relative to their diameters (i.e. the particle size distribution function, PSD) for the range of particle sizes important to light scattering (approximately 0.01 to 250 μm for this study). The primary objective of this study was to determine the variability in b * within the study area, i.e. the region where the main channel of the Mississippi River enters the Gulf of Mexico and mixes with marine water of the Louisiana continental shelf, and to characterize the change in b * relative to change in the percent organic fraction (%OM) of TSM and the slope parameter ζ of the assumed power law PSD. Additional objectives were to determine if b * and particle characteristics remained consistent within prescribed geographic regions within the study area. These regions included the location just outside the river mouth at Southwest Pass, main sediment plume extending from the river mouth, shelf waters not directly in the main plume and the region surrounding the Birdfoot Delta that receives river water through the myriad of outflows of the main channel. This characterization was done using statistical regression analysis of measured particle properties and through the use of modeling of particle optical properties using the Mie theory for homogeneous spheres.