A Citizen's Guide to Understanding and Monitoring Lakes and Streams

Chapter 2 - Lakes

Temperature in Lakes

Why Is It Important?

Temperature exerts a major influence on biological activity and growth. To a point, the higher the water temperature, the greater the biological activity. Temperature also governs the kinds of organisms that can live in your lake. Fish, insects, zooplankton, phytoplankton, and other aquatic species all have a preferred temperature range. As temperatures get too far above or below this preferred range, the number of individuals of the species decreases until finally there are none.

Temperature is also important because of its influence on water chemistry. The rate of chemical reactions generally increases at higher temperature, which in turn affects biological activity. An important example of the effects of temperature on water chemistry is its impact on oxygen. Warm water holds less oxygen than cool water, so it may be saturated with oxygen but still not contain enough for survival of aquatic life. Some compounds are also more toxic to aquatic life at higher temperatures.

Reasons for Natural Variation

The most obvious reason for temperature change in lakes is the change in seasonal air temperature. Daily variation also may occur, especially in the surface layers, which are warmed during the day and cooled at night.

In deeper lakes during summer, the water separates into layers of distinctly different temperature. This process is called thermal stratification. The surface water is warmed by the sun, but the bottom of the lake remains cold. You may have experienced this difference when diving into a lake. Once the stratification develops, it tends to persist until the air temperature cools again in fall. Because the layers don’t mix, they develop different physical and chemical characteristics. For example, dissolved oxygen concentration, pH, nutrient concentrations, and species of aquatic life in the upper layer can be quite different from those in the lower layer. It is almost like having two separate lakes.

When the surface water cools again in the fall to about the same temperature as the lower water, the stratification is lost and the layers mix. This process is called fall turnover. (A simple process also may occur during the spring as colder surface waters warm to the temperature of bottom waters and the lake mixes. This is called spring turnover.) The lake mixing associated with a turnover often corresponds with a large increase in turbidity. Watch for this change in your lake this fall.

Because the sun can heat a greater proportion of the water in a shallow lake than in a deep lake, a shallow lake may warm up faster and to a higher temperature. Lake temperature also is affected by the size and temperature of inflows (e.g. glacial fed stream or springs or a lowland creek) and by how quickly water flushes through the lake. Even a shallow lake may remain cool if fed by a comparatively large, cold stream.

Expected Impact of Pollution

Thermal pollution (artificially high temperatures) almost always occurs as a result of discharge of municipal or industrial effluents. Except in very large lakes, it is rare to have an effluent discharge. In urban areas, runoff that flows over hot asphalt and concrete pavements before entering a lake will be artificially heated and could cause lake warming, although in most cases this impact is too small to be measured. Consequently, direct, measurable thermal pollution is not common. However, since streams and rivers constitute a major source of flow to some lakes, these lakes may be indirectly impacted by thermal pollution via inflows.

Temperature is reported in degrees on the Celsius temperature scale (C). There is no numerical Washington State water quality standard for lake temperature. The standard reads there will be "no measurable change from natural conditions." Temperatures for three Western Washington lakes are shown below to provide examples of the range you may expect to measure. The three lakes shown represent an oligotrophic lake (Summit Lake, Thurston County), a mesotrophic lake (Blackmans Lake, Snohomish County), and a eutrophic lake (Black Lake, Thurston County). These same lakes are used throughout this chapter to provide values for comparison purposes.

Temperatures (degrees C^o) Measured in the Top Layer (Epilimnion) and Bottom Layer (Hyoplimnion) of Three Western Washington Lakes in June and September 1989.

The next section talks about dissolved oxygen in lakes.

Chapter Four provides information on how to measure temperature in lakes.

Temperature | Oxygen | pH | Secchi | Nutrients | Turbidity | Chlorophyll | Fecal Coliforms

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