The night shift

By Phoebe Hudson

Sampling nutrients

It’s 11 pm – time to wake up! I re-open my porthole and it’s now dark outside, with only the whitewash as a contrast against the dark sea background. I stumble around my cabin – getting dressed, brushing my teeth and hair – struggling to tell if I keep bumping into things because of my lack of balance in the morning or the ship’s swell!

I pop into the main lab on my way up to breakfast to check everything is going ok for the previous shift (and to work out how many CTDs we’ll be doing tonight)! It’s then time for a quick breakfast before my shift starts. My typical breakfast of porridge is almost as accurate as the radar measurement of significant wave height – with the milk sloshing back and forth as I put my porridge in the microwave. Then it’s back to the main lab to start the shift (and convince those on the previous shift that it’s time for them to go to sleep)!

As mooring recovery and deployment need to be done during the day, we as the night shift typically do back-to-back CTDs. Over the first few days of night shifts, when we were over the relatively shallow shelf, this reached up to 4 CTDs in one night! But it calmed down considerably over the next week to 1 or 2 CTDs a night, when we entered deeper water and each CTD deployment took considerably longer (~2-3 hours)! This was particularly true of the “caldip” moorings, used to calibrate sensors pre and post-mooring recovery/deployment, which have 5-minute stops at various depths.

It’s then time to prepare the CTD logsheet ready for the first CTD. If the CTD’s recovery is imminent or we’re lucky (and those on the last shift were bored) this will already be ready for the first CTD of the shift. It details the Niskin bottles fired and what depth they were fired at, as well as what Niskin bottles/depths we need to take water samples from such that we can analyse them for dissolved oxygen, and dissolved inorganic carbon (DIC), nutrients and measure their salinity. This logsheet also details the Niskins / depths from which duplicate or triplicate samples will be taken. We also write out a separate logsheet just for the dissolved oxygen samples that contains the bottle numbers of each sample (along with the associated Niskin bottle it will come from and the depth of that bottle) and another logsheet if we are also taking DIC samples.

Once the CTD is almost up, we pop on our waterproofs, steel toecap boots and hard hats and bring out everything we need to collect our samples. This includes 2 clipboards with all the logsheets, lots of numbered bottles with stoppers (for dissolved oxygen, DIC and nutrients), plastic tubes for collecting dissolved oxygen and dissolved inorganic carbon samples, a thermometer and spare beaker for measuring Niskin bottle water temperature, and Stanley (the carry crate that holds the pipettes with and in them, used for the dissolved oxygen samples).

We watch the CTD being recovered – on day 1, we all watch with avid interest and by day 10, it’s all just part of the routine. Once, we’re informed the CTD is securely back on board, it’s time to begin sampling! We start with dissolved oxygen – to limit the gas exchange of water in Niskin bottles and the surface air – making sure to close each bottle securely after sampling. First, we take each Niskin bottle’s temperature before rinsing the first sample bottle and filling it – hopefully without any bubbles!!! We then work our way around the CTD, starting with the first Niskin fired (at the greatest depth) and finishing with the shallowest Niskin.

If taking DIC samples, we then have someone (in our case me) follow that first person, making their way around the CTD rosette, collecting each sample. By the last day, we’d perfected the art of knowing when the DIC sampler should start, such that a Niskin bottle would never be open for 10 minutes but such that the DIC sampler never caught up with the dissolved oxygen sampler! The DIC samples then go back to the chemistry lab to be poisoned (and stop any phytoplankton still living in the sample from removing all the DIC!).

Next up is nutrients. Niskin bottle water goes through a filter to remove any large particles before entering the plastic nutrient tubes. These samples are then frozen, ready to be transported back to SAMS to be analysed.  

Finally is salinity – we obtain a sample from each bottle in order to be able to calibrate the conductivity reading from the CTD rosette. Each sample is collected and then the neck of the bottle must be dried (inside and out) to make sure no salt crystals can form on the top of the bottle and fall into the sample – artificially raising the conductivity (and salinity) measurement. The sample is then stoppered and capped, before going back into the crate of samples. Once a crate is full, it gets rinsed and carried to the salinometer room, where samples will later be analysed by the autosal salinometer. As conductivity measurements are very temperature dependent, the entire room is kept at a constant temperature (of 21C), such that all the salinity samples can equilibrate to this temperature before analysis.

Once we’ve successfully managed to collect all the water samples we need, we return to the main lab. We then need to scan all the logsheets to make sure we have a copy just in case anything happens to them. We also run a series of scripts to process CTD data, removing all the times when sensors were recording before and after the CTD was in the water and removing any spurious data spikes (particularly in salinity).

This cycle repeats until the waft of freshly cooked pastries informs me it’s almost breakfast time! I’m then faced with 20-30 minutes of temptation to run and grab a croissant/pain au chocolat whilst they are still fresh! It’s then time for breakfast and time to say good morning (/ good night / good day?) and to hand over to those on the day shift.

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