Yes you can often see the thermocline but I don't think that is on this picture. There is no thermocline right now as the water is homogenized top to bottom. That is likely plankton or sometimes a mud line shows like that.
A thermocline starts to develop in the early spring in still waters where the water can stratify. It starts on the surface as the cold winter water is warmed and warm water stays on the surface and cold water sinks. The thin layer on the surface gets thicker as the summer continues to warm it. The only oxygen entering the water is from the atmosphere when cold water sinks and caries it down at "lake turnover" in Sept. & Oct at this latitude. The dissolved oxygen is depleted in the lower level below the thermocline slowly throughout the summer from respiration and decay of organic material.