Window to the deep

Today our mission was a 40 minute snowmobile ride to Pegasus, a sampling area out on the thick sea ice where two large holes were drilled in last November. This required some "Kill Bill" action in my virus bodysuit to prepare for the ride.
At the ice, our plan was to send down 2 plankton tows to collect organisms in the water column. The sea floor was about 500 meters below us, but our lines only reached down around 100 m. We also sent down a CTD (Conductivity-Temperature-Depth), which is a device that directly measures the conductivity, temperature, and depth of the seawater, twice a second as you lower it down into the water.

Conductivity is a measure of how easily electric currents pass through water, and this value is used to calculate salinity. Electric currents pass more easily through water with high salt content than through fresh water...so once we know the conductivity, we know the salinity, measured in psu (practical salinity units).

The temperature is measured with a thermistor in the CTD housing.

Pressure is measured with a pressure gauge. This is useful for the depth calculation, since depth and pressure are directly related. As you could imagine, the deeper you go, the greater pressure there is at a given point. Because this relationship is constant, a measurement of pressure (in decibars) can be converted to depth (in meters). Furthermore, since the relationship between meters and decibars in approximately equivalent, at "X" meters in the water column, the pressure should be "X" in decibars. For instance, at about 500 m below the surface, the pressure is right around 500 db.

Combining these parameters: salinity (S), temperature (T), pressure (P), you can calculate the density (∂) of water at a given point. This is possible with an equation of thermodynamics called the "Equation of State of Water." This equation is used used to understand the physical properties of fluids (ex: seawater) and mixtures of fluids (ex: seawater and fresh water). This is useful when we try to understand the biology, and biological constraints, in different environments, where these parameters differ from what we are used to here at "atmospheric" pressure.
We had some slight problems with "traffic" at the holes...the Weddell seals that spend their lives under, on, around sea ice, had found our holes and have been using them as "breathing spots" since November. They came up for about 3-5 minutes at a time, taking deep slow breaths. At the holes, they alternated between checking us out and scouting below them...the males are known to bite the genitals off seals at breathe-holes, which obviously has a major affect on their ability to reproduce, and thus their contribution to the gene pool.