Yesterday — 30 days after most of us left our homes — we reached 30 °S and our science operations finally began. A lot of our science uses a piece of equipment known as a CTD, which is sort of the oceanographer’s bread-and-butter. CTD stands for conductivity, temperature, and depth, which are three of the mainstay measurements of the ocean: from conductivity, you can calculate salinity, and from temperature, salinity, and pressure, you can calculate the density of the seawater. The CTD can have many other sensors equipped, though — ours has several optical sensors, a sensor to measure the intensity of sunlight, and an oxygen sensor.
But there are many other quantities that we’re interested in that can’t be measured by sensors. To measure things like the amount of dissolved carbon in the water, or the number of plankton of a certain size, we need to collect water samples and analyze them, either using machines on board or once we get back to dry land. So surrounding the CTD are 24 bottles made out of heavy-duty plastic, tough enough to withstand the crushing pressures in the ocean’s depths. The switches controlling these bottles are triggered remotely when the CTD frame, called the rosette, reaches a certain depth.
The water was almost impossibly clear for our first cast — we could see the top of the rosette down to about 40 meters, or more than 130 feet! Like I talked about in my last post, the clarity of the water is because there aren’t that many plankton growing in this part of the ocean.
Once the rosette was back above the water and secured on deck, there was a bit of organized chaos while everyone collected their water samples. There’s a very specific order that people take their samples in — it’s strict enough that one person, the “bottle cop”, has to stand off to the side and tell people when they’re allowed to take water from a particular bottle.
The reason the order is so important is that some of the gases we’re studying, like oxygen, equilibrate very quickly with seawater. As soon as the bottle is opened, the gases in the water can start escaping, affecting our measurements. These fast-equilibrating gases are the first water samples to be collected, while things that are minimally affected, like salinity, are collected last.
We did find out today that it’s very hard to socially distance when you’re all collecting water from the same rosette. Luckily, the weather is nice enough here that we can take our water samples while the CTD is still out on the deck and we have fresh air blowing through (once it gets colder or stormier, we’ll move the CTD inside a hangar so that everyone can sample safely). And even luckier, we only have two more days of following our strict covid protocols! It’s going to be interesting to have to re-learn everyone’s face once everyone gets to de-mask.
Giuliana Viglione is a journalist and science communicator who has joined Senior Research Scientist Barney Balch’s research cruise to study the impact of coccolithophores in the Southern Ocean. On board the ship, she’ll be helping the team carry out experiments, document the research cruise, and conduct educational outreach with students across the country. She can be reached at firstname.lastname@example.org.