Help explain this sonar return from daylight changes

I have a Humminbird Fishing Buddy depth finder I use on my portable Bass Baby boat. I've noticed something unusual but don't understand what is causing it.

I have been fishing this same small reservoir several times and see the same thing happen. During the day I get lots of fish ID returns over deep water as well as what I thought was either weeds or a denser cold water thermocline. it seems too early for weeds to me.

Then after the sun goes down the returns occur in mid-depth. Same numerous fish ID returns and also what looks like a thermocline. Here is a picture after the sun went down and the returns moved up.

http://i794.photobucket.com/albums/y...g?t=1399920223

That #8 Might be Flight 370 eh?

Sorry, my explanation above may not be clear... :santa

That band of returns in the center will usually be along the bottom while the sun is out. After sunset I see it as shown in the middle.

Yes, it could just be the fish moving but I think it is some other phenomenon. Those cloudy looking returns are odd. This small reservoir doe snot have schools of forage fish either.

Perhaps it is algae, plankton or something like that, that rises towards the surface to feed after the sun goes down.

^^^^What he said

Zoo-plankton feeds at night alot.They rise toward the surface to feed on the Phytoplankton. I have seen my DI screen look like it had the white measles at night. I have talked to Tim Broadbent (Tn Wildlife) and Paul Rister (Ky Wildlife) about plankton stuff I see on my sonar screens. I still know nothing .