Entry 08
February 25 2025
Our first CTD, swath bathymetry system and laying moorings. Oh how I've missed these deployments
Today brought with it our first CTD! Through we weren’t looking to sample our full smorgasboard of filtering delights, it was very exciting to see that depth profile for the first time on this cruise, and allowed us to do a number of tests through our respective rigs to get a feel for filtering speed and biomass.
‘Filtering’ is not actually as straightforward as it sounds – there are a number of factors at play and it’s important to find a happy medium to get high quality samples. In my case, I need to sample enough volume to collect a high enough biomass to enable me to do genetics analysis, but I also need to make sure this happens quickly enough that the DNA and RNA doesn’t degrade. With that in mind, it takes time to push my samples through the filters – and this time increases with the amount of plankton in the water. So I need to have the pressure high enough that I can filter enough volume in a short enough time, without causing excessive shear stress that either dismantles a) my filtering rig, or b) the plankton I’m collecting! There aren’t really hard and fast rules about any of these things, it mostly comes with experience, which is why having days like this is so useful, as it enables us to trial run all these things.
In the afternoon it was time to put the first set of moorings in. These are a really awesome deployment – essentially a 4.3 km stretch of wire lined with various instruments such as temperature loggers and current meters. We lay them out over about four hours, horizontally on the surface so you can see them stretching behind us for miles. At the same time the ship’s swath bathymetry system continually maps the seabed to assess the best place to drop the mooring. Once the whole length is laid out, the bottom (ie the end still on the ship) is attached to an old anchor chain, and on the order this is dropped to the bottom of the ocean. As this happens the whole 4.3 km of mooring essentially moves towards you as it follows the anchor down to the bottom of the ocean, and is now totally vertical in the water. Overall this system measures temperature and currents in the water, and will be recovered in a couple of weeks, along with its data.