In all our monitoring programs we focus on Sylvan Lake water quality , yet the records show that those data don’t change very fast. In comparison, lake water temperature (T) does cycle dramatically as it can vary from near zero Celsius (C) under the ice and 4 C above the sediment in the winter, to as high as 23 C in a very warm summer. Those Ts and their rates of change must have an impact on the chemical and biological process: on the aquatic populations, the food chain, and on the general health of the lake.
Let’s walk through an annual cycle starting in a fall season. First, cold fall and winter weather removes heat from the surface layer. As the density of liquid water increases when it is cooled, the colder, denser, surface water sinks, mixes, and gradually lowers the T of the whole water column. Water reaches a maximum density at 4 degrees C, so the deeper areas of the lake never freeze right to the bottom. Eventually an ice layer forms on the surface and seals the surface of the lake from the atmosphere. The density and T gradients stabilize the water column as this first Alberta Environment graph of T measurements at different depths at the deep-water sampling station shows:
In the January-March period in the listed years between 1984 and 2002 the lake water at the 10 metre depth cooled to between about 1.2 and 3.3 C. Under the ice at 1 m depth the recorded Ts were between 0 and 2 C. Right at the bottom of the lake at the 16 metre depth, Ts reached about 4 C where water reaches its maximum density. At that point, both warmer or colder volumes of water will be more buoyant and will rise, mix, exchange heat, and equilibrate at a new level in the water column. That density-driven mixing makes the lake a dynamic place.
The fall and winter T conditions that caused the lowest vertical T gradient in 1984 and the highest one in 1999 are illustrated in these two graphs acquired from the Alberta Agriculture weather archives:
Temperatures in Fall 1983 and Winter 1984
Temperatures in Fall 1998 and Winter 1999
A simple explanation for the difference between the two lake T profiles is difficult to see however early rapid heat removal in 1984 by the very low November minimum Ts may have been the primary cause. In contrast, the 1998 fall season was relatively warm.
That is quite a difference in refrigeration. Cooling the 420 million cubic metres of Sylvan Lake water by an extra 2 C required removal of 3740 Terajoules of thermal energy!
All that cooling is reversed when warm surface winds transfer heat and solar energy is absorbed to first melt the winter ice and then heat up the lake water. The next graph contains a series of Alberta Environment T vs depth profiles for May to October 1996:
The first set of data for May shows that rapid warming and mixing had already occurred, and the lake had a constant T from top to bottom of about 5 C. The winter top-to-bottom T gradient had already been eliminated by density-driven mixing. The maximum T at 8 m, about half the total depth of the 16 m deep-water station, reached 19 C on August 23.
Here is the history of Sylvan Lake air T from Fall 1995 through 1996 that determined that year’s lake T cycle:
Hot 30 C weather continued through late August 1996 and that caused an additional 2 C of heating of the top 1 m of lake water as the 23-Aug data show in the previous figure.
Variability of April-August weather conditions have had a great influence on Sylvan Lake’s heating and cooling cycle. This graph shows how mid-August T profiles differed by 6 to 7 C between 1983-2002:
Lots of heat transfers in and out of Sylvan Lake during the annual cycle. For example in the Spring, to melt 0.5 metre of ice on the 42 million square metres of lake area would require 0.7 x 1016 Joules of energy. To warm up the 420 million cubic metres of lake water through a 15 C range would require an additional 2.8 x 1016 Joules. So, the total energy input would be 3.5 x 1016 Joules. An equivalent amount of thermal energy would be released by burning 0.73 million tonnes or 95 million litres of gasoline fuel worth more than $100 million at the pump. The sun provides it for free.
Outside the beach months of June to September Sylvan Lake’s natural heating and cooling cycle continues and affects the submarine conditions for aquatic life. There’s nothing we can do about it. Neither can the jackfish who swim around under the ice in cold water for 4-5 months. The rest of the members of the Sylvan Lake food chain have a really cool lifestyle too.
Acknowledgements: All the graphics in this post were extracted from the AXYS Consulting Sylvan Lake Water Quality Study 2005.