Color Measurement and Reflectance Spectroscopy of Total Suspended Solids for Remote Sensing and Environmental Monitoring of Aquatic Ecosystems.

Water color and water quality are related in complex and poorly understood ways. Suspended particulates, such as phytoplankton, bacteria, and soil particles, contribute significantly to the color and quality of natural water bodies by absorbing and scattering sunlight. There is currently no standard method to measure the color and optical properties of total suspended solids (TSS). Remote sensing algorithms estimate TSS concentrations from observed values of reflectance. There are spectral differences in TSS over time and space. A convenient, inexpensive, and quick method is developed and evaluated to quantify optical differences between different TSS. The technique links the traditional TSS laboratory method to the Hapke radiative transfer theory of closely packed particles. The Hapke bidirectional reflectance model is modified to incorporate the measured dry mass of TSS on the filter paper. The measured reflectances of a series of different TSS layer thicknesses (mass per filter area) enables calculation of the absorption and scattering properties of TSS particles. The specific extinction, absorption and scattering coefficients are calculated to classify TSS. The observed relationship between reflectance and TSS mass loading rate is modeled. Model parameters are determined by nonlinear least squares. The parameters, expressed as functions of wavelength (390 to 700 nm), permit optical classification and comparison of TSS from different locations and times. The extinction, absorption, and scattering efficiencies, the volume single-scattering albedo (w) and the effective single-particle absorption-thickness (espat) are also calculated with the model. This is the first time these parameters have been measured for TSS. Reflectance spectra may also be converted to CIE XYZ tristimulus values or Munsell colors. The method is evaluated using eutrophic lake TSS, Mississippi River TSS, wastewater bacteria, and algae (Chladymonas reinhardtii). A remote sensing model is presented based on the Hapke model parameters. The method enables quantification of the packaging effect of algae. The technique is easily incorporated into any TSS analysis and has applications in sediment tracing and nonpoint source pollution, laboratory methods to measure TSS pigments (chlorophyll), remote sensing, and water quality monitoring