The particulate scattering characteristics of coastal waters were examined at nine locations around the United States, including near-shore sites in the Gulf of Mexico and the Atlantic and Pacific oceans. The scattering measurements were used in conjunction with inversion models to estimate particle size distributions and bulk refractive indices of the suspended particles. The relationships between various scattering properties and chlorophyll concentration were also investigated and compared with previous relationships described for case I waters. Although the general patterns of scattering and particle characteristics in coastal waters were fairly consistent, fine-scale variability within the water column was substantial. Combining optical measurements with inversion techniques provided a more informative view of the environment and a better understanding of the nature of particle populations in the coastal ocean.
Abstract. During the BIOSOPE field campaign October–December 2004, measurements of inherent optical properties from the surface to 500 m depth were made with a ship profiler at stations covering over ~8000 km through the Southeast Pacific Ocean. Data from a ~3000 km section containing the very clearest waters in the central gyre are reported here. The total volume scattering function at 117°, βt(117°), was measured with a WET Labs ECO-BB3 sensor at 462, 532, and 650 nm with estimated uncertainties of 2×10−5, 5×10−6, and 2×10−6 m−1 sr−1, respectively. These values were approximately 6%, 3%, and 3% of the scattering by pure seawater at their respective wavelengths. From a methodological perspective, there were several results: – bbp distributions were resolvable even though some of the values from the central gyre were an order of magnitude lower than the lowest previous measurements in the literature; – Direct in-situ measurements of instrument dark offsets were necessary to accurately resolve backscattering at these low levels; – accurate pure seawater backscattering values are critical in determining particulate backscattering coefficients in the open ocean (not only in these very clear waters); the pure water scattering values determined by Buiteveld et al. (1994) with a [1 + 0.3S/37] adjustment for salinity based on Morel (1974) appear to be the most accurate estimates, with aggregate accuracies as low as a few percent; and – closure was demonstrated with subsurface reflectance measurements reported by Morel et al. (2007) within instrument precisions, a useful factor in validating the backscattering measurements. This methodology enabled several observations with respect to the hydrography and the use of backscattering as a biogeochemical proxy: – The clearest waters sampled were found at depths between 300 and 350 m, from 23.5° S, 118° W to 26° S, 114° W, where total backscattering at 650 nm was not distinguishable from pure seawater; – Distributions of particulate backscattering bbp across the central gyre exhibited a broad particle peak centered ~100 m; – The particulate backscattering ratio typically ranged between 0.4% and 0.6% through the majority of the central gyre from the surface to ~210 m, indicative of "soft" water-filled particles with low bulk refractive index; and – bbp at 532 and 650 nm showed a distinct secondary deeper layer centered ~230 m that was absent in particulate attenuation cp data. The particulate backscattering ratio was significantly higher in this layer than in the rest of the water column, reaching 1.2% in some locations. This high relative backscattering, along with the pigment composition and ecological niche of this layer, appear to be consistent with the coccolithophorid F. profunda. Moreover, results were consistent with several expectations extrapolated from theory and previous work in oceanic and coastal regions, supporting the conclusion that particulate and total backscattering could be resolved in these extremely clear natural waters.
.[1] The effects of particle fields including bubbles on the optical volume scattering function (VSF) were investigated in the surf zone off Scripps Pier as part of an ongoing effort to better understand the underlying dynamics in the VSF in the subsurface ocean. VSFs were measured at 20 Hz at angles spanning 10°-170°in 10°increments with a device called the Multiangle Scattering Optical Tool (MASCOT). Modification of the phase function was observed in passing suspended sediment plumes, wave-injected bubble plumes, and combinations of these particle populations relative to the background. Phase function enhancement in the 60°-80°range was observed in association with bubble plumes, consistent with theoretical predictions. VSFs were inverted to infer size distributions and composition using a least squares minimization fitting procedure applied to a library of phase functions, each representing a lognormally distributed subpopulation with refractive index and coating, where applicable. Phase functions representative of nonspherical mineral particle subpopulations were computed using discrete dipole approximation (DDA) and improved geometric optics method (IGOM) techniques for randomly oriented, asymmetric hexahedra. Phase functions for coated bubbles were computed with the Lorenz-Mie theory. Inversion results exhibited stable solutions that qualitatively agreed with concurrent acoustical measurements of bubbles, aggregate particle size distribution expectations, and anecdotal videography evidence from the field. Although a comparable inversion with a library that assumed spherical shaped particles alone provided less stable results with some incorrectly assigned subpopulations, several dominant subpopulation trends were consistent with the results obtained using nonspherical representations of mineral particles.Citation: Twardowski, M., X. Zhang, S. Vagle, J. Sullivan, S. Freeman, H. Czerski, Y. You, L. Bi, and G. Kattawar (2012), The optical volume scattering function in a surf zone inverted to derive sediment and bubble particle subpopulations,
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.