Sampling Seawater 101

By Jessica Bolin, University of the Sunshine Coast

Time is flying by – it’s day six of CAPSTAN Voyage #2, and we’ve now had the chance to explore different fields of oceanographic research during our group rotations: hydrochemistry, birds and mammals, plankton, geophysics, and sedimentology. Because we all have diverse backgrounds and areas of research, we’ve started to decide what our favourite stations are. My favourite, so far, is hydrochemistry, because we get to work with the CTD!

The CTD (conductivity, temperature, depth) rosette containing the CTD sensors and surrounded by 36 niskin bottles for discrete water sampling is deployed over the side of RV Investigator as part of the hands-on marine science training on RV Investigator through the CAPSTAN program
The CTD being lowered over the side

CTD stands for conductivity, temperature, and depth, and is one of the main pieces of scientific equipment that oceanographers use, because it measures changes in water properties throughout the water column. The CTD we’re using on board the RV Investigator holds 36 ‘Niskin’ bottles in a circular rosette frame, which is lowered from a huge boom on the starboard side of the ship, into the depths below. Upon deployment, each bottle’s plug is held open under tension by a spring-loaded metal hook. It’s an impressive piece of gear – the frame is taller than me (1.8 m), and each bottle can hold up to 12 L of seawater.

Bathymetric map showing the submarine shelf break canyons near Portland, Victoria in the Great Australian Bight.  Four major canyons are visible, one shaped like a Y is the main focus of our study.  The image shows shallower depths in red and greater depths in blue.  Bathymetric data was collected along with sediment and water sampling as part of the hands-on marine science training on RV Investigator through the CAPSTAN program.
Bathymetric map of our study area. Our site is the Y shaped canyon (third from the left). We’ve deployed the CTD at various depths along the canyon.

Our target site is a submarine canyon near Portland, Victoria, and we’re dropping the CTD at various sections along the canyon to further understand the circulation and other physical processes occurring within. As the CTD descends through the water column, sensors attached to the bottom of the frame sample the water’s properties, including temperature, oxygen, and conductivity; the latter which is used to calculate salinity. The data are pinged back to the ship’s operations room, where we all watch the vertical profiles of these parameters developing in real-time.

Fellow CAPSTAN student Sian sites in the operations room on board RV Investigator in front of several computer monitors to operate the CTD deployment.  She is watching the CTD parameters (temperature, depth, conductivity, oxygen, and chlorophyll) read out in real time to determine the depths at which to fire the niskin bottles and communicating with the winch operators as part of hands-on marine science training through the CAPSTAN program.
Sian sits in the operation room viewing the different CTD parameters in real time, and getting ready to fire the niskin bottles as the CTD ascends through the water column.

Once the CTD has nearly reached the bottom and starts ascending to the surface, each bottle is remotely ‘fired’ by an observer in the operations room at regular depth intervals. ‘Firing’ a bottle relays a signal to the CTD to release the hook on the target bottle’s plug, closing the bottle and trapping the water at that depth inside. After we have fired every bottle, the CTD is carefully retrieved by the crew and prepped for subsampling on deck… and the real fun begins!

CAPSTAN trainer Veronica Tamsitt samples a niskin bottle on the CTD rosette on board RV Investigator as part of the 2019 CAPSTAN voyage
Veronica sampling (very cold) seawater from a Niskin bottle

From each bottle, we’ll take three subsamples of water for further testing: one to test for nutrients, one for salinity, and one for dissolved oxygen, whilst also recording water temperature. From these subsamples, we can calculate the density of the seawater, which is a primary driving force for major ocean currents. It blows my mind to think that the water we’re working with has come from up to 2200 m within a submarine canyon, which in turn, has travelled along ocean currents all over the world!

CAPSTAN students Jessie, Imbi, and Jin Sol stop between sampling the niskin bottles on the CTD rosette for a selfie in front of the CTD on board RV Investigator with CAPSTAN trainer Veronica as part of hands-on marine science training
CTD selfie! The green team (Jessie, Imbi, and Jin-Sol) with trainer Veronica (far left)

It is safe to say that I’ve developed a new-found love for physical oceanography and all things ocean currents. The ocean is inherently dynamic, and constantly changes in real-time. Teasing apart the mechanisms underpinning the circulation within our site is both challenging and fascinating. Once we start processing and analysing our data, we’ll hopefully be able to pick up the signature of the Flinders Current that flows west along the Great Australian Bight, and perhaps internal waves from within the canyon. Also, with a bit of luck, *fingers crossed* we can pick up the signature of a deep-water ocean current, that Veronica Tamsitt – our token physical oceanographer on board – recently discovered in the Bight, and collect some much needed data to ground-truth the current’s existence. In short, we are discovering SUPER exciting stuff this voyage, so stay tuned!

Check out my group’s blog on The Field!

Jessica is a PhD student at the University of the Sunshine Coast. Follow her on Twitter @JessieABolin

Advertisements

6 thoughts on “Sampling Seawater 101

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s