January 5 – CTD Stations

Post by VIMS Graduate Student Mar Arroyo

1 – 5 Jan 2017
65° 7’ S, 121° 19’ E

The CTD rosette as it is lowered off the side of the ship.
The CTD rosette as it is lowered off the side of the ship.

The CTD rosette is a large metal frame with mounted sensors for Conductivity, Temperature, and Depth (along with any additional sensors such as chlorophyll, oxygen, turbidity, etc). Around the edges are 24 heavy duty plastic Niskin bottles that can hold 10 liters of seawater, each with spring-controlled caps on both bottle ends. The caps are hooked up to a trigger system that is electronically controlled from the instrument room on board the ship. The instrument room is essentially the brain of the operations. The CTD is lowered off the side by a cable to just above the sea floor, which could be over 3000 meters deep in the waters we are sampling!

View of the computer screens in the instrument room. A watchstander will ‘talk’ to the CTD from these computers.
View of the computer screens in the instrument room. A watchstander will ‘talk’ to the CTD from these computers.

Data from the CTD sensors is transmitted up the cable to computers on board. This allows us to see different features in the water column in real time, such as changing temperature or low oxygen. As the CTD descends, we choose what interesting depths (or bottles) we want to sample from. Bottles are closed on the way back up to the surface, capturing the water from that depth.

Me sampling for total CO2 from a Niskin bottle. After I collect the sample, I add an aliquot of mercuric chloride as a poison to kill the critters living in the seawater. This ensures that there will be no CO2 released into the water sample by respiration.
Me sampling for total CO2 from a Niskin bottle. After I collect the sample, I add an aliquot of mercuric chloride as a poison to kill the critters living in the seawater. This ensures that there will be no CO2 released into the water sample by respiration.

As soon as the CTD is back on deck, water sampling begins! Sampling at the rosette is like a strategic dance, with everyone taking their turn at each Niskin bottle at the right time. Volatile gasses, like oxygen and total CO2, are sampled first, followed by other parameters, such as total alkalinity, salinity, and nutrients. On this voyage, I’m sampling for total CO2 and total alkalinity.

Time is of the essence when sampling. We’re usually at the next station by the time sampling is finished and ready to put the CTD straight back into the water. Different stations are sampled around the clock. We’ve completed 20 stations so far in the Dalton Polynya!

Mar