[OldBaldGuy]

Received this message from Daryl late today:

I’ve been under the impression from several things Daryl has written in the past that stratification – and the subsequent fall “turn-over” – are largely limited to deep sand and gravel pits and to the larger reservoirs. Do lakes the size of Summit (or Zorinsky, or Burchard) stratify for prolonged periods each summer?

Water has different densities at different temperatures and that is why it can stratify into layers that may not mix for the entire summer. Most Nebraska waters are wind-swept enough that they do not stratify during the summer. Even some of our largest reservoirs do not stratify during the summer because the wind blows hard enough to break up any stratification that might begin for a day or two. Surprisingly sandpits are the Nebraska water bodies most likely to stratify because the pumping that created the sandpit creates a relatively deep body of water. Couple that with the fact that sandpits are relatively small and often protected by lots of trees and stratfication commonly occurs.

Zorinsky? Burchard? Summit? Nope, no stratification. Those waters may begin to stratify on a hot, calm summer day, but as soon as the wind kicks up a day or two later--no more stratification.

This leads to a second question: generally speaking, how deep is the thermocline? Does it vary widely from lake to lake or is it always found at about the same depth in each area of the state? Can you boat fishermen with sophisticated electronics see the thermocline on your screens?

How deep is the thermocline? It depends! Depends on the water body, water levels, inflow and outflow rates, weather patterns, etc., etc., etc. No, the thermocline will not necessarily be near the same level in different water bodies in the same area of the state. On waters that do stratify the thermocline very likely will not be the exact same level from one year to the next. For that matter, depending on a lot of factors the thermocline can actually move throughout the summer!

I have seen sandpits where the thermocline was so shallow that you could float in the middle of the pit, extend your toes down as far as you could reach and feel the difference in water temperature! A thermocline as shallow as 6-8 feet is not unusual in sandpits (By the way, most sandpits have little or no oxygen below the thermocline in the summer and therefore no fish below the thermocline. If you think fishing deep water is your best bet during the summer, well, good luck to you, there may literally be NO FISH deeper than 6 or 8 feet!)

Yes, the thermocline can be detected on depth-finders. Since there is a change in water density in those lower layers, you may be able to detect that on the depth-finder screen. In addition plankton and baitfish may concentrate at a level corresponding to the thermocline and those organisms can be seen on a depth-finder.

And a third: Twice in the last two weeks, first with Catfishsteve and later with Aaron Lee, we found bluegills suspended at Summit between 10 and 12 feet, but Aaron’s graph showed few if any deeper than 12. Are they relating to the thermocline, water temperature, oxygen content, something else, or “all of the above” when that occurs?

How about "none of the above"? When bluegills are suspended in open-water during the summer they likely are out there following swarms of zooplankton. Zooplankton relates to the phytoplankton or algae and the level that algae is suspended at depends on the level of light penetration.

I am not saying I know that for sure, but that is the first thing I would suspect.

Later,

Daryl Bauer
Lakes and Reservoirs Program Manager
Nebraska Game & Parks Commission
daryl.bauer@nebraska.gov