Day 5: Not your ordinary bottled water
By Chris La Rosa
To understand the ocean from a chemical, physical and biological standpoint it’s incredibly important to gather as much information about the seawater itself, as this has such a tremendous effect on everything else. Within the water column there can be huge variations in certain parameters, but how do you go about collecting samples to analyse? On this expedition Dr Dirk Erler and his team from Southern Cross University are utilising CTD (Conductivity, Temperature, Depth) instrumentation to acquire discrete samples from throughout the water column in the deep sea.
CTD being deployed from the RV Investigator.
How does it work? Seawater is collected in an array of 24 large cylinders called Niskin bottles, with each 24 L bottle about a metre in length. These heavy-duty, plastic cylinders have stoppers on either end. They are arranged around the outside of a metal frame known as a rosette. A number of sensors are attached to the rosette relaying information to the scientists in real time. Conducting a CTD deployment, commonly referred to as a cast, begins when the ship comes to a stop over the designated site where sampling is to occur. Once it has stopped, a large metal door opens on the side of the ship and a mechanical arm moves the rosette outward over the water. The rosette, containing the many Niskin bottles and other sensors, is lifted and carefully manoeuvred into position several metres above the water’s surface using a powerful winch and metal cable. Before it can be deployed, it must reach a sufficient distance away from the side of the RV Investigator and as it does the rosette sways with the pitch and roll of the ship before being gently lowered into the sea.
Lowering the CTD into sea
As the CTD rosette descends into the water column, both ends of the Niskin bottle are kept open allowing water to pass freely through until it reaches its final depth. On this current CSIRO voyage, that has been as deep as 2000m. The crew can watch the change in seawater characteristics as the rosette falls through the ocean. Technicians aboard the ship then go about closing the bottles remotely and capturing discrete water samples at specified depths. As the instrument is being retrieved, the researchers will communicate with the ship’s crew in order to stop the winch at each specific depth so that they may close the stoppers on the designated bottle and gather their water samples.
CSIRO hydrochemists and Southern Cross University researchers retrieving water samples from the CTD.
When the rosette is hoisted back aboard the ship and the exterior door is closed, the team of research scientists move quickly to retrieve the water samples and begin their analysis in the lab.
Dr Dirk Erler preparing water samples from the recent CTD deployment.
Analysis of water samples in RV Investigator’s onboard wet lab.
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