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Session Five  --  Seaweeds
Session Five  --  Seaweeds

Temperatures in the Ocean
Temperatures in the Ocean


Below a zone of surface mixing, ocean temperatures are stable -- and cold.

   On land, we are used to rapidly changing temperatures.  The air is cool in the morning, warm in the afternoon, and cooling again as evening comes.  We are used to this, and don't think about it, but think of what this must be like for other life forms.

     For ectotherms, that are the same temperature as the environment, the rate of metabolism and growth varies over the course of the day.  For endotherms (animals here) that heat their bodies by burning energy, the organism must constantly work to keep the internal temperature steady.  When we hear of people suffering hypothermia from exposure to cold, or dying of heat stroke in the desert, we know that their bodies are not able to adapt to the changing temperature of their environment.

    As we think about life forms in the ocean, it is helpful to think about temperatures in the water.  Temperatures affect living organisms, because life processes are chemical, and chemical reactions occur more quickly in warm water than in cold water.

     Life forms in the ocean are exothermic, which means that their cells are the same temperature as the water that surrounds them.  (Exceptions to this are whales, seals, and dolphins, which are animals who have returned to the ocean after their ancestors evolved as land animals).  The temperature of the water in which these life forms live is an important factor for them.  

     Respiration is the word that describes the processes that living organisms do to stay alive.  It includes using the food that they eat and breathing the gases that they get from the environment.  As respiration is a chemical process, it works more slowly in cold organisms than in warm ones.  This is especially important for exothermic (cold blooded) animals, who can get along with very little food in a cold environment.

     For organisms in the water, temperature fluctuations do not really present a problem.  Water has a high Specific Heat,  which means that you have to put a lot of heat into water to warm it up, or take a lot of heat out of water to cool it down.  On a hot day the surface of the ocean may get a tiny bit warmer -- but it will not get hot like the sand, or the blacktop on the path down to the beach. On a cold night the surface of the water may cool a little -- but not as much as the land beside it.  The temperature in an aquatic environment tends to be stable, and to change very slowly if it does change.   

     However, as we learned in The Transfer of Heat: Conduction, Convection, and Radiation, warm air, water, and melted rock tend to rise, and cold air, water and magma tend to sink.  So it is reasonable to wonder if the great depths of the oceans are also cold.

Yes, they are.

     The chart below tells you about temperatures in the ocean.  On the surface of the ocean the sun warms the waters, and winds and water turbulence stir the warm water and mix it with the cooler water below.  The mixing zone goes down to somewhere between 100 to 400 meters, depending on the latitude and the weather.  

     As you would expect, the surface sea water temperatures are close to freezing near the poles.  You have seen pictures of ice sheets and ice bergs in the high latitudes.  In summer the ice sheets break up and the icebergs float into waters in the temperate zones and melt there.  

     In the tropics, surface waters are warm, sometimes coming close to human body temperatures.  

     Over the whole earth, the average temperature of the surface waters is about 63o Fahrenheit (17o  Celsius).

     Below this mixing zone, the waters begin to get steadily colder.  The area in which they are cooling is called the thermocline.  No matter what the surface temperatures were, the water cools down to nearly 0 o Celsius (32o  Fahrenheit) and stays at that temperature.

     About 90% of the ocean water is below the thermoclime..  The cold water below the thermocline is stratified (lies in layers, like the sedimentary rocks) by density, which is dependent on the water's temperature and salinity (saltiness).

 Here is a chart modified from Windows to the Universe which shows the temperature pattern in the mid-latitudes.    

Temperatures are Fahrenheit or Celsius,

The Photic Zone is the area into which sunlight penetrates.  It might be this deep in the open ocean, but would be shallower closer to land.

This is the part where the water gets colder very quickly.  It is already very dark.

Photosynthesizers cannot live here, but fishes and other animals can spend time here.  Some of them come up closer to the surface to feed.

Down here the water is just a little above freezing.  It is stratified in layers by density.  The saltiest water is at the bottom of this zone.




Most of the abyssal plain is at about this level.  The deepest point in the ocean is 6.8 miles (10.9 Kilometers) deep.

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Windows to the Universe team. Temperature of Ocean Water. Boulder, CO: ©2000-04 University Corporation of Atmospheric Research (UCAR), ©1995-1999, 2000 The Regents of the University of Michigan, Aug 313, 2001.   Online. Available: . Nov. 22, 2003.

© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at .