Seeing light underwater

by srosengard

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My lab and port hole to the horizon outside! A maze of secured, non-leaking tubing and wires that go from seawater to ship to instruments to computer files.

See? I must have over three million data points now, and less than 3 days at sea have passed. In part, I owe this quantity to the relative “easiness” of characterizing light’s behavior in seawater- i.e., the fact that the measuring process can be automated by two key instruments that monitor light through the ship’s steady seawater intake. Hopefully, by the end of this cruise, you’ll agree with me that measurements like these are essential to understanding life in the ocean, and relatedly, the planet’s carbon cycle.

Visible light, like the ultraviolet radiation you block with sunscreen or heat emanating from a warm sidewalk in the city, is made of energy packets, or photons, that travel as waves. Waves with different energies have different crest-to-crest distances and humans see those differences as colors. So lower energy red light, for instance, would have the longest wavelengths, while violet light has shortest. Natural sunlight is a mix of light waves with many wavelengths between red and blue, each of which behaves differently in the ocean.

Our team seeks three parameters for light behavior in the ocean: absorption, scatter and attenuation. I am interested in all of these parameters, but in these last four months on the new job, absorption has been the most intuitive to learn. For our ac-s meter to measure the light absorption in the ocean, it has to shine its own beam of light into a tube full of incoming seawater from one end, and then monitor the amount of light that exits the other side. With absorption, less light exits the tube than had entered, as is always the case with seawater.

I am most fascinated not by absorption itself, but by what in seawater causes light to get absorbed. The options we have are: terrestrial minerals kicked up from the ocean floor or delivered by rivers, mineral shells produced by algae like coccolithophores, and organic carbon in the cells of living and dead algae (yes finally the connection to carbon!). At 5 meters in the Gulf of Alaska, we can rule out the first two things, which means that our absorption numbers primarily tell us about algae in the water.

Moreover, the ac-s meter measures absorption at >150 wavelengths per seawater sample (per second)! With so many wavelengths, we can start to tease apart absorption by different pigment groups in algae cells, which in turn mark distinct species that are made of different pigments. You have heard of pigments: the most famous is probably beta-carotene, but among oceanographers, chlorophyll makes the top for its role in photosynthesis.

It is challenging to take a cup of seawater and say how many of what types of algae are living there. It is also challenging to look at enough cups seawater and make global interpretations, how the algae fuel the food chain or turn the carbon cycle. These rich measurements that optical instruments make at sea take us one step closer to overcoming both challenges.

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