organisms are poikilothermic - i.e., "cold-blooded" - which
means they are unable to internally regulate their core body temperature.
Therefore, temperature exerts a major influence on the biological activity
and growth of aquatic organisms. To a point, the higher the water temperature,
the greater the biological activity. Fish, insects, zooplankton, phytoplankton,
and other aquatic species all have preferred temperature ranges. As
temperatures get too far above or below this preferred range, the number
of individuals of the species decreases until finally there are few,
or none. For example, we would generally not expect to find a thriving
trout fishery in ponds or shallow lakes because the water is too warm
throughout the ice-free season.
in the growth rates of cold-blooded aquatic organisms and many
biochemical reaction rates can often be approximated by this
rule which predicts that growth rate will double if temperature
increases by 10°C (18°F) within their "preferred" range.
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 that 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.
Temperature is reported in degrees on the Celsius temperature scale
for Natural Variation
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 warm during the day and cool at night. In
deeper lakes (typically greater than 5 m for small lakes and 10 m for
larger ones) during summer, the water separates into layers of distinctly
different density caused
by differences in temperature. Unlike all other fluids, however, as
water approaches its freezing point and cools below 4°C, the opposite
effect occurs and its density then begins to decrease until it freezes
at 0°C (32°F). This is why ice floats. This process is called thermal
stratification. The surface water is warmed by the sun, but the
bottom of the lake remains cold. You can feel 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. The
most profound difference is usually seen in the oxygen profile
since the bottom layer is now isolated from the major source of oxygen
to the lake - the atmosphere.
surface water cools again in the fall to about the same temperature
as the lower water, the stratification is lost and the wind can turbulently
mix the two water masses together because their densities are so similar
A similar process also may occur during the spring as colder surface
waters warm to the temperature of bottom waters and the lake mixes
turnover). The lake mixing associated with a turnover often corresponds
with changes in many other chemical parameters that
in turn affect biological communities. Watch for these changes in your
lake this fall and spring.
deceases exponentially with depth in the water
column, the sun can heat a
greater proportion of the water in a shallow lake than in a deep lake
and so a shallow lake can warm up faster and to a higher temperature.
Lake temperature also is affected by the size and temperature of inflows (e.g.,
a stream during snowmelt, 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.
Impact of Pollution
(i.e., 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 pavement 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. In running waters,
particularly small urban streams, elevated temperatures from road and
parking lot runoff can be a serious problem for populations of cool
or cold-water fish already stressed from the other contaminants in
urban runoff. During summer, temperatures may approach their upper
tolerance limit. Higher temperatures also decrease the maximum amount
of oxygen that can be dissolved in the water, leading to oxygen stress
if the water is receiving high loads of organic matter.
Water temperature fluctuations in streams may be further worsened by
cutting down trees which provide shade and by absorbing more heat from
sunlight due to increased water turbidity.
J.P. 1991. A citizen's guide to understanding and monitoring lakes
and streams. Publ. #94-149. Washington State Dept. of Ecology, Publications
Office, Olympia, WA, USA (360) 407-7472.
1989. NALMS management guide for lakes and reservoirs. North American
Lake Management Society, P.O. Box 5443, Madison, WI, 53705-5443, USA (http://www.nalms.org).