Author: Emma Pontes
Take a deep breath. The air you just inhaled contains about 20% oxygen, 78% nitrogen, and 2% of a few other minor gases. Some might assume that oxygen is only available to terrestrial air-breathers, however, this assumption couldn’t be further from the truth.
Oxygen (O2) generally exists in a gaseous state, but also exists in the world’s oceans as a dissolved gas. Fish and other ocean biology utilize the available dissolved oxygen just like humans do; taking up O2 and discharging carbon dioxide (CO2). Just like on land, the ocean is home to millions of photosynthetic organisms such as plankton, algae, and other underwater plants that take up CO2 and release O2 during a process called photosynthesis. Therefore, there is a constant ebb and flow of CO2 and O2 being ‘inhaled’ and released into ocean waters.
So what does this mean for ocean chemistry, and why do we care? Dissolved oxygen in the ocean is a sensitive indicator of climate-related changes. The dissolved oxygen concentration can be used to determine how much anthropogenic CO2 (carbon dioxide released by humans resulting from the burning of fossil fuels) is being taken up by the ocean. Just like oxygen, CO2 can dissolve in ocean waters, and most of human-created CO2 has been sequestered by our oceans. The uptake of anthropogenic CO2 by the world’s oceans is a leading cause of ocean acidification. Therefore, it is of high importance to determine the O2 concentration of various locations around the world’s oceans, not only to learn more about the how ocean biology is functioning, but also to examine the effects of ocean acidification.
Enter GOMECC-3, Ocean Acidification Research Cruise. In the past, research vessels have travelled our current route collecting the same data we are gathering now at the same locations. We can get an idea of how ocean chemistry is changing over time by comparing the data we get on this cruise, to the historic data sets collected on the same path we are on now.
Work days on the ship consist of lowering the CTD rosette (stands for conductivity, temperature, and depth) into the ocean at a predetermined location called a Station. The CTD is a large cylindrical ring of bottles, called Niskins, that are triggered to close and collect water samples at predetermined depths. The CTD is a useful tool for scientists onboard to get insight as to how ocean chemistry changes with depth.
My job is to collect water samples from the Niskins and analyze each sample for its dissolved oxygen concentration using a technique called Winkler Titration. This procedure requires the addition of chemicals to the water sample that act as a fixative; the chemicals bind to the oxygen in the water and create a solid precipitate that eventually sinks to the bottom of the water sample. You can think of it as ‘pickling’ the oxygen to preserve it, so that the sample can be analyzed anywhere from 1hr to 4 weeks after being collected. To learn more about the titration procedure, check out the peer-reviewed paper entitled ‘Determination of Dissolved Oxygen in Seawater by Winkler Titration Using the Amperometric Technique’ written by Dr. Chris Langdon in 2010, which basically serves as my lab manual on the ship. I am looking forward to collecting some meaningful data that will contribute to OA research as we continue our trip around the Gulf of Mexico!
Langdon, Chris. “Determination of dissolved oxygen in seawater by Winkler titration using the amperometric technique.” The GOSHIP Repeat Hydrography Manual: a Collection of Expert Reports and Guidelines, edited by: Hood, EM, Sabine, CL, and Sloyen, BM (2010).