Lake Turnover
“Once a lake stratifies, atmospheric oxygen, and oxygen produced by algae in the surface layer, have a hard time getting to the bottom of the lake.”......."I think of turnover as the lake taking a deep breath as everything is mixed. It’s like a fresh start every spring and fall.”
Robert Ladwig, Hydrologist
University of Wisconsin-Madison
The phenomena called “lake turnover” usually occurs twice a year in most Canadian lakes but usually only once a year in Hotel Lake. While we may not notice the turnover of the water in our lake in the fall, this is a very important event in the ongoing health of the lake. Even if we do not actually see the turnover, it's very likely that we will notice an unpleasant “fishy” smell all around the lake and also in the water that we draw out of the lake for domestic use. On rare occasions, you might look down on the glassy smooth lake surface and see these turnovers in progress. If viewed from an elevated position, the turnover can appear as dramatic, slowly billowing “clouds” of brown mud and sediment emerging from the depths. These “clouds” look like thunderstorms viewed from above, and as they slowly dissipate, the entire lake assumes a uniform sepia-brown colour which can last for days. The strong odours or “fishy" smell which can accompany turnover events are sometimes very intense, and in the worst cases may cause residents considerable discomfort. Water drawn from the lake may carry a vestige of this “fishy” smell for days in domestic water systems and tanks.
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Lake turnovers are of considerable interest to recreational anglers and fishermen. Small shallow lakes that cool quickly will turnover first, then deeper lakes will follow. The turnover moves highly oxygenated surface water to the bottom which spreads oxygen throughout the lake, but as the bottom water rises it carries sediment and decomposing organic matter upward. So profound are these turnovers that fish are temporarily affected and so is fishing. A fisherman fishing in a boat during a turnover may experience the lake turning brown all around the boat and an odour rising from the water. As a result, fishermen often wait several weeks after a lake’s turnover before resuming fishing.
November 25, 2020 Hotel Lake turnover. The colouration of the water is seen in daylight and in shadow.
There is a great deal of information about lake turnovers world-wide. The following is an excerpt from a National Geographic article which provides a basic understanding of the limnology of these events.
National Geographic: Lake turnover is the seasonal movement of water in a lake. Shallow lakes experience very little lake turnover, while large, deep lakes experience major changes as waters of different temperatures mix.
Limnology is defined as the study of the biological, chemical and physical features of lakes and other bodies of fresh water. There are three basic ways that limnologists classify lakes: how many nutrients lakes have, how a lake’s water mixes, and what kinds of fish live in them.
When lakes are classified by the amount of nutrients they have, limnologists are using the trophic system. Generally, the clearer the water in the lake, the fewer nutrients it has. Lakes that are very nutrient-rich are cloudy and hard to see through; this includes lakes that are unhealthy because they have too many nutrients. Lakes need to have a balance of nutrients.
Lakes can also be classified by how the water mixes, or turns over from top (epilimnion) to bottom (hypolimnion). This is called lake turnover. Water in some lakes, mostly shallow ones, mixes all year long. These lakes have very little lake turnover.
Deep lakes experience lake turnover on a large scale. The middle layer, the thermocline, mixes and turns over throughout the year. It turns over due to climate, nutrient variations, and geologic activity such as earthquakes. However, major lake turnover happens during the fall and spring, when the lake’s cold and warm waters mix and readjust. Most lakes that experience lake two turnovers are dimictic lakes, meaning their waters mix twice a year, usually in fall and spring. Hotel Lake which generally experiences one turnover a year is classified as a monomictic lake
Lake temperature stratification changes with the seasons. During the summer, the epilimnion, or surface layer, is the warmest. It is heated by the sun. The deepest layer, the hypolimnion, is the coldest. The sun’s radiation may not reach this cold, dark layer.
During the fall, the warm surface water begins to cool. As water cools, it becomes more dense, causing it to sink. This cold, dense water sinks to the bottom of the lake. It forces the water of the hypolimnion to rise.
