Today, we’ve been at sea for exactly a month, and have just finished sampling at the final, Northeast station. As Espen described yesterday, there’s a palpable change in the atmosphere onboard with everyone’s thoughts turning to their lives back on land. Today, however, it was decided by the captain that we would return to Fairlie, on the Clyde, instead of Aberdeen. This is because of the threat of a storm to the north of the British Isles at the weekend, and also because we are running short of fresh water. Unfortunately, the change in port means a lot of last-minute logistical problems, as people have to change details of how to transport themselves, their equipment, and samples back to where they live and work.
Although the end is in sight, there is still a lot of interesting science being carried out onboard. Overnight, Andy Dale and Colin Griffiths carried out “Yo-Yo CTDs” which, as the name suggests, is when the CTD is continually raised and lowered throughout the water column. This was carried out for 16 hours, in an area close to where we have deployed the long-term mooring. The Yo-Yo CTDs examined how temperature and salinity varies throughout the water column over a complete tidal cycle, and will fill in the gaps in the data provided by the mooring.
As the moon travels round the earth, it pulls the ocean, which uses up a lot of energy and causes tides. This process is slowing the moon down, and causing it to move closer to the earth, which it may one day collide with (a very long time in the future). Although this is a small effect, it is measurable. Tides, in turn, lose energy because of friction, which is very big along ridges compared to the ocean margins, although the importance of ridges in this process wasn’t fully appreciated until recently. From the density (which is a function of temperature and salinity) anomalies identified in their data, Andy and Colin can calculate the amount of energy that is being “extracted” from the tide by the Mid-Atlantic Ridge. This is also part of their ongoing research which aims at understanding the circulation of deep water in the North Atlantic.
At the same time as the CTDs were being carried out, the EK60 echo-sounder was being used to examine how the zooplankton zonation changed overnight. It is well known that zooplankton moving closer to the sea surface at night, generally to feed on phytoplankton, then back down again during the day. Given their small size (often only a few millimeters long), some zooplankton migrate over very large distances indeed. Last night, we observed two zones of zooplankton, one which is normally at around 300 m depth during the day, and one which is normally found at 150 to 50 m, which both moved to the surface during the night, then back down again as the sun came up.
After the long CTD deployment was complete, it was time to retrieve the two landers that were still on the seafloor: the PAL lander and the Amphipod trap. The Amphipod trap, which is like a very small lobster creel, came back full of small scavengers, which will be examined in great detail by Tammy Horton in Southampton, who specializes in these fascinating creatures. This marked the last physical sample that we will obtain during this cruise.
Following the return of the landers, we are now carrying out a final EK60 transect. This will finish around midnight, when the ship will finally turn towards the Clyde and start the journey home.
National Oceanography Centre, Southampton.