MadSci Network: Earth Sciences

Re: How is the acidity and temp. of lakes affected by the weather conditions?

Date: Sun Apr 18 01:50:54 1999
Posted By: Anna Steding, Grad student, Ecological Engineering, University of California/Department of Civil and Environmental Engineering
Area of science: Earth Sciences
ID: 923975201.Es

First, temperature: Changes in temperature in the lake or pond happen because of changes in the heat content of the lake. Heat is GAINED by the absorption of sunlight. So, if you are in an area where there is more sunlight (for example in the tropics, or during the summer in the temperate zones) then you will have a higher temperature in your lake or pond. Heat is LOST through conduction and evaporation to the air. So, if the air is cooler than the lake, the lake will tend to lose heat to it through conduction, thereby lowering the temperature. Or, if the surrounding air is warm or if there is a lot of wind, more evaporation can occur. This would cause the temperature to decrease… provided that the lake is losing more heat through evaporation than it is gaining from the sun.

WIND can cause something interesting to happen to temperature in a lake (if it is deep enough) that doesn't happen in a pond. Sunlight heats the water of the lake near the surface and so a layer of less dense warm water forms above the more dense, cooler water underneath. (One of the important properties of water is that its density INCREASES with a DECREASE in temperature -until you get as low as 4 degrees Celsius, where the maximum density occurs.) The wind mixes the water of the lake - but the wind can only mix the shallow layers of a lake (for example, the first 5 meters of a 25 meter lake). Because of the combination of sunlight heating the shallow waters and the wind mixing the shallow waters, you get a region of warm water (called the epilimnion) trapped above the cool water (called the hypolimnion). So, imagine you are a fish travelling from the surface of the lake to the bottom. At the surface you will feel the warm water (for example 15 degrees Celsius) and as you continue down through the epilimnion the temperature of the water will remain the same. Then, all of sudden at about a 5 meter depth, you will encounter much cooler water (for example 7 degrees Celsius). You are now in the hypolimnion, and you have just passed the thermocline, which is the boundary between the epilimnion and the hypolimnion. This whole process of warm water forming on top of cool water is called stratification.

The speed of the wind, as well as the length of the lake over which the wind passes, will determine how deeply the wind can mix the lake. This will determine, in turn, the depth of the thermocline, which will be different for each lake. The temperature in the epilimnion and hypolimnion will be different for each lake, and will depend on the amount of heat the lake has absorbed. Since a pond is very shallow, the wind is able to mix all of the water in it, and so stratification does not happen. It also doesn't happen in shallow lakes for the same reason.

Now, acidity: This is a little more tricky. The most important natural factor that controls acidity in a lake is the amount of dissolved carbon dioxide in the water. More carbon dioxide means more carbonic acid, which means a lower pH and higher acidity. Less carbon dioxide means just the opposite: higher pH and higher alkalinity. The important creatures that control the level of dissolved carbon dioxide in lakes and ponds are ALGAE. Algae are photosynthetic, meaning that they consume carbon dioxide and water in the presence of sunlight to produce some carbohydrates and oxygen. So, when they are consuming carbon dioxide the pH of the lake will increase. This happens during the day, but the pH falls back down during the night when the algae are not photosynthetically active. The amount of photosynthesis that algae undertake is due, in part, to the availability of sunlight. More sunlight means you will get bigger increases in pH during sunlight hours. (The amount of photosynthesis also depends on how many nutrients are in the water, but levels of nutrients don't depend heavily on weather patterns).

Acidity can also be affected by human activities through acid rain. Whether or not acid rain falls on a lake will depend on the local weather patterns. For example, let's say that a coal-burning power plant is located a few kilometers from a lake. The power plant is emitting nitrogen and sulfur compounds that, when dissolved in water, become acids and lower the pH of the rainwater. So, any rain clouds that come in contact with the power plant emissions will become acid rain clouds. If the wind blows the rain clouds AWAY from the lake, then there won't be very big changes in the lake's pH because of acid rain. If the wind blows the acid rain clouds TOWARD the lake, then there will be acid rain and the pH of the lake will decrease.

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