PS113 - Weekly Report No. 5| 4th -10th June 2018

The Final Strai(gh)t

[11. June 2018] On 5th June after leaving Las Palmas on Gran Canaria we again reduced our speed for 5 hours so that we could tow the Triaxus/topAWI for one final test. Thereupon followed a final comparison station with CTD and the optical measurements package. Thereafter, we were supposed to head to Bremerhaven with all haste and without further ado.

However, on the 6th June at 13:20 Polarstern was informed by the Maritime Rescue Coordination Centre in Lisbon of a Mayday call from an English yacht and reacted immediately with a change of course towards the supposedly stricken vessel – 32 miles to the south, against the direction of our homeward course. From 18:20, Polarstern carried out a systematic search in the allocated area. At 19:15 the search was called off by the Coordination Centre without further explanation. Polarstern resumed her homeward course, the first 32 miles now for the third time. It goes without saying that we should respond to such a call for help at sea.

Fig. 1: CTD-Team monitoring the incoming data. (Photo: Alfred-Wegener-Institut)

Fig. 2: Radar-Antenna used by WaMoS. (Photo: Alfred-Wegener-Institut)

Fig. 3: Quality control of the WaMoS-Data. (Photo: Alfred-Wegener-Institut)

Fig. 4: Filtration of the seawater samples for genetic analysis. (Photo: Alfred-Wegener-Institut)

To recoup the lost time and arrive punctually in Bremerhaven Polarstern is now putting in extra effort. So are the scientists on board, busy with packing and stowing of equipment, tidying up and cleaning the laboratories, as well as writing reports. Some groups are nevertheless continuing their underway measurements until shortly before arrival in Bremerhaven.

During virtually all the cruise the computer system “Rutter s6 WaMoS” was used to process radar images of the sea surface to calculate the sea and swell as well as the surface currents. The method needs wind and waves; if there is little wind or heavy precipitation it does not work. The work with this WaMoS system was principally to investigate the influence of environmental conditions on the quality of the measurements. Amongst other things the currents were compared with those measured in the depth range 20 to nearly 300 m with the acoustic Doppler current profiler (ADCP) mounted in the ship’s keel. Throughout the greater part of the cruise track both systems showed very similar current speeds and directions. Regarding this comparison it should be noted that WaMoS measures the current in the uppermost 5 m of the water. From this, it can be understood that the Equatorial Undercurrent, which flows with 1 m/s towards the east at depths between 50 and 200 m, was measured by the ADCP but not by WaMoS. In the Spanisch EEZ around the Canaries only WaMoS delivered current measurements, as we had switched off the ADCP to avoid acoustically disturbing the bottle-nosed whales which occur there.

Almost all the cruise up to this morning samples of sea water were taken every three hours from the supply pumped into the ship at 11 m depth. From these samples particles and cells were filtered for genetic investigation. Even if no living matter is visible to the naked eye, there are on average a billion single celled organisms in a litre of seawater. They make up a majority of the oceanic biomass. These marine microbes are not only responsible for the majority of the oceanic primary productivity, but they also influence the chemical composition of our atmosphere, our climate, recycle nutrients and break down pollutants. Because different species of microbes have different effects, it is important to know the species composition and variety of these smallest organisms. Using the genetic fingerprint of marine microbes, their occurrence and species composition can be decoded. How and whether this corresponds to the various ocean regions which we have crossed during our cruise will become apparent from laboratory investigations ashore.

This is my last Weekly Report – for this cruise, and forever, as my retirement is gradually approaching.

With Best Wishes from on board Polarstern to all relatives, friends and colleagues at home; we are looking forward to being reunited with you.

Volker Strass with Harry Leach, Katrin Hessner and Cora Hörstmann