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We describe the underwater light field of the Strait of Georgia in spring and summer, using apparent optical properties (reflectance, attenuation coefficient of downwelling irradiance, the average cosine of downwelling irradiance, and the attenuation of scalar irradiance). Both the attenuation and reflectance of photosynthetically available radiation (PAR; 400-700 nm) are highest in the turbid waters of the Fraser River plume, due to scattering by mainly inorganic particles and absorption by coloured dissolved organic matter, phytoplankton, and other organic particles. Light is most diffuse in the surface waters of the plume and least diffuse at depth and away from the plume. Throughout the Strait, blue and red wavelengths are attenuated most rapidly resulting in a green peak of reflectance, the portion of the electromagnetic spectrum that penetrates the most deeply. PAR is attenuated to 1% of its surface intensity within 6-22 m in the spring and 4-23 m in the summer. For red and blue light, the depth of 1% penetration is never deeper than 9 m. All of the visible radiation, with the exception of some green light, is absorbed within the outflowing layer (15-30 m) that is exported from the Strait with the estuarine circulation. The rapid extinction of light helps to explain the very shallow distribution of phytoplankton.

Oil slicks can be visually detected through remote sensing techniques because of sharp image contrast variations between the oil slicks and surrounding water. These contrast variations are usually due to the dampening of the water surface roughness caused not just by oil, but possibly also by freshwater runoff and biogenic surfactants (also called "biogenic lookalikes"), such as those due to phytoplankton blooms. Floating macroalgae can also alter the texture of the water surface and contribute to look-alikes. Using methodologies we developed and implemented in previous studies of oil spills using hyperspectral optical imagery, we have tested several algorithms for biogenic look-alikes and oil slick characterization from optical and RADAR sensors in order to improve operational monitoring of marine coastal areas for oil pollution. With the opportunity to use imagery acquired over the Deepwater Horizon oil spill in the Gulf of Mexico in 2010, we have demonstrated promising utility of optical imagery to assist in differentiating oil from RADAR look-alikes in low wind situations. We have demonstrated that the interpretation of false positives for oil slicks in RADAR imagery can be adequately assisted by the analysis of optical imagery. Furthermore, oil spill extent and slick thickness can be mapped and characterized using spaceborne imagery. This represents a major improvement over local observations of oil spill for emergency and mitigation actions by improving response time and providing a synoptic view of the impacted areas.

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