In situ methods for measuring the inherent optical properties of ocean waters

Abstract

In situ attenuation (488 and 660 nm), absorption (488, 676, and 750 nm), backscattering (at 488 nm), and stimulated fluorescence were determined as functions of depth in oceanic and coastal waters off Oregon, using both commercial instruments and recently developed prototypes. The inability to perfectly constrain scattered light in these instruments necessitates correction to retrieve accurate optical coefficients. Because scattering is usually one order of magnitude higher than absorption, the correction is most critical for absorption coefficients. A correction procedure based on values obtained with the absorption meter in the infrared region (750 nm) was used to correct the measured spectral absorption coefficients. Realistic values of the backscattering coefficient were obtained from a prototype instrument, although accurate calibration could not be performed for this study.These measurements resulted in the first‐ever data set ofin situ profiles of attenuation, absorption, scattering, backscattering, and the backscattering ratio. The blue and red attenuation coefficients were tightly correlated, with a relationship varying from site to site. The red‐to‐blue absorption ratio varied both from site to site and vertically, indicating changes in the relative concentrations of chlorophyllous (phytoplankton) and nonchlorophyllous (biogenous or mineral) particles. The backscattering ratio also appeared to be very sensitive to vertical changes in particle composition. The in situ scattering coefficients were compared to those estimated with a Mie scattering model, using measured particle size distributions and particulate absorption coefficients as input parameters. By allowing the real part of the refractive index of particles to vary over the range of realistic values, convergence between measured and modeled values was obtained.