®
THERMOCLINE
identifying & understanding the
Winter 2019
page
10
by Mike Gorman
O
ut cruising the lake this past weekend – I
was carefully watching my electronics, but
I wasn’t looking for structure or fish. I was looking for the thermocline (a horizontal layer of water
in a lake with a particularly steep temperature gradient).
Thermocline depth is a key piece of information
when looking for bait and the bass following them.
So, in order to edify everyone – I thought it prudent
to put together a brief lesson on limnology, so we can
better understand this key piece of information and all
become better anglers.
Most deep bodies of water in temperate latitudes,
including lakes, rivers and the ocean, exhibit (during part
of their annual cycles) a characteristic thermal structure
in which a well-mixed, warm upper layer is separated
from a relatively colder bottom region.
When a water body exhibits such a thermal
structure, it is said to be stratified, and the layer of
intense temperature gradient separating the almost
homogeneous upper layer from the colder bottom
waters is termed the thermocline.
Before we get too far along, lets define some key
terminology.
Epilimnion - The surface layer of a lake during
the period of summer stratification.
Metalimnion (thermocline) - A horizontal
layer of water in a lake with a particularly steep temperature gradient. During the summer stratification period it is the intervening layer between the epilimnion and the hypolimnion, but during winter, with ice cover, it may be near the surface (not something we typically observe in western waters).
Hypolimnion - The deep-water region in a lake
below the thermocline or metalimnion.
Dimictic – Waters that circulate twice each
year, in spring and fall and are directly stratified in summer (warmest water on top) and inversely stratified (warmest water on bottom at 4°C) in winter under ice.
Warm monomictic - Circulate once a year in
the winter at or near 4°C and are thermally stratified the remainder of the year; not ice covered.
Oligomictic - Thermally stratified much of the
year but cooling sufficiently for rare circulation periods at irregular intervals; not ice covered.
Mixing – Temperature gradients (density) become
similar allowing water from the upper layer and lower layers to “mix.”
The graph depicts real thermographic profile data illustrating stratification during peak summer months as well as developing strata in spring (May) and nearly isothermal conditions in fall (October).
In the thermocline, there is approximately a 9°C decrease in temperature in a depth range of approximately 15-feet during August. This is a considerable variation in temperature.
**Note the depth – unlike some mid-western and southern lakes and reservoirs, the thermocline is deeper and not really reachable by crankbaits or other similar patterns often described in bass fishing articles.
Often the thermocline can be detected via sonar as picked up by the concentrations of life and density differences in water (see example). Sometimes it is clearly evident – others you need to adjust the sensitivity settings on your sonar unit. There are a number of resources in older issues of this magazine and at Westernbass.com and other sites like YouTube.
FUNCTION AND PROCESS
During the spring, most temperate lakes are nearly homothermal (similar temperature throughout the water column).
As weather warms and daylight period and solar radiation increase, surface water warms at an increasingly rapid rate. Colder, denser water settles to the bottom beneath this warming surface layer. As the heating continues, a point of inflection develops in the temperature profile, and a well-mixed upper layer, with relatively intense temperature gradient at its bottom boundary is formed.
Developed thermoclines will be strongest and most distinct in mid-to-late summer.
Dissolved oxygen, critical to habitat suitability for fishes, is primarily produced through production by