I don't know, tc ..... that looks more like plankton than a thermocline. Usually the thermocline level shows up as a streak across the screen at a given depth.
I've seen similar screens like that on my DI units .... at night, but the filled in area was from the surface down to 15ft (and the boat was close to a dock with a large halogen light shining into the water from the edge of the dock). The only time I've seen similar in the day is when the lake was flooded (muddy) and had settled out for a week or so.