Seasons aren't the
only thing causing temperature variations in streams!
The heat of the day can warm them up; and stormwater running off of hot pavement
can cause dramatic temperature increases in streams.
Read more about how temperature can impact streams here.
Why is it Important?
Besides being important to swimmers and fishermen, most
aquatic 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, aquatic insects, zooplankton 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 particular, warm water impairs their reproduction-
trout eggs need cool, well oxygenated water to develop properly,
but it may also directly stress adult fish.
This is also a major concern for the brook trout populations in Duluth's
urban streams since water temperatures during heat waves and
low flow periods in summer are physiologically stressful. In
Miller Creek, in the Miller mall area of northern Duluth, intensive
studies have shown that trout are near their thermal maximum
in summer and that runoff water that heats up on parking lots
and in shallow sedimentation ponds contribute to the problem.
The removal of shoreline trees and shrubs associated with commercial
development also contributes to the problem by removing natural
to find out more about Miller Creek studies.
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.
Changes 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.
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. Warmer temperatures increase the solubility of salts
in water but decrease (just the opposite !) the solubility
of gasses in water. Warm soda pop going flat is an example
of this effect. Another 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
invertebrates or certain fish. Some compounds are also more
toxic to aquatic life at higher temperatures. Temperature
is reported in degrees on the Celsius temperature scale (°C).
Aquatic scientists usually measure temperature with electronic
thermometers such as a thermistor, a semiconducting material
that smoothly decreases in electrical resistance with increasing
temperature. Water temperature can be precisely determined
by maintaining a constant current through the thermistor and
measuring the voltage drop across it.
Although air temperatures reported in the news
media in the U.S. are given in degrees Fahrenheit, scientists
and the rest of he world usually record temperatures in Celsius,
because this is the unit designated by the International System
of Units. To convert from ° F to ° C, use this equation:
T °C = [T °F - 32 °F] * [5 °C / 9 °F]
Where T °C = temperature in Celsius and T° F = temperature in Fahrenheit
For example, the freezing temperature of water
is 32 °F; this translates to 0 °C. The boiling temperature
of water is 212 °F, or 100 °C.
10° C translates to 50 °F
20° C translates to 68 °F (just about perfect for
30° C translates to 86 °F
40° C translates to 104 °F (Whew ! - the melting point
of most Duluthians is 31 °C)
Reasons for Natural Variation
The most obvious reason for temperature change
in streams, wetlands and lakes is the change in seasonal air
temperature. Daily variation also may occur, especially in
the surface layers, which warm during the day and cool at
Streams, unless very large and slow moving,
differ from lakes in that water density is usually uniform
enough with depth that little depth variation temperature
is evident. This greatly contrasts with deeper lakes (typically
greater than 5 meters for small lakes and 10 meters for larger
ones) that thermally stratify. People who want to know a lot
more about how lakes work should visit Water-on-the-Web (WOW)
Expected Impact of Pollution
Thermal pollution (i.e., artificially high temperatures) in
larger streams usually 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 with smaller streams
such as Duluth, runoff that flows over hot asphalt and concrete
pavement before entering a stream or pond will be artificially
heated and can cause significant warming. In running waters,
particularly small urban streams during low flow periods, 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. Since trout
eggs require cool, well oxygenated water, reproduction may be
directly impaired by this pollutant in addition to its effects
on adult and juvenile fish survival. Another less well understood
indirect effect relates to disease. Since bacteria and other
disease causing organisms grow faster in warm water, the susceptibility
of aquatic organisms to disease in warm water increases as well.
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.
Michaud, 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.
Moore, M.L. 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).
Holdren, C., W. Jones and J. Taggart. 2001. Managing lakes and
reservoirs. North American Lake Management Society, P.O. Box
5443, Madison, WI, 53705-5443, USA and Terrene Institute, 4
Herbert Street, Alexandria, VA 22305, USA (http://www.terrene.org)