Tuesday, May 12, 2015

Color by Number: Ocean Optics

Have you ever stopped to think about the color of the ocean? I mean, really stopped to think about why the ocean looks the way it does? If you’ve been to a variety of beaches, you’ve probably noticed that some waters are crystal blue, while others are emerald green, and still others can be yellowish-brown to even red! What causes these colors, and what can they tell us about the ocean?

And so begins our dive into the realm of Optical Oceanography. As you might expect, many different things contribute to ocean color, but we can group them into five basic categories:
  • Water – obviously the most abundant contributor, it actually slightly changes color based on temperature and salinity!
  • Phytoplankton – the plants of the sea! As diverse as they are abundant, they influence ocean color based on their size and the amount and type of pigments such as chlorophyll.
  • Non-Algal Particles – all other particles in the ocean. This includes dirt, sand, and the clumps of organic material that sink to depth and make marine snow.
  • Colored Dissolved Organic Material – the fraction of dissolved organics that interact with light. I like to think of this as brewed tea, but so watered down that it has a light yellow color. It can also be called gelbstof, German for “yellow stuff”.
  • Bubbles – formed from breaking waves, phytoplankton photosynthesis, or zooplankton respiration, they are highly reflective and change the light field both above and below them. For this reason, they are very annoying when they get into our instruments when we’re trying to measure everything else! Pick your favorite drum solo and tap on the instruments to help push them along.


Lots of research has been done to characterize all the different ways these 5 constituents absorb and reflect all the different colors of light, and then some. With this information, we can look at how light availability changes with depth and piece together the puzzle to find the amount of each thing at different depths.

One of the instruments we use to do this (and my personal favorite one to work with) is the Compact Optical Profiling Spectrometer, or C-OPS. Norm Nelson and I deploy it midday when the CTD is out of the water so that we can match time/data with some ocean color satellites passing overhead. C-OPS has two clusters of sensors on it. One looks down at the spectrum of the light bouncing back up at it from below, and the other looks up and measures light coming down. It measures many different wavelengths as it sinks and sends this data back to us in real time. Clivarians who come to watch us often hear me yelling out the depth and attitude to Norm, who adjusts deploying the cable accordingly. Since we just want to measure light straight up and down, we don’t want any tilt as it sinks, which is sometimes pretty difficult with all these subsurface currents pushing it around. After we pull C-OPS up, we’ll go look at the profile and measure how fast each color of light seems to “disappear” with depth. By modeling these changes, we’ll be able to get a high-res map of many different constituents at once.

Norm Nelson (left) and James Allen (right) pose with the C-OPS prior to yesterday's deployment. Photo by Samantha Siedlecki. 
Our ultimate goal is to be able to monitor the amount and distribution of all these different constituents everywhere in the global oceans many times per year. With this information, we’ll be able to model and investigate the ocean’s role in climate change in much greater detail. However, it’s physically impossible for us to sail everywhere at once, so we must rely on using our optical profiles to train ocean color satellites to see what we see, a process called “groundtruthing”. Good data is vital, which is why we chose to work with CLIVAR, the gold standard of ocean analysis and observation. P16N in particular gives us a very nice range of data from productive equatorial upwelling to the gyre of the North Pacific. With this data, satellites measuring ocean color from orbit will be better equipped to tell us how the ocean is changing on many different scales of time and space. Talk about seeing the ocean in a new light!

--James 

This post was written by James Allen, a 2nd year PhD student at the University of California, Santa Barbara who studies ocean optics and biogeochemistry. James is primarily interested in ocean color, and hopes to eventually study the global size distribution of phytoplankton to better inform ocean-atmosphere models of Earth’s Carbon Cycle. On this cruise, James has been working with Norm Nelson, a researcher at UCSB who specializes in field observations of ocean optics. Norm is working to understand the distribution and dynamics of CDOM, which provides information on deep ocean circulation and the fate of dissolved organic material in the ocean interior. 


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