What do the optical people do on Ronald H. Brown?

Author: Shuangling Chen

Finally, it is time for the optical guys to talk about something! Yes, it is us (Shuangling Chen & Yingjun Zhang, Fig. 1), Ph. D students from Dr. Chuanmin Hu’s Optical Oceanography Lab in College of Marine Science, University of South Florida (http://optics.marine.usf.edu/).

Fig. 1. The optical guys from College of Marine Science, University of South Florida (left: Shuangling Chen, right: Yingjun Zhang).

Simply speaking, we measure ocean color. When sunlight gets into the ocean, it is attenuated with depth due to absorption and scattering by the constituents in water. Different constituents (such as Colored Dissolved Organic Matter (CDOM), phytoplankton, inorganic suspended matter) have different absorption and scattering characteristics, and that is basically why we see different ocean colors.

Since we got on board the first day, the most frequent question we were asked about was: “What is a HyperPro?” After it got clear, the question becomes, “Hey, HyperPro today?” I am so glad that people on the ship care so much about what we do, and I believe you, who is reading this blog, must be also curious about it!

The “Giant” HyperPro we brought is a Satlantic free-falling HyperPro II (left in Fig. 2), ~1 m long and 25 pounds heavy. It is a hyperspectral radiometer with a wavelength range of 350-800 nm. An independent surface reference system (right in Fig. 2) is also included to provide downwelling irradiance (Es) during casts. It is mounted high on the vessel to avoid any potential shading.

Fig. 2. HyperPro profiler and the surface reference system.

The profiler, surface reference system, and a GPS device, are connected to a laptop via a deck unit for synchronous data transfer. Once all the pieces are connected, before the deployment, we need to do a pressure tare to record the reference pressure and a dark measurement to record the offset values of the sensors.  Then it is ready to go!

Since we measure light in the water, we do not want the solar light to be changed much by an unpleasant cloud. Therefore, usually we would deploy when the sky is clear and sunny. Also considering the satellite overpass time during a day, we would prefer to deploy during local 10 am – 4 pm. Besides, to avoid the ship’s shadow, and depending on the strength of the current, the ship may need to move slowly (~0.1 knots) to keep away from the profiler.

Usually, Yingjun is responsible for the deployment, and I’m in the lab to control the laptop for data logging and checking the depth and tilt of the profiler and communicating that back to Yingjun (Fig. 3). It sounds quite simple and easy, right? Well you’d be surprised at how much labor and coordination it needs, especially when you take into account the water pressure and currents. And that’s mainly why I say the HyperPro is a “Giant”. For stations over 200 m deep and if time allows, we need to cast to 50-70 m twice and then cast to 15 m 5 times, deploying and recovering by hand and often working against currents and water pressure at depth. It takes lots of labor (Thanks, Yingjun, you did a great job!)! One more thing to consider is the communication cable and preventing it from getting tangled. The survey tech on deck, with whom I communicate via radio during casts always helps to unravel it (Daniel and Josh, thanks! We really appreciate that!! See, we are doing science together!).

Fig. 3. HyperPro deployments on deck and operations in the lab.

It is very hard work, but we really enjoy it! Look at the fancy data we collected (Fig. 4) and see how the light is attenuated at different wavelengths and depths, isn’t that cool?

Fig. 4. An example of data collected at station 002 on July, 20th, 2017.

In addition to the HyperPro, we also carried 4 other optical instruments (Fig. 5): 1) a handheld spectrometer, to measure remote sensing reflectance; 2) a handheld sunphotometer, to measure light absorption in the ozone column; 3) an ALFA underway system, to measure chlorophyll fluorescence; and 4) a water filtration system, to filter water samples from the CTD or underway seawater line for measurements of particulate absorption, CDOM, and chlorophyll pigments.

Fig. 5. Other instruments that we worked on.

I just realized that the cruise is going to end in 4 days. How time flies! Flipping over days past, it is the outstanding leadership of our conscientious and considerate chief and co-chief scientists Leticia and Denis and the awesome teamwork of our lovely and responsible crew members on Ronald Brown that makes all the science go smoothly. I believe all the scientists are collecting very interesting data, and science will never stop!