Lake Temperature

Developing a temperature profile of the lake is crucial to many aspects of your study. During the summer, lakes do not have uniform temperature from top to bottom, nor does temperature change uniformly with depth. Rather, they develop "stratification" which simply means two layers of water: an upper warmer and less dense layer that is fairly uniform in temperature and a lower colder and more dense layer that is also fairly uniform in temperature. Between the two is a region of sharp temperature change called the thermocline. In many respects, the two layers are isolated from each other. The sharp temperature and density barrier prevents mixing of the two layers. Consequently, nutrients may accumulate as organisms die, settle to the bottom and decompose. Decomposition in the bottom waters will also reduce dissolved oxygen critical for fish life, even though the surface waters are saturated with oxygen. There are many other differences that make the measurement of the temperature with depth (temperature profile) critical to understanding a lake.

The thermocline depth changes throughout the summer and will differ between lakes. The size of the lake, its orientation to the prevailing wind and the weather (temperature and wind) determine the depth of the thermocline. In spring, after ice out, the thermocline begins to form near the surface as the lake warms. Increased warming will increase the thermocline depth. As the summer progresses, the thermocline deepens. By late summer, the temperature difference between top and bottom layers is greatest. But even when the summer ends and air and surface water temperatures cool, the thermocline continues to deepen, although the temperature differential between top and bottom layers begins to get smaller. Eventually, the temperature difference gets so small that the wind can force the thermocline to the bottom of the lake and completely mix the lake's water.

The upper layer of lake water is where most of the algal growth occurs because it receives sufficient light for photosynthesis and the bottom layer is where most of the effects of decomposition, such as oxygen depletion, are observed. The volume of these respective layers will determine much of the chemistry and biology of the lake (translation: can the lake support trout? Is the lake becoming eutrophied?). Consequently, we want to be able to determine the depth of this layer.

One could take a sample every meter of so from top to bottom, but this would take quite awhile. To save time, we suggest a different approach. We will use the fact that the upper and lower layers are each relatively uniform in temperature.

Sampling Method

Equipment:

  • Depth Electronic Thermometer or Max-Min thermometer or:
  • Alcohol Thermometer
  • Calibrated Line
  • Field data sheet and pencils

Measurement Protocol:

Alcohol thermometer:
Take a surface measurement on the shady side of the boat by holding the thermometer for two minutes at elbow's length below the surface.

If you collect a bottom dissolved oxygen sample, take another temperature reading as instructed in the DO protocol.

Record both of these temperature readings on your lake field sheet.

Max-Min thermometer or electronic temperature probe:
Measure the temperature at one meter intervals from top to bottom

Always wait two minutes for the temperature for the thermometer to reach water temperature before raising it to the surface.

Record temperature readings and depths on the lake field sheet.

Notes:
If there is less than a 2°F between the top and bottom, the lake is not stratified. Record the top and bottom measurements; they are all that is necessary.

If there is more than a 2°F between the top and bottom, take temperature readings every meter to help you find the thermocline (where the temperature changes a lot over a small depth).

When you take your first sample of the year, you will have no idea where the thermocline lies, if there is one; but, after that, you can be reasonably assured that it will be in the same place or lower. The smaller and more wind-sheltered your lake, the more certain you can be. So, for the next sampling period, start your search where you last found the thermocline and work down from there. Lakes do have an interesting habit of sloshing about (both temperature layers behaving a little differently) when exposed to wind, so be a little flexible about your search. In late fall, the temperature difference between top and bottom layers will get smaller and smaller, and consequently harder to find. Luckily, the thermocline is less necessary to find at this time, too. Use your own judgment on how hard to search for the thermocline in late fall.

Hopefully, this process will help you find the thermocline with a minimum number of samples. If you discover a better way, we are all ears!

Quality Control

To ensure that field and lab thermometers are accurate, use a couple of quality control checks:

  1. Calibrate your thermometers yearly (or more often) against a Standard Thermometer--a thermometer certified by the National Institute of Standards and Technology. Professional labs have Standard Thermometers (EAL has one at UMass Amherst). To calibrate your thermometer, place it first in a room-temperature water bath along with the Standard thermometer. Take 10 separate readings of each thermometer over a period of time (say, one hour) and compare the means. Your thermometer should agree within 1°C of the Standard Thermometer. Then repeat this process using an ice water bath.
  2. Check your thermometer each day it's going to be used: Look closely at the thermometer to ensure that it is not cracked or damaged in any way and that the alcohol column is unbroken.