Oxygen, that thing we all need to breathe…

OxygenWe all do it subconsciously. Breathe in… Breathe out… We probably only really notice we are doing it when we get to the top of a flight of stairs, or when we go up in altitude and realize that the oxygen availability has changed, but we all know that oxygen is critical to life.

It is the same in the oceans as it is on land. Whales come up to the surface to breathe – the exciting moment when we see their spouts. Fish use their gills.

In recent years, the oxygen levels in our oceans have been changing dramatically. You may have heard about the dead zones in the Gulf of Mexico, where rainfall runoff from crops washes nutrient-rich waters into the Mississippi River and out into the ocean. Those nutrients are quickly used up by algae, which grow to larger numbers than normal (called an ‘algal bloom’). When those algae die, they sink to the bottom of the ocean, where they decompose. The process of decomposition uses the available oxygen; in extreme cases all the oxygen is used up, creating dead zones. This low-oxygen water is called ‘hypoxic.’  You may have heard the term ‘hypoxia’ before, which indicates when oxygen is low enough to affect marine life, generally lower than 64 micromoles per kg (or 2 milligrams per liter).  Dead zones can lead to dramatic events, such as massive fish kills.

Hypoxia is also linked to the pH (the acidity) of our waters. When oxygen is used up by decomposition or respiration, the same process produces CO2, leading to lower pH waters. Many research efforts, including ours on this cruise and the West Coast OA and Hypoxia Panel, recognize this link and study both together.

Although we see dramatic changes in oxygen in some coastal areas, there is also a lot of natural variation in oxygen levels just beyond our continental shelf, where we are also sampling on this cruise. As our sensors move from the surface of the ocean down to the bottom, they see oxygen levels changing from oxygen-rich waters at the surface down to low oxygen levels at depth, sometimes as low as zero. This oxygen depth-gradient is called a vertical profile, and the vertical profile at different sample stations can be very different from one location to another, depending on the conditions at each site.

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CTD water column profile for station 21. Depth (as a function of pressure) is represented on the Y-axis; oxygen concentrations are shown in red. Notice how the concentration of oxygen starts out high (above 200 micromoles per kg) at the surface, but rapidly drops (i.e. shifts to the left) as the depth increases. Figure: Sigrid Salo

One of the oceanographic features  we always look for in a vertical profile is the oxygen minimum zone (or OMZ); this is the region of the water column that has the lowest oxygen level. It may be surprising, but this is not necessarily near the sea floor.  Often we see the lowest oxygen concentrations somewhere in the middle of the water column, and then closer to the bottom of the ocean the oxygen concentration starts increasing again. The OMZ is created by bacterial respiration, when bacteria use up the oxygen in the water around them for metabolism. As you move below the OMZ in the deep ocean, there is less bacterial respiration and some inflow of oxygen-rich water, leading to higher oxygen concentrations than in the OMZ above. Low oxygen areas are usually the areas that organisms avoid, for the obvious reason: lack of oxygen. However, scientists have learned that some organisms, such as the Humboldt squid, have developed a unique way to use the oxygen minimum zone as a refuge for protection from predators who typically avoid the OMZ.

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Dale Hubbard, taking a water sample from the CTD. Focused and jolly. Photo: Emma Hodgson

To measure oxygen levels onboard, we use a method called a “Winkler titration.” Once the CTD comes up with water samples from different depths, the race begins. Oxygen is the first thing that needs to be sampled from the Niskin bottle. This is because oxygen is a gas, and as soon as air is let into the top of the Niskin bottle (while water goes out through a stopper at the bottom of the bottle into our sample jars), oxygen from the air can start to infuse into the seawater and change its oxygen levels, giving us an inaccurate measurement. After the water is sampled, the oxygen level is measured by our onboard oxygen-gurus, Dale Hubbard or Carrie Weekes. In a Winkler titration, the dissolved oxygen in the sea water sample is bound to a combination of a manganese and a strong base (the opposite of an acid).  A solution containing iodide ions is also added to the sample. When the oxygen binds to the manganese, an orange-brown solid compound is formed – that is what you see in the picture below! The more brown, the more oxygen.

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Oxygen bottle samples from line 21, bottles from left to right are ordered going from deep to shallow waters. Photo: Emma Hodgson

After the formation of the brown solid, a strong acid is added to the sample flask. The acid causes the brown solid to re-dissolve. This in turn generates iodine, which is much more easily measured than oxygen. Oxygen is present in the sample in direct proportion to the amount of iodine. A chemical called thiosulfate is added to the sample according to the amount of electrical current generated by the iodine. The thiosulfate converts the iodine back to iodide; we can measure this conversion from the electrical current the iodine generates. This addition of thiosulfate is known as a titration, because the amount of thiosulfate added depends on the amount of iodine. In reality, we are actually titrating iodine, not oxygen, but because the amounts of iodine and oxygen are directly proportional to one another, titrating the iodine and measuring how much iodine is there allows us to then calculate how much oxygen is present.

But you might be asking, how do oxygen levels off the continental shelf change with increased warming from climate change? In the past 50 years we have seen OMZs (the low oxygen areas) increase in volume and get closer to the surface; this means that the region of the ocean from the surface down to these low oxygen zones is becoming compressed, reducing the available habitat for some marine organisms. Researchers have observed the increase in volume of oxygen minimum zones and project that this will worsen going into the future.

Author: Emma Hodgson

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