D7 Optics and Physical Biogeochemistry

Topic

Ocean optics and biogeochemistry study how light interacts with seawater constituents and the implications of this interaction for both biological productivity and the carbon cycle. Sunlight drives all marine photosynthesis: photosynthetically active irradiance—the portion of the solar spectrum ranging from 400 to 700 nanometers—penetrates the ocean and attenuates exponentially with depth, following the Beer-Lambert law. The attenuation coefficient governing this light loss depends on three factors: phytoplankton concentration, the amount of colored dissolved organic matter, and the quantity of suspended mineral particles. It is this combination of factors that results in two bodies of water—despite receiving the same amount of surface light—exhibiting completely different light profiles at depth.

Chlorophyll fluorescence is the primary tool for directly measuring phytoplankton concentration in the water: when chlorophyll absorbs blue light (at a wavelength near 470 nanometers), it responds by emitting red fluorescence (at a wavelength near 685 nanometers). It is precisely this red emission that allows a sensor to measure the amount of phytoplankton present without making direct contact with the water. Today, these fluorescence sensors are integrated into both autonomous floats equipped with optical instruments and ocean gliders, enabling the acquisition of continuous phytoplankton abundance profiles rather than isolated, discrete measurements.

Light backscattering, measured simultaneously at multiple wavelengths, allows for the estimation of the total suspended particle content—both organic and inorganic—and, specifically, the amount of particulate organic carbon present. The more particles suspended in the water, the more light is backscattered toward the sensor; this direct relationship allows backscatter to serve as an optical proxy for particle concentration—a value that would otherwise require water filtration to measure.

Transmissometry, by contrast, measures the amount of light passing through a fixed distance in the water—typically around twenty-five centimeters—and uses this transmitted fraction to derive the light attenuation coefficient. This coefficient is simply the sum of two components: the light absorbed by the water and the light scattered by it. It is precisely this sum that allows the attenuation coefficient to encapsulate, in a single figure, both the light absorbed by the water and the light diverted without being absorbed.

Particle cameras, meanwhile, automatically photograph everything floating in the water—ranging from particles barely a tenth of a millimeter in size to organisms measuring one hundred millimeters. This enables the tracking of zooplankton and organic aggregate distributions with a level of detail unmatched by any single previous optical sensor. When combining these techniques, the expected pattern is for fluorescence to peak at the top of the pycnocline—where both light and nutrient levels remain simultaneously sufficient—and for a significant portion of the particulate organic carbon formed there to eventually be exported to deep waters. Thus, from the irradiance entering at the surface to the particulate carbon that eventually sinks, ocean optics traces—wavelength by wavelength—the same path taken by light within the carbon produced and transported by the ocean.

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