Temperature and salinity charcterize the physical properties of the water column

The figure below shows vertical profiles of temperature (red line) and salinity (blue line). These two variables show the typical vertical stratification in summer. There is a sharp thermocline at about 22 m, where the temperature drops from 15°C to 4°C in just 2-3 m. You can also see that a secondary thermocline has developed further up between 10 and 15 m, with warm surface water of 18°C above the "summer water" of about 15°C. Such multiple thermoclines can develop when warm and stable weather prevails for a few days.

You can see that also see that salinity is fairly constant at 6-7 until about 70 m, and then increases. This is the salinity discontinuity layer, or "halocline". The temperature profile below 70 m more or less follows the halocline, meaning that the water body below the halocline is warmer than the overlying "winter water". This relatively warm, high saline water is a "remnant" of earlier salt water intrusions from the North Sea. It is, nonetheless, old, stagnant water containing no oxygen anymore.

Usually, warm and low saline water is less dense that cold and high saline water, which produces a stable stratification. Thermo- and haloclines act as a physical barrier, so that mixing between surface and deep water is hampered.

Go on to the oxygen and hydrogen sulfide profiles...