…and water sample collection

During some of the CTD profiles we also take water samples. As I am in the ‘CTD command centre’ almost full time during the sections, I asked Pat Ryan to write something about the water sampling. Here is her story:

Water Sample Collection, by Pat Ryan, University of Delaware

One of the ways oceanographers study the sea is through the collection of water samples.  Chemical oceanographers evaluate the characteristics of water to provide information on the source of the water, contaminants and conditions of the sea to sustain life among other things.

The collection of water aboard the Larsen is done in a contraption we call a rosette.  It’s comprised of a number of Niskin bottles (large cylindrical plastic bottles), in our case 12, held upright in a large round metal frame.  Our rosette is about as tall as I am (170 cm).  As the device is lowered into the ocean, all of the bottles are open at both the top and the bottom so that there are more cylinders than bottles. As the rosette is lowered, water flows freely through the open bottles as if they were pipes… stoppers at each end are connected to a tension-based triggering device that can close both ends instantaneously – thereby capturing the seawater residing at the depth of the rosette at the time of firing.  On the deck, connected via wire to the rosette, is a computer that sends a signal to each of the bottles when the operator wants to capture water. 

The CTD rosette on its way down, bottles open, in Nares Strait 2009.

Our rosette is typically lowered at a controlled rate to the bottom of Nares Strait collecting samples on its descent and then quickly brought back up.  Attached to the rosette is also a CTD device that continuously provides salinity, temperature and depth information to the operator.  Physical oceanographers use these data to analyze aspects of the sea and for the water collection purpose this information is vital to collect samples (that is to trigger the closure of bottles) at specific depths.

All of the heavy work of wrangling the rosette off and then back onto the deck of the ship is done by the deck crew of the Henry Larsen.  Garbed in bright orange jumpsuits and hard hats, the crew pilot the heavy and cumbersome rosette to a gentle landing after each cast.  They are a great group of guys who regale us with tales of the sea, and places back home (Newfoundland for most of them) and make us laugh to the point that we sometimes even forget how cold our hands are!

While the deck crew is managing the heavy work of the rosette, one of the science team members, Jo, mans the winch, hoisting, lowering and then raising the rosette up as directed by another member of our team, Renske.  She operates the computer, carefully monitoring the graphics which tell her where the rosette is at all times.  It’s Renske who determines when each of the bottles fire.

As soon as the rosette is safely back aboard, the piddlers get to work.  We are members of the science crew assigned to get the samples from each Niskin bottle into the pre-labeled sample bottles.  First, we check that the bottles have fired properly and that there are no signs of leaks that would indicate a sample was compromised.  Then a group of us (3 is a nice number – as it speeds up the process and is not so many that there are traffic jams on the deck) set about the task of filling various glass and plastic vials with very cold ocean water, these will be shipped to a lab for analysis when we port.  The label in each bottle indicates the location and depth at which the sample was collected as well as which type of analysis is to be performed.  On this trip we are sampling for O-18, Barium, Salinity and Nutrients.   Protocols to insure the integrity of the sample for each analysis are followed – some of the samples must be immediately frozen and kept at very cold temperatures.  Others require specialized gaskets to prevent oxygen transfer.  Duplication of some samples provides for quality control check performance.   Every time the rosette is deployed, we typically fill about 50 bottles.  Each of the bottles is rinsed with the sample several times prior to being filled – as we dump this rinse water onto the deck, this process can lead to wet shoes and socks when the wind is blowing hard as it was yesterday. In the Arctic, this water piddling as it is “affectionately” dubbed can be chilly, wet work.  Your hands can feel like you might not get sensation back for days and on a windy day (as we had yesterday) the water can be blowing in sheets to soak you.

That being said, it can also be an enjoyable group endeavor.  Our lead scientist often joins us filling bottles and Renske will lend a hand as time permits.  There is a frenzy of activity when the rosette alights upon the deck that feels like work but the camaraderie of the piddlers and the tendency to easy laughter among the group actually makes it some of the fondest (if perhaps coldest) memories of my Arctic experience.


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