Upper mixed layer, UML

The principal features of the vertical T,S structure of the Black Sea waters are shown in Fig. 3. The upper mixed layer (UML) of the Black Sea in the warm period of the year has a thickness less than 10 m (see Fig. 3a). At this time, it is underlain by the layer of the seasonal pycnocline (ther-mocline) this layer is also thin (10-20 m) but features high vertical gradients of temperature (0.2-0.3 °Cm 1) and, correspondingly, of water density... 

Phytoplankton can be exposed to an in situ field of radiation by using UV-transparent (see section 11.2.3) bottles hanging from a line or tubes placed in trays (Figure 1) which are incubated at different depths in the water column [18,20]. One disadvantage with this approach is that phytoplankton cells are kept at a fixed depth for the entire incubation period (e.g., few hours), thus receiving a constant proportion of the surface incident radiation. In the water column, however, cells are moving within the upper mixed layer (UML) and thus exposed to a variable field of irradiance [21] (see Chapter 4). So far, few studies have addressed the importance of mixing rates on the phytoplankton photosynthesis [21-23], and with the exception of the experiments performed by Marra [24] on the effects of PAR, we are not aware of such studies done under in situ conditions. 

---