Are lakes warmer at the bottom?

The bottom layer is the coldest, staying at around 39–45 degrees F (4.0–7.4 degrees C). Since light does not penetrate to the bottom, photosynthesis is limited to the top layer. Because of the warmer waters and more plentiful food supply, almost all creatures spend the summer months in the upper layer.



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Lakes have layers Water temperatures also play a role in water density. Warm water is less dense meaning it is lighter and stays toward the top of the lake. The colder, heavier water is found at the bottom.

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In summer, the top of the lake becomes warmer than the lower layers. You've probably noticed this when swimming in a lake in summer - your shoulders feel like they're in a warm bath while your feet are chilled. Since warm water is less dense that colder water, it stays on top of the lake surface.

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When the upper water layers warm in the summer months, they become separated from deep water by a transition zone known as a thermocline. In a thermocline, the temperature decreases rapidly with small increases in depth. This phenomenon linking temperature change with depth is called temperature stratification.

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But, in winter some lake surfaces can get very cold. When this happens, the surface water becomes more dense than the deeper water with a more constant year-round temperature (which is now warmer than the surface), and the lake turns, when the colder surface water sinks to the lake bottom.

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When water cools down to 0 degrees Celsius (0°C), ice begins to form and floats on top of warmer water; water at the bottom of a lake or river is typically 4°C (if it is not frozen). During the summer, the opposite occurs as warmer water floats on top of colder water. Many factors can affect water temperature.

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Deep lakes only mix in spring and fall, and the bottom of deep lakes stays cold and dark because light cannot reach the bottom. Shallow lakes, in contrast, mix all summer because light reaches the bottom of the lake and warms the whole water column.

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The Profundal Zone or Hypolimnion (cold water region) can be found below the Euphotic Zone. This zone is located under the thermocline where the sunlight does not penetrate the water. Again, the size of this zone depends on the age and water clarity of the pond or lake.

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If you are getting 50-55 degrees Fahrenheit, turnover may be nearing or just starting. By 45-50 degrees Fahrenheit the lake is making the change rapidly, and fall turnover is complete at around 40.

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Because water warmer than 40oF is more buoyant than cooler water, spring warming creates a shallow, warmer layer of water late in the day on the downwind side of a lake. Other factors being the same, the north side will warm more quickly than the south side of the lake.

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We now have a micro (small) scale circulation — the lake breeze. This lake breeze that develops in the spring can cause a 30 degree difference in temperature over a few miles. It can oscillate back and forth, where you a warm one minute, but cold the next. We see this on a grand scale as well around the globe.

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Turnover is like being at the mall … or at a fair. Turnover is a natural way the lake cleans up harmful bacteria and algae. It carries dead algae down into the depths of the lake where there is less sunlight, helping to prevent algae growth.

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