Oregon State University

College of Earth, Ocean, and Atmospheric Sciences

Uptake of Oxygen at the Seafloor

May 1, 2013

Andrea Albright, a Master's student in chemical oceanography, works with Clare Reimers to measure the uptake of oxygen by organisms living in the seafloor sediment. Reimers' research group has built a benthic oxygen exchange rate lander (BOXER) to quantify the uptake of oxygen by the seafloor in hypoxic areas off the coast of Oregon.

Reimers, co-PI Tuba Ozkan-Haller, and their team have been conducting research at sea and performing controlled waveflume experiments in the O.H. Hinsdale Wave Laboratory on campus–quantifying benthic oxygen fluxes and testing assumptions about fine-scale processes.

The goal is to create a reliable method for measuring the consumption of oxygen in a range of aquatic environments, in order to answer questions about hypoxia (low oxygen) zones and to quantify oxygen and carbon cycles in a changing environment.

Measuring Oxygen

Oxygen currently makes up about 20% of the atmosphere, which is due to plants that produce oxygen to make carbon-based organic matter through photosynthesis. On other side of the equation, animals, and most other organisms, breathe in oxygen and breathe out carbon dioxide. The interplay between these processes affects the proportions of oxygen and carbon dioxide in the atmosphere and the ocean. One major undertaking by oceanographers is quantifying how different elements–oxygen and carbon, in this case–move between different reservoirs in the ocean.

As Albright notes, "We can observe that the nearshore environment is changing, and we know that there is some uptake of oxygen by the seafloor. However, we don't know how much or how it varies. Until we have really robust measurements of oxygen–over this period of time, in these conditions, and know that this much oxygen was taken up–we can't say what exactly is changing.

"Questions that we're interested in answering include: How much oxygen is the seafloor taking up? Does the amount consumed depend on how much oxygen is already in the water? What happens when there is a bad year for hypoxia?"

Experiment Design

Oxygen is very difficult to measure across different interfaces. In the past, benthic fluxes were estimated by pushing a large chamber of known volume into the seafloor, and then measuring the decrease in dissolved O2 over time. However, this type of measurement was unrealistic as water normally flows freely along the seafloor, which the box chamber prevented.

Reimers and Albright are using a technique called eddy correlation to get an accurate measurement of O2 uptake in real conditions at the seafloor. Eddy correlation was developed several decades ago to measure fluxes of carbon dioxide from the land to the atmosphere, and has recently been adapted for use underwater by measuring current velocities and oxygen together very rapidly in the same sampling volume above the seabed. To take these measurements, the Reimers group has built a frame that can withstand being deployed in the ocean and have also created ultra-sensitive microelectrodes that can measure the fine-scale changes in oxygen just above the seafloor in a high-energy, fast-mixing environment.

Along the Oregon coast, these researchers have focused their measurements from April to October–the beginning and end of upwelling season, when winds drive currents that bring deeper, colder, and more nutrient-rich waters to the surface. They have concentrated on locations at 45 and 80 meters depth, around deposition centers where rivers bring sediment onto the continental shelf and oxygen uptake is believed to be highest. Typical research cruises are scheduled for ~ 1-10 days, and the BOXER system is deployed for 24 hours at a time at different sites along the coast.

At the Hinsdale Wave Lab, researchers can accurately measure and control, the height, direction, and period of waves, as well as the depth and orientation of equipment. Using that information, the Reimers' group is studying how eddy correlation measurements are affected by wave pumping or the direction the sensor and instrument frame are pointed.

Albright's Path to OSU

Albright's background as an undergraduate was in chemistry. After taking every chemistry class offered by her college, she decided that being tied to the lab bench was not for her, and took some time off. After working on a sailboat–and loving it–she decided to explore ways to mix oceans and chemistry, and started looking into graduate programs for chemical oceanography. She contacted Reimers, was accepted to OSU, and moved out to Oregon.

Albright describes the most valuable thing that she has gotten out of this program is "a concrete understanding of how the world works. Chemistry is great for looking at chemical reactions in a laboratory, but when I came to OSU, I realized that I didn't really know much about the chemistry of the Earth.

"The people who are most successful in this field are the people who are able to apply methods from more than one academic discipline. On Earth, everything–physics, chemistry, and biology–is happening all together, all the time. It's much more difficult to separate cause and effect, because the system already works: the clouds form, the rains fall, runoff goes into the rivers and ocean, and so on.

"It's just been incredible to see how that system works–what processes people have already measured and quantified, and understanding that the system is so complicated that we really can't describe it all yet."

The Future

For Albright, the future includes finishing her Master's (~ Fall 2013) and then finding an interesting project to work on. "Career planning is not my strong suit. That might be why science appeals to me. I want to put my energy into contributing something that will get used and that I can talk to other people about."

Albright sees the eddy correlation method being used more widely underwater. She notes, "Once you can go anywhere and measure oxygen precisely, we can start to think about where we want to take measurements on a long-term scale and where we just need to take a few measurements to fill in the picture."

Andrea on Elakha

Andrea Albright on the deck of R/V Elakha at the start of a research cruise out of Newport, Oregon. The BOXER lander is deployed underwater to measure oxygen variations near the seafloor. (photo: courtesy, Andrea Albright)

Hinsdale test

The benthic oxygen lander (BOXER) set up in the O.H. Hinsdale Wave Research Laboratory, in preparation for measuring oxygen variations due to wave motions in a controlled environment. (photo: Andrea Albright)

sensors

Microelectrodes used to measure high frequency oxygen changes underwater. A silicon membrane at the tip keeps water from entering, but still allows oxygen to diffuse into the sensor. Inside there is a gold-plated platinum wire that consumes oxygen via a known electrochemical reaction. The amount of current required to reduce the oxygen crossing the membrane is proportional to the amount of oxygen in the water surrounding the sensor. These microelectrodes were constructed by Kristina McCann-Grosvenor in Clare Reimers's lab and have a response time of less than 1 second. (photo: Andrea Albright)

Deployment from Wecoma

Deployment of the lander from R/V Wecoma. (photo: Andrea Albright)


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