One of the more interesting features of the perennially ice-covered lakes of the McMurdo Dry Valleys in Antarctica is the fact that very little mixing of the water column’s take place.  In Lake Hoare, the water is pretty much the same density throughout and is well mixed, except near the deepest  point in the lake, where slightly denser, warmer water (4°C) occurs. The water becomes anaerobic here as well and hydrogen sulfide is produced by the microorganisms present.  Just at the interface between the oxygen rich water and the anaerobic water is the region called the chemocline.  Here is a gradient between the oxygen rich water and the oxygen poor water, and its also the home of purple sulfur bacteria that consume the hydrogen sulfide.  This video will take you into the cloud of microorganisms present in these density layers.  You will pass through to the anaerobic bottom (flat and black with white particles of sand sprinkled on top) and then back up through the various layers into the dark blue water leading to the surface some 27 meters above.  Keep in mind, the ice on top the lake is almost 4 meters thick and light levels at these depths is very low. This video is best viewed in a DARK room!  Enjoy.