Under Saharan Dust

After this stop, Polarstern continued towards Cap Verde with calm weather and rather low visibility due to dust plumes from the Sahara. The whole way, the dust clouds were present resulting in some spectacular sunsets. On this part of the cruise, samples were taken only from the moving ship along the cruise track. Every morning an XBT (Expendable Bathythermograph) was deployed to record the thermal stratification of the water columns (Fig.1), and water samples were taken from the sea surface for plankton analysis. The students were very busy in their groups to work on the samples from the last stations on the Ampere Sea Mount and to analyse the data. Five groups were formed at the start of the cruise who investigate oceanography, phytoplankton, zooplankton, remote sensing and one group works on communication. In each group six students work together and rotate to a new task after six days (Fig. 2). This way all students have a chance to experience the different subjects.

So far we have encountered three significant water masses: Mediterranean Outflow Water (MOW), North Atlantic Central Water (NACW) and North Atlantic Deep Water (NADW) that can be distinguished by their temperature and salinity characteristics. The signature of the MOW gradually decreased on the way to Cape Verde. The region we were crossing is an “ocean desert” with deep blue colour. There is very little plankton in the surface waters and only at the thermocline some phytoplankton is present. The CPR (Continuous Plankton Recorder) which is towed continuously at 20 m depth behind the ship shows only occasionally somewhat greener colours on its silk filter and collects only few zooplankton. When we approached the Cape Verde Islands more plankton was found and the filters became greener.

Finally the Cape Verde Islands appeared at the dust-veiled horizon (Fig. 3). At Mindelo, the capital of Cape Verde, the two technicians who successfully repaired the satellite antenna and an inspector of the ships agent left the ship. Within the over 3000m deep central region between the islands of the archipelago we carried out a night and a day sampling station to investigate the diurnal migration of the plankton. The ship’s lights at night attracted large squid and many flying fish; large Salps and Sargassum drifted by. It became obvious that the ocean around the archipelago of Cape Verde is much more productive than the surrounding ocean.

Another encounter at sea happened at Cape Verde: Maria S. Merian just left Mindelo en route to the Canary Island and we exchanged greetings from ship to ship by radio. Four birthdays were celebrated on Saturday night, 14th November, on the helicopter deck and all were happy to join and celebrate together.

Best regards

Karin Lochte