In this logbook entry, we want to take a closer look at the oceanographic depth sounder that has already been mentioned several times. It is the most important tool in measuring oceanography, with which horizontal stratification and currents of water masses can be observed. This information is needed to explain physical mixing processes and the distribution of dissolved substances, which in turn form the boundary conditions for the occurrence of marine life in certain regions and water layers. As can only be hinted at here, the physics and chemistry of the ocean are thus very closely intertwined with biology, the life in the sea. In the marine sciences, therefore, a cross-disciplinary approach is necessary.

Oceanographers often call this depth probe "CTD" because of its combination of sensors, which stands for electrical conductivity (conductivity), temperature and depth (depth). Seawater conducts electricity very well because of its salinity. Conversely, the total content of salts dissolved in seawater can be determined by determining the electrical conductivity.

The variation of water temperature and salinity over depth, i.e., the depth profile is crucial for the stratification or vertical mixing and movement of water masses. The combination of water temperature and salinity results in a certain water density. The following applies: cold water is heavier than warm water and high-salinity water is heavier than low-salinity (sweeter) water. Stable water stratification occurs when water of lower density lies above a mass of water of higher density. This can occur, for example, when surface water is diluted by freshwater inputs from rivers, rain, or melting ice.

In contrast, various dynamic processes can lead to unstable stratification. For example, the cooling of near-surface water masses in polar regions or the formation of sea ice, which freezes as freshwater and precipitates its salt in the process, can increase the salinity of the remaining seawater. Lateral mixing of one water layer into another can also create a higher density water layer at the sea surface. Such unstable stratification usually results in vertical movement (advection) of water masses.
The horizontal or vertical transport of water masses also transports dissolved substances that are important for marine life. An example is oxygen, which is taken up by the contact of seawater with the atmosphere and transported by advection to the seafloor, thus enabling "aerobic" life there. Conversely, in "upwelling" areas, nutrients are transported from the bottom to the top, thus again causing new algal growth in the surface ocean, which serves as the food base for many other marine organisms.
Modern CTD probes are often supplemented with additional sensors, e.g. for measuring oxygen content, pH, turbidity or chlorophyll content. The CTD probe that Sea & Sun Technology provided us with for the Ocean Change expeditions can measure oxygen and chlorophyll (actually green pigment in water) in addition to the standard variables of temperature, salinity and depth.

This measuring device is a technical masterpiece: highly precise individual sensors are arranged in a very small space on the underside of the device. They are controlled by a computer, which - like the data memory and battery supply - is housed in a pressure-stable titanium casing. Connected to a notebook, the probe can be programmed and its data read out. To switch the device on and off, a magnetic pin is simply held against a marker, so that we do not have to work with the notebook on deck in bad weather.

In the CTD profiles published on GEOMAR's BELUGA page, it is often easy to observe that chlorophyll values and oxygen saturation follow a similar course. This is due to the fact that algae release oxygen during their photosynthesis, in which they absorb sunlight and CO2. Statistically, every second breath worldwide comes from algae in the sea.

Die Profile zeigen auch gut, wie die Wassersäule bei starkem Wind oder an Stellen mit starken Strömungen durchmischt wird: in diesem Fall bleiben die Messwerte von Temperatur und Salzgehalt über den durmischten Tiefenbereich gleich. Oftmals lässt sich in unseren CTD-Profilen meist direkt unterhalb einer durchmischten Oberflächenschicht eine sog. Sprungschicht erkennen. Hier ändern sich v.a. die Messwerte für Temperatur, Sauerstoff und Chlorophyll sehr schnell. In den schottischen Gewässern, die wir derzeit befahren, wird Wasser aus dem Atlantik in die Nordsee und ihre Randbereiche eingemischt. Durch die Topographie des Meeresbodens bedingt, ist diese Durchmischung teils so intensiv, dass wir selbst in flachen Bereichen zwischen den Inseln Strudel an der Wasseroberfläche beobachten konnten. Am südlichen Kap des Mull of Kintyre und besonders eindrucksvoll am Coreywreckan. Diese Strudel sind teilweise so stark, dass sie kleineren Schiffen gefährlich werden können.