During the winter, in Canada's north temperate region, a lake's epilimnion is coldest because it is exposed to wind, snow, and low air temperatures. The hypolimnion is the warmest. It is insulated by the earth. This is why there is ice on lakes during the winter, while fish swim in slightly warmer, liquid water beneath.
During the spring, the ice melts. Cold surface water sinks to the bottom, forcing the warmer, less dense water upward causing a turnover.
Lake turnovers are primarily influenced by seasonal temperature changes which cause thermal stratification layers to either remain stable or to become unstable and turnover. Other influencers are wind and heavy-cold rain and melting ice.
Wind, in particular, is a natural factor which can influence turnovers or cause mixing of the water in Hotel Lake. For instance, a prevailing wind from the south-west will penetrate the gap between Pender Hill and Harbour peak. Winter storms will sometimes drive strong winds over the lake for several days. As the wind passes through the gap, it appears to veer slightly and the result is a steady flow of air across the lake surface. This pushes the the surface water in the direction of the wind and when it arrives at the far shore, it has no where to go but to curl down and reverse direction thus causing a significant mixing or an overturning of the lake water. The mixing effect of wind is removed when ice forms on the lake.
A cold winter will sometimes cause ice to form on a lake and, in a strange twist of physics, cause the warmest water in the lake to be at the bottom and the coldest on top.
As winter approaches and the air gets colder it cools the surface water on a lake. The cooled water generally becomes more dense (water molecules move closer together) and as a result the cooled water sinks and is replaced by warmer water from below. Eventually, as winter continues, all the water in the lake cools to about 4 degrees C.
As water cools below 4 degrees C, the “sinking” phenomena suddenly reverses. Below 4 degrees C, water molecules begin to move further apart and in so doing become less dense causing the cooled water to float or remain at the surface. As the surface water reaches the freezing point, it has already formed a light and stable layer floating on the top of the lake; all warmer water has been forced below. Further cooling by winter air above the lake, causes the top layer of water to freeze and form ice, which floats on the lake's surface, because ice is significantly less dense than the water below. Providing the average ambient air temperatures remain below freezing, the ice continues to float on the surface and thicken.
So... does Hotel Lake ever freeze over?
A final look at BC coastal lakes and why they are so different from most Canadian lakes.
Lakes that experience winter ice cover in Canada’s north temperate or continental regions are generally classified as dimictic, a limnological term which means a lake that experiences two periods when lake water de-stratifies and mixing occurs (turnover) throughout its depth.
On the other hand, BC coastal lakes, influenced by the warming effect of the Pacific Ocean generally remain ice-free all year. During the autumn when water temperature drops to about 4 degrees C, BC coastal lakes thermally destratify and thus lake water can mix throughout the winter; this phenomena is referred to as monomictic which means one turnover or mixing per annum.
The pattern of hydrology also differs in BC coastal lakes from continental lakes. Continental lakes receive the highest inflow of water (freshet) in the spring and early summer as a result of melting ice and rainfall.
In BC coastal lakes the highest inflow of water is delivered from rain storms between November and March. Total rainfall in BC coastal areas can reach 300 cm/year.
During the period when this highest inflow of water is being delivered, major nutrient loads are also delivered into BC Coastal lakes. Simultaneously, flushing occurs to remove excess water.
In BC coastal lakes the high flushing rates during winter can cause lake nutrient levels to be extremely low, or ultra-oligotrophic.
BC coastal lakes generally begin thermal stratification in March and gradually, with sunshine and warm weather, the lake becomes more stable through the summer. In the late fall, cooling of the lake causes de-stratification which allows turnover to occur. According to British Columbia Lake Stewardship Society, "Warm monomictic lakes have temperatures that do not fall below 4°C. They do not freeze, circulating freely in the winter at or above 4°C, and stratify directly in the summer. They are found mostly in warm regions of temperate zones." BC coastal lakes, such as Hotel Lake, are usually classified as Warm Monomictic.