The Kimberley, Part 2 – A Biologist Attempts Water Chemistry

The invitation to go to Camden Sound came only a few short weeks before the trip was due to take place, so I had no time to waste. After I got the ok from my supervisors I had to work out what I was going to do. I wanted to sample oxygen and strontium isotopes, and trace elements. Oxygen and strontium are both measures of salinity but oxygen can also vary due to things like latitude, depth and temperature. We have had some interesting results from our comparison of oxygen and strontium in the otoliths so trying to understand their relationship in the water itself will be a great help. Trace elements are supposed to vary geographically and are used as ‘tags’ to locate fish to specific sites, but this can only be done when you know the trace element composition at each site.

I emailed around to find labs that could analyse the samples and that could tell me how to collect them. Oxygen isotopes were sorted fairly quickly – I only needed 20ml water filtered through a 0.45 𝜇m filter and stored in a glass vial. Strontium should have been equally straightforward except for my chemical ineptitude which meant that I had multiple back-and-forths between the lab to try and work out what type of container I needed as there were multiple types of plastic available and I had no idea which would be acceptable. All of this was compounded by the fact that I needed to make sure I could get the sampling equipment before I left which isn’t always easy if the suppliers have to bring things in from the east coast. Eventually we got it sorted and oxygen and strontium samples were a go. For the trace element sampling I was getting questions about anions and cations and various other things I had no idea about and I just didn’t have time to work it out so they will have to wait for someone else. But strontium and oxygen are a great start.

My sampling equipment arrived on at the end of the week and I got myself sorted. It was pretty basic and hopefully foolproof in its simplicity:

Sampling equipment for one sample

Collecting the samples was nicely straightforward. For samples in deep water we used a ship-based CTD, an instrument that measures conductivity (a measure of salinity), temperature, and depth. It can also collect water samples on command in 10L niskin bottles which are then brought back onto the ship and can be collected in the comfort of the lab.

Collecting water samples from the livewire CTD with my ever-helpful assistant Becky

For samples in shallow water we had to use a more primitive form of niskin bottle. No-one could remember the last time they had been used and their lack of maintenance was showing when we took them out for a practice run. Niskin bottles are essentially tubes with spring-loaded lids on each end. These portable ones attach to a weighted rope at the required length above the weight which then goes into the water. Once the bottom has been touched by the weight a ‘messenger’ – a small weight - is sent down the rope which hits a button which triggers the lids to close. Or that’s how they’re supposed to work. The first one wouldn’t work, the second one almost worked then broke, but third time lucky and we got our first water samples. From then on things worked smoothly.

The niskin bottle set and ready to be deployed

Sampling consists of pouring water into ‘pre-contaminated’ (which just means rinsed with the water that we’re sampling) 125ml plastic bottles for the strontium analysis and into syringes with filters attached which then squeeze water into 20ml glass vials. We used a CTD at each site to take conductivity (a proxy for salinity), depth and temperature for each site and once back on the boat the samples get placed in the fridge ready to send to the labs for analysis.

Collecting water samples with the help of Marcus

We sampled at 13 sites over the course of the six days we were in Camden Sound. Even allowing for travel time to and from each site, preparing the equipment and doing the camera surveys this left rather a lot of downtime. So what did I get up to when I wasn’t advancing science? Well, I was having a rather good time…