Water layers on a sonar unit .

Heres something I noticed yesterday , that I have not noticed before . The first thing I always  do is check  the thermocline . Yesterday it was showing strong at thirty foot deep . Most of the fish in the lake were showing  in the ten to fifteen foot range and all the fish I caught were shallower than ten foot . Heres the strange part, on the down scan there was another layer at ten foot , it wasnt strong like the thermocline but it was there . It showed up on the downscan and not the normal screen . Dont know what caused it . algae bloom maybe ? I'm fairly new with downscan and maybe its so sensitive that  this is normal .