by Claudia Thölen, Jochen Wollschläger and Michelle Albinus
Every day the ship stops for two stations where we can lower our instruments into the water and take samples or measurements. In the morning after breakfast, I usually go back on deck every day to see if I can help Kai and Neeske with the CTD and the water sampler. CTD means Conductivity, Temperature & Depth.
This is a sensor unit that is lowered into the water column along with a water bottle rosette for water samples, usually the entire device is just called “CTD”. The CTD is the first instrument to go into the water each day and is also often called the standard oceanographic instrument. The data from the CTD is sent live to Kai’s, the CTD operator, computer and the bottles can be closed remotely at various depths. While many of the scientists on board also have other equipment of their own, most are often busy with processing the water samples from the CTD in the lab for hours.
Once the CTD is ready, I join Michelle in the universal lab and take over the preparation of our photometer René for the duration of the CTD cast. We asked the crew’s system administrator if he could mirror the CTD’s screen into our lab. This way we can observe live how the temperature and salinity in the water column develops and if there are different density layers. Kai closes the water rosette bottles in the agreed depths or in depths where the signals from the sensors are particularly exciting. Once the CTD is back on deck, Neeske and I grab our two sample containers and return to deck. There, the first people are already scurrying around the CTD with their sample containers to take their samples. We have a set order, because some scientists work with gases and have to get to the bottles first so that the samples are not biased. We are also some of the first up with our optical parameters and tap off our samples. After we have brought the samples back to Michelle in the lab, Neeske supports her with the filtration (for suspended material and chlorophyll – the most important photosynthesis pigment from microalgae in the water), while I continue to work on René with the first samples.
Meanwhile, on deck, one or two different plankton nets are being deployed by other groups on board. As soon as these are ready, it’s time again for Neeske and I to go back on deck. This time armed with a black and white disk and a color scale. This disk is called Secchi disk which is lowered into the water column until it is just out of sight to estimate the depth of visibility. The Forel-Ule color scale is then used to estimate water color at half the depth of visibility. These measurement techniques have been around for a very long time, over 150 years, so they are very valuable for long-term trends of these parameters. The viewing depth and water color give us clues about the turbidity of the water and possible water constituents such as sediment (brown) yellow matter (yellow) and chlorophyll (green). If the water is turquoise blue, it is a sign of less content and life in the water. Neeske and I register the Secchi disk with the bridge. This is important for any device or object that goes into the water from the ship. The bridge logs everything. “Bridge for lab?” – “The bridge is listening!” – “The Secchi disk is going into the water!” – “All right, Secchi can go out.” And then the Secchi goes out. Neeske lets the disk into the water and we watch together as it slowly becomes barely visible. Then we measure the depth and lower the disk again to half depth to estimate the water color.
When the measurement is finished, we go back to the radio first: “Bridge for the lab, Secchi is done, next is the Profiler.” – “Okay, Profiler can go out.” After the announcement, Jochen, Kai and I go to the afterdeck, while Neeske disappears to the Profiler room to the Profiler computer. On the afterdeck two sailors are already waiting for us and help to get our next device ready. Normally it starts to rain right now. This has somehow happened over the last few days. But our measurements still work. The Profiler measures the light field under water. It is connected to our winch on deck via a strong Kevlar cable. The sailors bring a crane into position so that we can attach our pulley. My job here is to control our Profiler winch. It’s quite simple, as it’s just in and out, heave and set, and regulate the speed. Jochen holds the cable tight and I lower it a bit. Kai heaves the Profiler overboard and carefully lowers it into the water. When it is caught by the current, it drifts away and is held tight like a kite in the wind for now. We radio to Neeske that we are ready and when Neeske gives the command “Drop!” we release enough cable to make the Profiler fall as straight as possible. The sensors now measure what intensities of which wavelengths of light are still reaching the profiler during the fall. When Neeske says “Stop”, I stop profiling and Kai pulls on the cable to slow down the fall. Then the cable is heaved again with the winch. We repeat the whole process three times. If all casts were good, the Profiler can go back on deck and we continue with the next device.
Under another crane, BOP is already waiting for us. BOP is our Bio-Optical Package which measures fluorescence, light absorption and light scattering with many different sensors while being lowered into the water column. While Jochen takes over the control on the PC, Kai is responsible for lowering the cable. Neeske or I also help with the cable so that it doesn’t get tangled when we heave it again. In the afternoon, the one of us who isn’t busy with the cable can start rinsing the Profiler. Also, BOP will be rinsed with fresh water when it is back on deck. Also, the CTD bottles can be emptied again, as all people have tapped their samples by this time. After the morning station, the CTD will be strained directly again and also flushed with fresh water after the afternoon station. When we are done with BOP, it is time to go back to the lab and assist Michelle with the photometer René. Then it’s usually almost lunchtime (11:15) and since we are already totally synchronized to the ship’s meals, we then quickly go down to the mess hall with growling stomachs.