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!

Plankton communities and incubation experiments on GOMECC-3

Author: Mrunmayee Pathare

image 1
The microscopic phytoplankton that we study.

Our lab comprises some of the biological sampling being conducted aboard the Ronald Brown on the GOMECC-3 cruise. We study tiny ocean organisms called plankton which range in size from microscopic phytoplankton that use photosynthesis to produce energy, to millimeter sized copepods that can be seen by the naked eye “jumping” to catch their prey.

Phytoplankton form the base of ocean food webs, they are the tiny plants of the ocean, floating in the water column turning carbon dioxide into energy. Phytoplankton fix organic carbon found in the atmosphere and dissolved in the water into energy that is transferred through the food web by bigger organisms eating the smaller organisms. Most of these tiny organisms are eaten, but those that are not eaten fall to the ocean floor, drifting thousands of meters down the water column to be decomposed by bacteria. Phytoplankton fix 45 gigatons of inorganic carbon per year, and are an integral part of the mechanism removing CO2 from the ocean (fixing it), and turning it into food that gets passed up through the food chain, or falls to the sea floor as marine snow.

image 2
A copepod predator that eats prey plankton.

On this cruise, we will be looking at the plankton communities in the top 5 meters of the Gulf of Mexico and who is eating whom. We are conducting a 24-hour incubation on a series of light and dark bottles containing seawater sampled by the CTD. Some of these bottles will contain only phytoplankton and small grazers, and some of them will contain phytoplankton and copepods. This set up will give us a snapshot of predator-prey dynamics at the base of the food chain (who is eating whom), how carbon moves through the base of the food chain in different conditions within the Gulf of Mexico (how much is being eaten and how it changes in different parts of the Gulf of Mexico). We also have some oxygen optodes fixed inside these bottles that will let us measure the amount of respiration taking place in the bottles during their incubation.

image 3
Gluing the optodes (tiny orange circle on the forceps) inside the bottles took a surprising amount of contortion and skill!

To simulate the environment that we are taking these little critters from, we rigged up an incubation tank on the back deck of the ship. We had to get creative with the materials and the location, and then strap it down securely so it won’t move when the Gulf decides to throw bad weather at us.

The tank simulates the natural environment of the ocean and there is sea water constantly trickling through a hose to keep up the circulation and make sure the water inside the bottles doesn’t turn into plankton soup or get the photosynthesizing plankton fried by the sun.

We are conducting a total of 8 of these incubations over the course of the cruise, and although the results will be analyzed after we return from the cruise we are very excited to study the plankton communities of the Gulf of Mexico and contribute to the better understanding of carbon fate and transport.