Studying Water Temperature at Depth
By Joel Pasternack, CLWA Board Member

Canandaigua Lake has changed in many ways over the past 50 years.  While the lake is still beautiful for recreation, and a source of drinking water, there have been disturbing signs.  Invasive plant species, mussel infestation, foam, Harmful Algae blooms, and fish die-off in 2013 and 2014, all point to undesirable changes in our lake.  The Temperature at Depth Project focuses on one of several factors that are thought to contribute to these changes in the lake.

All aquatic life forms are sensitive to the temperature of the water.  Temperature affects where fish reside and where cyanobacteria grow.  Surface temperature, while a useful measure, does not reflect overall water temperature.  To monitor the lake, it is necessary to measure the temperatures at various depths.

In 2022, CLWA Citizens Science Committee expanded the Secchi disc monitoring program to include measurement of temperature at depth. Seven trained volunteers recorded water temperature with a device used by fishermen.  As the device is lowered through the water column, it records the temperature every meter (3.28 feet).  The measurements were taken across the length and breadth of the lake.  The volunteers, with their location for measurement, are as follows: Brian and Dolores Perkins off Granger Point at the south end, Alan Krautwurst off Vine Valley, Dee Crofton off Seneca Point, Wade Sarkis off Black Point, Scott Hill off Tichenor Point, Joel Pasternack between Deep Run and Pelican Point, and Doug Merrill off the Canandaigua Water Treatment Plant.  The data collected builds on 17 years of data compiled by Professor John Halfman of Hobart and William Smith College.

Some basic science helps to understand temperature measurements in a fresh water lake.   Water is most dense at 39 degrees Fahrenheit (4 degrees Centigrade).  The heaviest water sinks to the bottom, so throughout the year, the water in the deepest regions of the lake is 39 degrees.  In a typical winter, the water temperature doesn’t vary much with depth.  In summer, the surface water warms and a transition zone develops between warm water above and cold water below.  This transition zone is called the thermocline, which is demonstrated as a steep decline on a curve of temperature vs. depth.  Monitoring the thermocline is key to understanding the overall temperature of the lake.  A shallow thermocline indicates that most of the water column is cold, a deep thermocline indicates a warmer lake overall.  The thermocline is both a physical and biological transition zone.

The graph below (Figure 1) of measurements taken between Deep Run and Pelican Point, from April 5 to August 3, 2022, shows the development of a thermocline.  Notice that in April and early May, the curves are flat with little change in temperature at increasing depth.  This year, by May 15, the surface water had warmed above 50 degrees and a thermocline developed.  Notice the curves demonstrate steep descent of temperature between 20 and 40 feet in early June, and between 40 and 60 feet from June 29 through August 3.  Also notice that below 100 feet all the curves from April through August come together reflecting the deep water which remains close to 39 degrees.

As the seasons progress into autumn, the surface cools and the thermocline is deeper.  In November or early December, there is no longer a thermocline as the surface layers have cooled down, close to 39 degrees F.  The graph below (figure 2), of temperature measurements from September 24 through November 15, shows the thermocline getting colder and deeper in autumn.  If we had readings on November 30 there would no longer be a thermocline.

 

The readings at depth form the basis for following water temperature from season to season, from one part of the lake to another, and from year to year.

We observed that the south end of the lake off Granger Point was consistently cooler than other sites from Vine Valley northward.  This difference has been observed in prior years.  Professor John Halfman explains, “…The southern end of the lake has a smaller surface area to volume ratio than the northern end.  The extensive shallow (above thermocline depths) portions of the northern end will warm from direct sunlight and light reflected off the lake floor, the latter is missing at the southern end.”

Recreational users of the lake notice year-to-year variation in lake temperature.  Using the temperature at depth profile, we can evaluate the overall heat in the lake from year to year.  Professor Halfman’s data, added to our data, shows warming of the lake over the past 18 years.  The graph below shows temperature readings taken in July in the Northern half of the lake.

Notice that the surface readings down to 25 feet are almost identical in 2019 (light blue) and 2022 (green).  Also, note that the surface readings in 2005 (dark blue) and 2016 (yellow) are also similar.  However, because of the depth of the thermocline and the shape of the curves, the lake was warmer in 2022 than 2019, and warmer in 2016 than 2005.

To assess the heat in the lake, we used the temperature vs. depth curves to calculate the average temperature of water from the surface to 131 feet deep.  The results are shown in the bar graph below (figure 4).  The average water temperature in 2009 and in 2013 is higher than in 2005, despite the surface temperatures being higher in 2005.  The above graph shows the dark blue curve, representing temperature readings in 2005, higher than the orange (2009) and gray (2013) curves down to about 30 feet.  If only temperature down to 30 was considered, the lake would seem warmer in 2005 than in 2009 and 2013.

 

Water temperature is an important factor affecting the chemistry and biology of the lake. Warm water holds less dissolved oxygen for aquatic life, and warm water potentiates bacteria and algae growth.  Cold water is preferred by some fish species.  For example, we have learned from Peter Austerman, biologist with the Division of Fish and Wildlife, NYS DEC, “…trout do prefer colder water. Lake trout are generally found below the thermocline in water less than 50 degrees. Brown trout and rainbow trout are usually in slightly warmer water – 55 to 65 degrees…”

We have shared our data with scientists studying algae blooms, and with the Canandaigua Lake Watershed Council (CLWC).  CLWC is composed of leaders of the municipalities in the Watershed.  Through CLWC our data is available to the six water purveyors who use the lake to provide drinking water.   Our data is of interest to water purveyors because it is desirable to have the intake pipe in cooler water below the thermocline.

We look forward to continuing our work monitoring water temperature and sharing our observations with scientists and community leaders.