Summary: It's Time to Think about the Thermocline
During summer the thermocline often becomes a factor that can influence bass habitat
use and movements. A thermocline is a section of lake water where the water
temperature drops drastically across a small change in water depth. This occurs because
warm, surface layers of water are much less dense than the cooler, deeper waters. Warm
and cool waters separate due to their difference in density, so that the warm layer (called
the epilimnion) is essentially perched on top of the cool water layer (the hypolimnion),
with little mixing between the two.
The thermocline is the relatively thin layer of water lying in between the warm
epilimnion and cool hypolimnion, and in this region the water temperature drops rapidly
with every foot of increasing depth. You may have actually seen the thermocline on your
depthfinder, because the density change in the water here is strong enough to deflect
sonar from today's sensitive electronics.
During a hot summer day you might expect that fish would prefer the cool hypolimnion,
but dissolved oxygen levels become an important part of this discussion. Most dissolved
oxygen enters water through production by phytoplankton (microscopic plants) or rooted
aquatic plants. The warm epilimnion supports nearly all of a lake's phytoplankton and/or
aquatic plants, meaning that it is in this zone where most of the oxygen is produced.
Below the thermocline, light becomes limiting and aquatic plants cannot continue to
grow, so oxygen is not produced. Because the cooler hypolimnion does not support plant