Where is the thermocline?
Posted: Sun May 28, 2006 10:22 am
I’ve been attempting to better understand precisely what and where the thermocline is because of its importance to successful fishing.
As you know, as one descends in water, temperatures usually drop. Less light is able to penetrate, so there is less conversion of light to heat as depth increases. Generally, as water cools it becomes more dense and sinks down while the warmer less dense water rises. This process results in lake stratification into three layers:
1. The epilimnion layer – the top warmer layer of the lake.
2. The metalimnion layer – the middle layer that is characterized by rapid temperature changes; and
3. The hypolimnion layer - the bottom cold layer.
Some internet references use the terms metalimnion and thermocline synonymously while others claimed that the thermocline is the layer where the temperature decreases most rapidly, one degree centigrade or more with each meter of depth. Others yet claimed that the thermocline is the complete stratum of water between the top warmer layer and bottom colder layer. Which is it?
Here is a set of actual water temperatures versus depth recorded during a Fall Walleye Index Netting study.
Water Depth--------Water Temp--------Water Depth
Meters----------------° C------------------Feet
0.5--------------------18-------------------1.6
1----------------------18--------------------3.3
2----------------------18--------------------6.6
3----------------------18--------------------9.8
4----------------------18--------------------13.1
5----------------------18--------------------16.4
6----------------------18--------------------19.7
7----------------------18--------------------23
8----------------------18--------------------26.2
9----------------------18--------------------29.5
10---------------------15--------------------32.8
11---------------------12--------------------36.1
12---------------------10--------------------39.4
13---------------------9---------------------42.7
14---------------------8---------------------45.9
16---------------------7---------------------52.5
18---------------------6.5-------------------59.1
20---------------------6---------------------65.6
24---------------------5---------------------78.7
28---------------------5---------------------92
32---------------------5---------------------105
36---------------------5---------------------118
40---------------------5---------------------131
Is the thermocline the water layer from 9 meters to 16 meters or is it more precisely the water layer between 9 meters and 11 meters because that’s where the largest temperature decline per meter took place (i.e. 6 degrees C in 2 meters)?
Secondly, in the same lake at the same time of year as the above water temps were recorded, my sonar screen displayed the typical thermocline foot print (dark line across the screen with shaded area beneath). The dark line was at approx 25 meters (80 feet) and the shaded area layer beneath that dark line was about 3 meters (10 feet) thick. I believe that I should be able to see a thermocline at 9 meters (30 feet). Could it be that I don't have my sonar sensitivity set high enough to see the higher thermocline, or is it that my sonar unit only indicates a thermocline when the water density reaches a specific level (e.g. the density of the coldest water in the lake (4 or 5 degrees C))?
I’d appreciate any input. Thanks in advance.
As you know, as one descends in water, temperatures usually drop. Less light is able to penetrate, so there is less conversion of light to heat as depth increases. Generally, as water cools it becomes more dense and sinks down while the warmer less dense water rises. This process results in lake stratification into three layers:
1. The epilimnion layer – the top warmer layer of the lake.
2. The metalimnion layer – the middle layer that is characterized by rapid temperature changes; and
3. The hypolimnion layer - the bottom cold layer.
Some internet references use the terms metalimnion and thermocline synonymously while others claimed that the thermocline is the layer where the temperature decreases most rapidly, one degree centigrade or more with each meter of depth. Others yet claimed that the thermocline is the complete stratum of water between the top warmer layer and bottom colder layer. Which is it?
Here is a set of actual water temperatures versus depth recorded during a Fall Walleye Index Netting study.
Water Depth--------Water Temp--------Water Depth
Meters----------------° C------------------Feet
0.5--------------------18-------------------1.6
1----------------------18--------------------3.3
2----------------------18--------------------6.6
3----------------------18--------------------9.8
4----------------------18--------------------13.1
5----------------------18--------------------16.4
6----------------------18--------------------19.7
7----------------------18--------------------23
8----------------------18--------------------26.2
9----------------------18--------------------29.5
10---------------------15--------------------32.8
11---------------------12--------------------36.1
12---------------------10--------------------39.4
13---------------------9---------------------42.7
14---------------------8---------------------45.9
16---------------------7---------------------52.5
18---------------------6.5-------------------59.1
20---------------------6---------------------65.6
24---------------------5---------------------78.7
28---------------------5---------------------92
32---------------------5---------------------105
36---------------------5---------------------118
40---------------------5---------------------131
Is the thermocline the water layer from 9 meters to 16 meters or is it more precisely the water layer between 9 meters and 11 meters because that’s where the largest temperature decline per meter took place (i.e. 6 degrees C in 2 meters)?
Secondly, in the same lake at the same time of year as the above water temps were recorded, my sonar screen displayed the typical thermocline foot print (dark line across the screen with shaded area beneath). The dark line was at approx 25 meters (80 feet) and the shaded area layer beneath that dark line was about 3 meters (10 feet) thick. I believe that I should be able to see a thermocline at 9 meters (30 feet). Could it be that I don't have my sonar sensitivity set high enough to see the higher thermocline, or is it that my sonar unit only indicates a thermocline when the water density reaches a specific level (e.g. the density of the coldest water in the lake (4 or 5 degrees C))?
I’d appreciate any input. Thanks in advance.