Toward Closure Between Single Particle and Bulk Optical Properties During the Coastal Mixing and Optics Experiment Book uri icon

abstract

  • Concurrent measurements of particle and bulk optical properties were collected at a single location on the New England Shelf during late summer 1996 (August 16-September 7) and spring 1997 (April 24-May 13), as part of the Coastal Mixing and Optics Experiment. Single particle measurements of forward angle light scattering, 90-degree light scattering, and red and green fluorescence, were made by analyzing discrete 5-ml water samples on a ship-board flow cytometer (FCM). Bulk optical measurements of absorption (particle and dissolved) and scattering were made using in-situ absorption and attenuation meters (ac-9's). Total absorption by phytoplankton measured by FCM was determined using an empirical chlorophyll fluorescence calibration. Comparison of the FCM-based estimates of phytoplankton absorption with those measured spectrophotometrically on filtered samples suggest that most of the absorption was by particles less than 5 micrometers in diameter. We applied Mie theory specific to our FCM configuration to determine total scattering cross-section, size, and refractive index for single particles. During the stratified period of late summer, there was a vertical gradient in absorption and scattering cross- section of phytoplankton cells with lower values in surface waters. In contrast, in the spring, cells often had larger cross-sections in surface waters. During both seasons, the eukaryotic phytoplankton contributed significantly to total scattering in near-surface waters, while non-phytoplankton particles dominated deeper in the water column. Synechococcus were only important to scattering in late summer. Lack of closure in our current analysis of scattering is most likely because bacteria have not yet been taken into account.

publication date

  • January 2000