Optical scattering properties of phytoplankton: Measurements and comparison of various species at scattering angles between 1u and 170u

We describe the results of a laboratory scattering experiment performed to investigate differences in the optical scattering properties between five common phytoplankton species (Dunaliella tertiolecta, Isochrysis galbana, Nanochloropsis, Skeletonema costatum, and Thalassiosira weissflogii). Data sets were taken at scattering angles 1u to 170u at incident polarizations parallel and perpendicular to the scattering plane. Differences between the species were especially apparent in the slopes at near-forward (1u–10u) angles and the enhanced scattering at nearbackward (150u–165u) angles. There were also notable differences observed between the depolarization ratios, especially at scattering angles between 70u and 110u. The data are shown to be not only helpful in differentiating several species from each other, but also in separating the phytoplankton from bubble/hydrosol contributions to the ocean’s inherent optical properties. The results presented here motivate the development of in situ large-angle polarimetric scatterometers and associated retrieval algorithms. Phytoplankton has a dominant influence on the optical properties of the open ocean (Bricaud and Morel 1986). Remote sensing techniques such as those used in Seaviewing Wide Field-of-view Sensor (SeaWiFS) and Moderate Resolution Imaging Spectroradiometer (MODIS) take advantage of the effects of cells on ocean color when interpreting reflectance measurements. These properties are also interesting from a biological perspective because they affect the light harvesting abilities of the cells. Phytoplankton is a vital component of both the global and local ecology; its effects on the environment include nutrient depletion, production of photosynthetic byproducts, and changes in the optical properties of the surrounding waters (Garver and Siegel 1997, Moisan and Mitchell 2001). The ability of the ocean to absorb greenhouse gases is also affected by the type of phytoplankton dominating an area, and a better understanding of how this process works is required (Bissett 2001). In spite of all this, little data are available on the scattering behavior of phytoplankton cells themselves, although it seems likely that scattering will differ between species depending on size, composition, shape, pigmentation, etc. (Morel 1987). Studies do exist addressing the volume scattering function (VSF), or scattered radiance given input irradiance, but knowledge of the optical scattering matrix (which includes polarization effects) is crucial. Generally, past phytoplankton studies have not fully characterized individual species for a range of angles from forward (approaching 0u) to backward (near 180u) angles for differing polarizations. For example, Slade and Boss (2006) and Agrawal (2005) consider forward angles ,20u, whereas Vaillancourt et al. (2004) and Ulloa et al. (1994) study primarily backscattering. Large-angle experiments have been performed, of course: Volten et al. (1998) covers intermediate angles between 20u and 160u, Witkowski et al. (1998) looked at the large-angle scattering matrix for a single species, and Shao et al. (2006) recently presented compelling theoretical as well as experimental results in the 40u to 140u range (albeit without considering polarization effects). Chami et al. (2006) discuss spectral aspects of scattering at large angles. A review of phytoplankton scattering basics is included in Kokhanovsky (2006). Our study measures the normalized scattering functions of five common species of phytoplankton (Dunaliella tertiolecta, Isochrysis galbana, Nanochloropsis, Skeletonema 1 Corresponding author. Acknowledgments We thank the University of Maryland’s Chesapeake Biological Lab (CBL) for growing the phytoplankton cultures used in this study. We also thank Kyle Ball and David Hadka of Penn State ARL for helping to prepare the manuscript. This work was supported by NAVAIR under Contract N00421-01-C-0223 PO 0002 at Patuxent River, Maryland. This project was administered through the Pennsylvania State University’s Applied Research Lab (Penn State ARL). Limnol. Oceanogr., 53(1), 2008, 381–386 E 2008, by the American Society of Limnology and Oceanography, Inc.