The water's clarity has a great deal to do with the depth that the light can penetrate into the water. It probably has a much greater effect on how deep submergent vegetation can grow than on the water's temperature.

But the warming of the water is totally dependent on the sunlight heating the air and water.

Not really sure how the fish relate to the thermocline. Most thermoclines that I have seen while scuba diving in lakes are about 10 to 20 ft down.

I can tell you about one dive I made in a lake that was used for public swimming. I am talking about the old Wes Lake on the NW side of Evansville, IN.

I use to work there as a life guard when I was younger and was good friends with the owners. All the life guards were allowed to swim there after hours. So in the morning before the lake opened to the public I would often get my scuba gear out and go for a dive. The lake is about 5 or 6 acres in size. They had installed concrete docks for diving boards and a big tall trapeze. The docks were build on 25 ft tall concrete columns. There were four columns holding up the Trapeze dock.

One morning I was diving by one of these concrete columns. I held onto one of the docks ladders that was next to the column and was gradually allowing myself to sink down toward the bottom. I had gone down about 10 ft to 15 ft and could see some bass hanging right next to the column. My head was 10 to 15 ft below the surface but my feet were 21 ft beneath the surface and touching the colder water below the thermocline.

When you dive down you can feel the colder water.

I think that anyone that's dove down into a lakes depths knows that you can feel the colder water below. I use to free dive down to the bottom of the lake underneath the trapeze to search for drowning victim.

So this day I found the 1 and 2 lb sized largemouth bass suspended and associating very close to the concrete pillar (vertical Structure). The pillar was about 2ft by 2ft in size.

Now this was in an area of the lake that had the bottom deepened. The clay bottom was dug out to make this area of the lake deep enough to keep people from hitting bottom when the dropped off the trapeze and diving platform. The diving platform was about 4 meters high. At times when I went off the tepees and entered the water just right I could easily reach the bottom of the lake which was around 25 to 30 ft deep at that point.

Most of the other parts of the lake were about 15 ft deep. The lake bottom out in the middle part of the lake was muck. You could be swimming along the bottom out there and not see more than two feet around your face mask. A few times I was scuba diving out in the middle of the lake just exploring the bottom. I didn't see many fish out there. One in a while I would see a small crawdad flipping out of my view and leaving a mud plume behind him. There were a lot of crayfish living out on the bottom of this lake. So the bass had a lot to eat.

I fished this lake from the shore and from a row boat and also from the concrete dock. Once while fishing from the dock in the early spring when the lake was closed I dropped an unweighted small 4" long plastic worm down near the pillar where I had seen the bass suspended. Sure enough I saw a bass come out of the gloom and hit the worm. I could see him opening his mouth and sucking in the small plastic worm. I was not quick enough to set the hook and he spit the hook out fast. It happened so fast. And the worm had not sunk down more that 5 ft.

Not sure what the fish did when there were 2000 people swimming on that lake on the Fourth of July Weekend.

Now I often would walk around that lake in the mornings during the spring time and catch bass on my fly rod. Never did catch anything really big but I did have a lot of fun hooking and releasing a lot of small bass. I caught a lot of bluegills too. The fishing was best on the secluded parts of the lake. I didn't really fish near the swimming areas as I didn't want to have a hook come of and get into that area. I didn't want to be stepping on any fishing hooks when I went swimming,,, if you know what I mean.

All I really know for sure is that most fish that require a lot of oxygen to survive will need to stay above the thermocline in the hot summer months.

The area below the thermocline is called the hypolimnium and it's where all the dead plants and dead animals fall. These organisms are eaten by bacteria. The bacteria are consuming up oxygen in the process of living and after the summer goes along they eventually can deplete the dissolved oxygen in these deeper waters. Since the lake is thermally stratified and not mixing there is no way to get new oxygen into the depths until the lake turns over in the fall. Not until the water from top to bottom reaches the same temperature will the lake become unstable enough to turn over and mix again.

You can go across most lakes and use the depth finder to see how deep the fish are suspended. They may be suspend anywhere above the thermocline but most game fish won't be below the thermocline

Also remember that fish move vertically as the day progresses and turns to night and back again.

Many of the small macrovertibrates are effected by the sunlight. They tend to go deeper during the daylight hours and then rise back up toward the surface at night. Daphnia do this. And those fish that feed on the Daphnia will follow this vertical migration too.

I think that the fish are trying to get below the area where the sunlight penetrates to hide and make it easier for them to catch a meal.

Like a lion hiding in tall grass these game fish like to hide in the shadows. From the depths they can look up and see any small fish above them that are back-lighted by the sunlight.

When you are scuba diving in really clear water you can see what this looks like from a fish's view point. Looking up from the bottom on a sunny day you can see the bright sky above. And this is the reason why the bottom of a fish is light colored. If you are at the surface and looking down at the lake bottom you see a darker colored bottom in some cases. This is why the top (dorsal) part of a fish is darker in color a lot of times. The dark back of the fish blends in with the dark bottom and the light colored top blend in with the brighter sky above the.