Lochaber Lake, located in northeastern Nova Scotia, is a headwater lake that drains into the St. Mary’s River system. The lake is fed by eight streams and is drained by one outlet at its southern end. Lochaber is at the southern extent of the Antigonish highlands, and is underlain by sedimentary deposits of the Knoydart Formation, primarily mudstone, siltstone, and shale.
Land uses in the watershed include seasonal and permanent residences, and agriculture. The waters of Lochaber are clear, and macrophytic vegetation thrives along the coastline. The lake has the shape of an elongated oval, with a total shoreline of 18 km, a width of 0.75 km, and a mean depth of 21.8 m. Given the lake shape and depth, the littoral zone( the area of shoreline where land is subject to wave action) dominates Lochaber Lake. Historically, the watershed of Lochaber lake was covered by a dense stand of Acadian forest, with a mix of red spruce, yellow birch, balsam fir, sugar maple, red pine, eastern white pine, eastern hemlock, and American beech. Two centuries ago, much of this forest was cleared for farming. However, farming declined during the mid 19th century and many of these fields are now covered with secondary successional forest species, predominantly softwoods.
Lochaber Lake is a DIMICITIC LAKE , which means it's water mixes from the surface to bottom twice each year.
Density Stratification and Temperature Changes with the Season
Water is less dense as a solid than as a liquid. In freezing tempeatures ice floats, while water at temperatures just above freezing sinks. Water is most dense at 4°C and becomes less dense at both higher and lower temperatures. Because of this density-temperature relationship, many lakes in temperate climates tend to stratify, or separate into distinct layers.
In Spring, the water near a lake’s bottom will usually be at 4°C just before the lake's ice cover melts in the spring. Water above that layer will be cooler, approaching 0°C just under the ice. As the weather warms, the ice melts. The surface water heats up and therefore it decreases in density. When the temperature (density) of the surface water equals the bottom water, very little wind energy is needed to mix the lake completely. This is called turnover. After this spring turnover, the surface water continues to absorb heat and warms. As the temperature rises, the water becomes lighter than the water below. For a while winds may still mix the lake from bottom to top, but eventually the upper water becomes too warm and too buoyant to mix completely with the denser deeper water.
It is useful to visualize a more extreme example of density stratification. Imagine a bottle of salad dressing containing vegetable oil and vinegar. The oil is lighter (more buoyant) than the vinegar which is mostly water. When you shake it up you are supplying the energy to overcome the buoyant force, so the two fluids can be uniformly mixed together. However, if allowed to stand undisturbed, the more buoyant (less dense) oil will float to the top and a two-layer system will develop.
As Summer heat arrives, the temperature (and density) differences between upper and lower water layers become more distinct. Deep lakes generally become physically stratified into three identifiable layers, known as the epilimnion, metalimnion, and hypolimnion (Figure ). The epilimnion is the upper, warm layer, and is typically well mixed. Below the epilimnion is the metalimnion or thermocline region, a layer of water in which the temperature declines rapidly with depth. The hypolimnion is the bottom layer of colder water, isolated from the epilimnion by the metalimnion. The density change at the metalimnion acts as a physical barrier that prevents mixing of the upper and lower layers for several months during the summer.
The depth of mixing depends in part on the exposure of the lake to wind and the lake’s size. Smaller to moderately-sized lakes (50 to 1000 acres) reasonably may be expected to stratify and be well mixed to a depth of 3–7 meters in north temperate climates. Larger lakes may be well mixed to a depth of 10–15 meters in summer
During Autumn, the weather cools and so does the epilimnion, reducing the density difference between it and the hypolimnion. As time passes, winds mix the lake to greater depths, and the thermocline gradually deepens. When surface and bottom waters approach the same temperature and density, autumn winds can mix the entire lake; the lake is said to "turn over." As the atmosphere cools, the surface water continues to cool until it freezes. A less distinct density stratification than that seen in summer develops under the ice during winter. Most of the water column is isothermal at a temperature of 4°C, which is denser than the colder, lighter water just below the ice. In this case the stratification is much less stable, because the density difference between 0°C and 4°C water is quite small. However, the water column is isolated from wind-induced turbulence by its cap of ice. Therefore, the layering persists throughout the Winter.
This pattern (spring turnover — summer stratification — fall turnover — winter stratification) is typical for temperate lakes. Lakes with this pattern of two mixing periods are referred to as dimictic. Many shallow lakes, however, do not stratify in the summer, or stratify for short periods only, throughout the summer. Lakes that stratify and destratify numerous times within a summer are known as polymictic lakes.
Much less common are holomictic lakes that circulate incompletely resulting in a layer of bottom water that remains stagnant.
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Questions:
1. What is the width of the lake at this point?
2. Estimate the depth of the lake at the shoreline. How does this depth compare to depth 3-4 m from shoreline?
3. What temperature would you expect the water to be at the bottom of the lake 150 m from GZ? How did you conclude this?
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